IIT JEE 2012 Solved Paper by Prabhat Gaurav

Solutions of IIT-JEE-2012
CODE
PAPER 1
Time: 3 Hours Maximum Marks: 210
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12. Section I contains 10 multiple choice questions. Each question has four choices (A), (B), (C) and (D) out of which
ONLY ONE is correct.
13. Section II contains 5 multiple choice questions. Each question has four choice (A), (B), (C) and (D) out of which
ONE or MORE are correct.
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inclusive).
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incorrect answer in this section.
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8
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IITJEE-2011-Paper 1-CPM-2
PAPER-1 [Code – 8]
IITJEE 2012
SECTION I : Single Correct Answer Type
This section contains 10 multiple choice questions. Each question has four choices (A), (B), (C) and (D) out of
which ONLY ONE is correct.
1. In the determination of Young’s modulus 2
4MLg
Y
d
 
= 
π l
by using Searle’s method, a wire of length L = 2m
and diameter d = 0.5 mm is used. For a load M = 2.5 kg, an extension l = 0.25 mm in the length of the wire
is observed. Quantities d and l are measured using a screw gauge and a micrometer, respectively. They
have the same pitch of 0.5 mm. The number of divisions on their circular scale is 100. The contributions to
the maximum probable error of the Y measurement
(A) due to the errors in the measurements of d and l are the same.
(B) due to the error in the measurement of d is twice that due to the error in the measurement of l.
(C) due to the error in the measurement of l is twice that due to the error in the measurement of d.
(D) due to the error in the measurement of d is four times that due to the error in the measurement of l.
Sol. (A)
0.5
L.C. 0.005
100
= = mm
Y 2 (d)
Y d
∆ ∆ ∆
= +
l
l
3
3
0.005 10 1
500.25 10
−
−
∆ ×
= =
×
l
l
3
3
(d) 2 0.005 10 1
2
d 500.5 10
−
−
∆ × ×
= =
×
2. A small mass m is attached to a massless string whose other end is fixed at P as shown in the figure. The
mass is undergoing circular motion in the x-y plane with centre at O and constant angular speed ω. If the
angular momentum of the system, calculated about O and P are denoted by OL
r
and PL
r
respectively, then
z
P
O m
ω
(A) OL
r
and PL
r
do not vary with time.
(B) OL
r
varies with time while PL
r
remains constant.
(C) OL
r
remains constant while PL
r
varies with time.
(D) OL
r
and PL
r
both vary with time.
Sol. (C)
PART - I: PHYSICS

IITJEE-2012-Paper 1-PCM-3
PL
r
O m
ω
OL
r
3. A bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. Refractive index n of
the first lens is 1.5 and that of the second lens is 1.2. Both the curved surface are of the same radius of
curvature R = 14 cm. For this bi-convex lens, for an object distance of 40 cm, the image distance will be
R = 14 cm
n = 1.2n = 1.5
(A) −280.0 cm (B) 40.0 cm (C) 21.5 cm (D) 13.3 cm
Sol. (B)
T
1 1
P (1.5 1) 0 (1.2 1) 0
14 14
   
= − − + − −   
−   
=
0.5 0.2 1
14 14 20
+ =
f = +20 cm
1 1 1
v 40 20
− =
−
1 1 1 1
v 20 40 40
= − =
∴ v = 40 cm
4. A thin uniform rod, pivoted at O, is rotating in the horizontal plane with
constant angular speed ω, as shown in the figure. At time t = 0, a small
insect starts from O and moves with constant speed v, with respect to the
rod towards the other end. It reaches the end of the rod at t = T and stops.
The angular speed of the system remains ω throughout. The magnitude of
the torque (| τ
r
|) about O, as a function of time is best represented by which
plot?
ω
v →
O
z
(A)
O
T
t
| τ
r
|
(B)
O
T
t
| τ
r
|
(C)
O
T
t
| τ
r
|
(D)
O
T
t
| τ
r
|
Sol. (B)
τ = ω 2 2dI d
(C mv t )
dt dt
= ω +
= mωv2
2t.

5. A mixture of 2 moles of helium gas (atomic mass = 4 amu) and 1 mole of argon gas (atomic mass = 40
amu) is kept at 300 K in a container. The ratio of the rms speeds rms
rms
v (helium)
v (argon )
 
 
 
is
(A) 0.32 (B) 0.45 (C) 2.24 (D) 3.16
Sol. (D)
vrms =
3RT
M
Required ratio = Ar
He
M 40
10
M 4
= =
= 3.16.
6. Two large vertical and parallel metal plates having a separation of 1 cm are connected to a DC voltage
source of potential difference X. A proton is released at rest midway between the two plates. It is found to
move at 450
to the vertical JUST after release. Then X is nearly
(A) 1 × 10-5
V (B) 1 × 10-7
V (C) 1 × 10-9
V (D) 1 × 10-10
V
Sol. (C)
qE = mg
q(V/d) = mg
V =
mgd
q
=
27 2
19
1.67 10 10 10
1.6 10
− −
−
× × ×
×
=
28
9
19
10
10 V
10
−
−
−
=
7. Three very large plates of same area are kept parallel and close to each other. They are considered as ideal
black surfaces and have very high thermal conductivity. The first and third plates are maintained at
temperatures 2T and 3T respectively. The temperature of the middle (i.e. second) plate under steady state
condition is
(A)
1/4
65
T
2
 
 
 
(B)
1/4
97
4
 
 
 
T (C)
1/4
97
2
 
 
 
T (D) 1/4
(97) T
Sol. (C)
σA (2T)4
+ σA (3T)4
= σ 2A(T′)4
16T4
+ 81T4
= 2 (T′)4
97 T4
= 2 (T′)4
(T′)4
=
97
2
T4
∴ T′ =
1/4
97
T
2
 
 
 
3T2T

8. A small block is connected to one end of a massless spring of un-stretched length 4.9 m. The other end of
the spring (see the figure) is fixed. The system lies on a horizontal frictionless surface. The block is
stretched by 0.2 m and released from rest at t = 0. It then executes simple harmonic motion with angular
frequency ω = π/3 rad/s. Simultaneously at t = 0, a small pebble is projected with speed v form point P at
an angle of 450
as shown in the figure. Point P is at a horizontal distance of 10 m from O. If the pebble hits
the block at t = 1 s, the value of v is (take g = 10 m/s2
)
450
v
x
P
10 m
O
z
(A) 50 m / s (B) 51 m / s (C) 52 m / s (D) 53 m / s
Sol. (A)
0
2vsin 45
1
g
=
∴ v = 50 m / s .
9. Young’s double slit experiment is carried out by using green, red and blue light, one color at a time. The
fringe widths recorded are βG, βR and βB, respectively. Then,
(A) βG > βB > βR (B) βB > βG > βR (C) βR > βB > βG (D) βR > βG > βB
Sol. (D)
λR > λG > λB
∴ βR > βG > βB
10. Consider a thin spherical shell of radius R with centre at the origin, carrying uniform positive surface
charge density. The variation of the magnitude of the electric field | E(r)
r
| and the electric potential V(r)
with the distance r from the centre, is best represented by which graph?
(A)
O rR
V(r)|E(r)|
→ (B)
O rR
V(r)|E(r)|
→
(C)
O rR
V(r)|E(r)|
→ (D)
O rR
V(r)|E(r)|
→
Sol. (D)

SECTION II : Multiple Correct Answer(s) Type
which ONE or MORE are correct.
11. Consider the motion of a positive point charge in a region where there are simultaneous uniform electric
and magnetic fields 0
ˆE E j=
ur
and 0
ˆB B j.=
r
At time t = 0, this charge has velocity v
r
in the x-y plane,
making an angle θ with the x-axis. Which of the following option(s) is (are) correct for time t > 0?
(A) If 0 ,θ = ° the charge moves in a circular path in the x-z plane.
(B) If 0 ,θ = ° the charge undergoes helical motion with constant pitch along the y-axis.
(C) If 10 ,θ = ° the charge undergoes helical motion with its pitch increasing with time, along the y-axis.
(D) If 90 ,θ = ° the charge undergoes linear but accelerated motion along the y-axis.
Sol. (C, D)
If 90 ,Bθ = °
r
exerts no force on q.
If 0 ,10 ;θ = ° ° the charge particle moves in helix with increasing pitch due to E
r
along y-axis.
12. A cubical region of side a has its centre at the origin. It encloses three fixed point charges, −q at
(0, −a/4, 0), +3q at (0, 0, 0) and −q at (0, +a/4, 0). Choose the correct options(s)
a
y
x
z
–q
–q 3q
(A) The net electric flux crossing the plane x = +a/2 is equal to the net electric flux crossing the plane x =
−a/2
(B) The net electric flux crossing the plane y = +a/2 is more than the net electric flux crossing the plane y =
−a/2.
(C) The net electric flux crossing the entire region is
0
q
ε
.
(D) The net electric flux crossing the plane z = +a/2 is equal to the net electric flux crossing the plane x =
+a/2.
Sol. (A, C, D)
Net flux through the cubical region
0 0
q 3q q q− + −
= =
ε ε
The flux passing through the faces
a
x ,
2
−
=
a
x
2
= + and
a
z
2
= + are same due to symmetry.
13. A person blows into open-end of a long pipe. As a result, a high pressure pulse of air travels down the pipe.
When this pulse reaches the other end of the pipe,
(A) a high-pressure pulse starts travelling up the pipe, if the other end of the pipe is open.
(B) a low-pressure pulse starts travelling up the pipe, if the other end of the pipe is open.
(C) a low-pressure pulse starts travelling up the pipe, if the other end of the pipe is closed.
(D) a high-pressure pulse starts travelling up the pipe, if the other end of the pipe is closed.
Sol. (B, D)
At the open end, the phase of a pressure wave changes by π radian due to reflection. At the closed end,
there is no change in the phase of a pressure wave due to reflection.

14. A small block of mass of 0.1 kg lies on a fixed inclined plane PQ which makes an angle θ with the
horizontal. A horizontal force of 1 N acts on the block through its centre of mass as shown in the figure.
The block remains stationary if (take g = 10 m/s2
)
θ
1N
PO
Q
(A) θ = 45°
(B) θ > 45° and a frictional force acts on the block towards P.
(C) θ > 45° and a frictional force acts on the block towards Q.
(D) θ < 45° and a frictional force acts on the block towards Q.
Sol. (A, C)
At 45θ = ° , mg sin θ = 1 cos× θ
At 45θ > ° , mg sin θ > 1 cos× θ (friction acts upward)
At 45θ < ° , mg sin θ < 1 cos× θ (friction acts downward)
15. For the resistance network shown in the figure, choose the correct option(s)
Q
2Ω 2Ω
4Ω4Ω
T
SP I2
I1
4Ω
2Ω
1Ω1Ω
12V
(A) The current through PQ is zero. (B) I1 = 3A
(C) The potential at S is less than that at Q. (D) I2 = 2A
Sol. (A, B, C, D)
Nodes P and Q are equipotential and nodes S
and T are equipotential from wheatstone bridge,
no current passes through PQ and ST.
1
12
I 3A
4
= =
2 1
12
I I 2A
6 12
 
= = 
+ 
I2
I1
12 V
6Ω
12 Ω

SECTION III : Integer Answer Type
This section contains 5 questions. The answer to each question is single digit integer, ranging from 0 to 9 (both
inclusive).
16. A circular wire loop of radius R is placed in the x-y plane centered at the origin O. A square loop of side
a(a<<R) having two turns is placed with its centre at z = 3R along the axis of the circular wire loop, as
shown in figure. The plane of the square loop makes an angle of 45° with respect to the z-axis. If the mutual
inductance between the loops is given by
2
0
p/2
a
2 R
µ
, then the value of p is
45°
R
O
y
x
z
3R
a
Sol. (7)
2
20
3
IR
2 a cos45
2(8R )N
M
I I
 µ
° 
φ  = =
2 2
0 0
1/2 7/2
a a
8R2 R2
µ µ
=
So P = 7
17. An infinitely long solid cylinder of radius R has a uniform volume charge density ρ. It has a spherical
cavity of radius R/2 with its centre on the axis of the cylinder, as shown in the figure. The magnitude of the
electric field at the point P, which is at a distance 2R from the axis of the cylinder, is given by the
expression
0
23 R
16k
ρ
ε
. The value of k is
P
R
R/2
2R
y
x
z
Sol. (6)
( )
3
2
0
4 R ˆK j
ˆ 3 8(j)
E
2 (2R) 4R
 
ρ π − 
λ  = +
πε
r
2
0
ˆ ˆR (j) K R( j)
E
4 R 24
ρπ πρ −
= +
πε
r
Kˆ ˆE K R(j) R( j)
24
= ρπ + ρπ −
r
P
y
x
z
0
23 23ˆ ˆE K R (j) R(j)
24 96
= ρπ = ρ
ε
r
⇒ k = 6

18. A proton is fired from very far away towards a nucleus with charge Q = 120 e, where e is the electronic
charge. It makes a closest approach of 10 fm to the nucleus. The de Broglie wavelength (in units of fm) of
the proton at its start is: (take the proton mass, mp = (5/3) × 10-27
kg; h/e = 4.2 × 10-15
J.s / C;
9
0
1
9 10
4
= ×
πε
m/F; 1 fm = 10-15
)
Sol. (7)
2
15 15
1 K(Q)e K(120e)e
0 mv
2 10 10 10 10− −
+ = =
× ×
9 19 2
27 2
15
1 5 9 10 120 (1.6 10 )
10 v
2 3 10 10
−
−
−
× × × ×
× × =
×
9 38
42
9 6 10 120 2.56 10
v
50 10
−
−
× × × × ×
=
×
13
v 331.776 10= ×
h
mv
λ =
15 19
27 13
4.2 10 1.6 10
5
10 331.776 10
3
− −
−
× × ×
λ =
× × ×
34
21
4.2 4.8 10
57.6 5 10
−
−
× ×
=
× ×
13
0.07 10−
= ×
15
7 10−
λ = × = 7fm
19. A lamina is made by removing a small disc of diameter 2R from a bigger disc of uniform mass density and
radius 2R, as shown in the figure. The moment of inertia of this lamina about axes passing though O and P
is IO and IP respectively. Both these axes are perpendicular to the plane of the lamina. The ratio IP / IO to
the nearest integer is
O
2R
P2R
Sol. (3)
( )
2 2
2 2
p
4mR m R m
I m 4R 5R
2 4 2 4
   
= + − +   
   
2
p
1 5
I mR (2 4)
8 4
  
= + − +  
  
2 2
p
11 37
I mR 6 mR
8 8
 
= − = 
 
...(1)
2 2
2
0
4mR m R m
I R
2 4 2 4
   
= − +   
   
2 2 2
0
1 1 3 13
I mR 2 mR 2 mR
8 4 8 8
      
= − + = − =      
      
...(2)
So
p
0
I 37 / 8
3
I 13/ 8
= (Nearest Integer)

20. A cylindrical cavity of diameter a exists inside a cylinder of diameter 2a as shown in the figure. Both the
cylinder and the cavity are infinity long. A uniform current density J flows along the length. If the
magnitude of the magnetic field at the point P is given by 0
N
aJ
12
µ , then the value of N is
O
a
2a
P
Sol. (5)
2 2
0 0(J a ) (J a / 4)
B
3a2 a
2
2
µ π µ π
= −
π  
π 
 
0 05 Ja NJa
B
12 12
µ µ
= =
So N = 5

IITJEE 2012
SECTION – I: Single Correct Answer Type
21. A compound MpXq has cubic close packing (ccp) arrangement of X. Its unit cell structure is shown below.
The empirical formula of the compound is
(A) MX (B) MX2
(C) M2X (D) M5X14
Sol. (B)
1 1
X 8 6 4
8 2
= × + × =
1
M 4 1 2
4
= × + =
So, unit cell formula of the compound is M2X4 and the empirical formula of the compound is MX2.
22. The carboxyl functional group (–COOH) is present in
(A) picric acid (B) barbituric acid
(C) ascorbic acid (D) aspirin
Sol. (D)
COOH
O C
O
CH3
(Asprin)
OH
NO2
O2N
NO2
(Picric Acid)
NHNH
O
OO
(Barbituric Acid)
O O
OH
OH
OH
OH
(Ascorbic Acid)
23. As per IUPAC nomenclature, the name of the complex [Co(H2O)4(NH3)2]Cl3 is
(A) Tetraaquadiaminecobalt (III) chloride (B) Tetraaquadiamminecobalt (III) chloride
(C) Diaminetetraaquacobalt (III) chloride (D) Diamminetetraaquacobalt (III) chloride
Sol. (D)
[Co(H2O)4(NH3)2]Cl3
Diamminetetraaquacobalt (III) chloride
PART - II: CHEMISTRY

24. In allene (C3H4), the type(s) of hybridization of the carbon atoms is (are)
(A) sp and sp3
(B) sp and sp2
(C) only sp2
(D) sp2
and sp3
Sol. (B)
2 2
sp sp sp
2 2H C C CH= =
25. The kinetic energy of an electron in the second Bohr orbit of a hydrogen atom is [a0 is Bohr radius]
(A)
2
2 2
0
h
4 maπ
(B)
2
2 2
0
h
16 maπ
(C)
2
2 2
0
h
32 maπ
(D)
2
2 2
0
h
64 maπ
Sol. (C)
As per Bohr’s postulate,
nh
mvr
2
=
π
So,
nh
v
2 mr
=
π
21
KE mv
2
=
So,
2
1 nh
KE m
2 2 mr
 
=  
π 
Since,
2
oa n
r
z
×
=
So, for 2nd
Bohr orbit
2
o
o
a 2
r 4a
1
×
= =
( )
2 2
22 2
0
1 2 h
KE m
2 4 m 4a
 
 =
 π × 
2
2 2
0
h
KE
32 ma
=
π

26. Which ordering of compounds is according to the decreasing order of the oxidation state of nitrogen?
(A) HNO3, NO, NH4Cl, N2 (B) HNO3, NO, N2, NH4Cl
(C) HNO3, NH4Cl, NO, N2 (D) NO, HNO3, NH4Cl, N2
Sol. (B)
5 2 0 3
3 2 4H NO , NO, N , N H Cl
+ + −
27. For one mole of a van der Waals gas when b = 0 and T = 300 K, the PV vs. 1/V plot is shown below. The
value of the van der Waals constant a (atm.litre2
mol–2
) is
(A) 1.0 (B) 4.5
(C) 1.5 (D) 3.0
Sol. (C)
θ
van der Waals equation for 1 mole of real gas is,
( )2
a
P V b RT
V
 
+ − = 
 
but, b = 0 (given)
( )2
a
P V RT
V
 
⇒ + = 
 
∴ PV
1
a RT
V
= − × + …..(i)
y = mx + c
Slope = tan (π – θ) = –a
So, tanθ = a =
21.6 20.1
3 2
−
−
=1.5
or,
24.6 20.1
tan 1.5
3 0
−
θ = =
−
28. The number of aldol reaction(s) that occurs in the given transformation is

3
conc.aq. NaOH
CH CHO 4HCHO+ →
OH OH
OHOH
(A) 1 (B) 2
(C) 3 (D) 4
Sol. (C)
CH3 C H
O
OH CH2 C
CH2
H
CH O
OH
( )i OH−
( )ii H C
O
H
(aldol)
C CH O
CH2
CH2
CH2OH
OH
OH
( )
conc.NaOH HCHO
crossed Cannizzaro reaction
+
←+H C
O
O
2 2HO CH CH CH O− − − =
( )
( )
( )
i OH
ii H CH O
Aldol
−
− =
→ ( )
( )
( )
i OH
ii H CH O
Aldol
−
− =
→
OH OH
OHOH
29. The colour of light absorbed by an aqueous solution of CuSO4 is
(A) orange- red (B) blue-green
(C) yellow (D) violet
Sol. (A)
Aqueous solution of copper sulphate absorbs orange red light and appears blue (complementary colour).
B G
Y
O
R
V
30. The number of optically active products obtained from the complete ozonolysis of the given compound is
CH3 CH CH C
CH3
H
CH CH C
H
CH3
CH CH CH3
(A) 0 (B) 1
(C) 2 (D) 4
Sol. (A)

CH3 CH CH C
CH3
H
CH CH C
H
CH3
CH CH CH3
3 2O / Zn / H O
CH3 CH O
(achiral)
+ O CH C
CH3
H
CH O
(achiral)
+ O HC C
H
CH3
CH O
(achiral)
+ CH3 CH O
(achiral)
SECTION II : Multiple Correct Answer (s) Type
31. Which of the following hydrogen halides react(s) with AgNO3(aq) to give a precipitate that dissolves in Na-
2S2O3(aq)?
(A) HCl (B) HF
(C) HBr (D) HI
Sol. (A, C, D)
( )3 3HX AgNO AgX HNO X Cl, Br, I+ → ↓ + =
( )2 2 3 3 2 3 2
AgX 2Na S O Na Ag S O NaX + → + 
32. Identify the binary mixture(s) that can be separated into individual compounds, by differential extraction, as
shown in the given scheme.
Binary mixture containing
Compound 1 and Compound 2
( )NaOH aq
→
( )3NaHCO aq
→
Compound 1
Compound 1 +
+ Compound 2
Compound 2
(A) C6H5OH and C6H5COOH (B) C6H5COOH and C6H5CH2OH
(C) C6H5CH2OH and C6H5OH (D) C6H5CH2OH and C6H5CH2COOH
Sol. (B, D)
(A) Both are soluble in NaOH, hence inseparable.
(B) Only benzoic acid (C6H5COOH) is soluble in NaOH and NaHCO3, while benzyl alcohol (C6H5CH2OH)
is not. Hence, separable.
(C) Although NaOH can enable separation between benzyl alcohol (C6H5CH2OH) and phenol (C6H5OH) as
only the later is soluble in NaOH. However, in NaHCO3, both are insoluble. Hence, inseparable.
(D) α-phenyl acetic acid (C6H5CH2COOH) is soluble in NaOH and NaHCO3. While benzyl alcohol
(C6H5CH2OH) is not. Hence, separable.
33. For an ideal gas, consider only P-V work in going from an initial state X to the final state Z. The final state
Z can be reached by either of the two paths shown in the figure. Which of the following choice(s) is(are)
correct? [Take ∆S as change in entropy and w as work done]

P(atmosphere)
V(liter)
Z
X Y
(A) x z x y y zS S S→ → →∆ = ∆ + ∆ (B) x z x y y zw w w→ → →= +
(C) x y z x yw w→ → →= (D) x y z x yS S→ → →∆ = ∆
Sol. (A, C)
( )X Z X Y Y ZS S S entropy S is a state function, hence additive→ → →∆ = ∆ + ∆   
X Y Z X Yw w→ → →= (work done in Y → Z is zero as it is an isochoric process)
34. Which of the following molecules, in pure form, is (are) unstable at room temperature?
(A) (B)
(C) O (D) O
Sol. (B, C)
Compound and
O
being antiaromatic are unstable at room temperature.
35. Choose the correct reason(s) for the stability of the lyophobic colloidal particles.
(A) Preferential adsorption of ions on their surface from the solution
(B) Preferential adsorption of solvent on their surface from the solution
(C) Attraction between different particles having opposite charges on their surface
(D) Potential difference between the fixed layer and the diffused layer of opposite charges around the
colloidal particles
Sol. (A, D)
Lyophobic colloids are stable due to preferential adsorption of ions on their surface from solution and
potential difference between the fixed layer and the diffused layer of opposite charges around the colloidal
particles that makes lyophobic sol stable.
SECTION III: Integer Answer Type
This section contains 5 questions. The answer to each question is a single digit integer, ranging from 0 to 9 (both
inclusive).
36. 29.2 % (w/w) HCl stock solution has density of 1.25 g mL-1
. The molecular weight of HCl is 36.5 g mol-1
.
The volume (mL) of stock solution required to prepare a 200 mL solution of 0.4 M HCl is
Sol. (8)

Stock solution of HCl = 29.2% (w/w)
Molarity of stock solution of HCl
29.2 1000 1.25
36.5 100
× ×
=
×
If volume of stock solution required = V ml
29.2 1000
V 200 0.4
36.5 80
× × = ×
⇒ V = 8 ml
37. The substituents R1 and R2 for nine peptides are listed in the table given below. How many of these
peptides are positively charged at pH = 7.0?
H3N CH
H
CO NH CH CO NH CH CO NH CH
H
COO
R2R1
Peptide R1 R2
I H H
II H CH3
III CH2COOH H
IV CH2CONH2 (CH2)4NH2
V CH2CONH2` CH2CONH2
VI (CH2)4NH2 (CH2)4NH2
VII CH2COOH CH2CONH2
VIII CH2OH (CH2)4NH2
IX (CH2)4NH2` CH3
Sol. (4)
Peptides with isoelectric point (pI) > 7, would exist as cation in neutral solution (pH = 7).
IV, VI, VIII and IX
38. An organic compound undergoes first-order decomposition. The time taken for its decomposition to 1/8 and
1/10 of its initial concentration are t1/8 and t1/10 respectively. What is the value of
[ ]
[ ]
1/8
1/10
t
10?
t
× (take log102 =
0.3)
Sol. (9)
1/8
2.303log8 2.303 3log2
t
k k
×
= =
1/10
2.303 2.303
t log10
k k
= =
1/8
1/10
2.303 3log 2
t k
10 10 9
2.303t
k
× 
    × = × = 
  
 
 
39. When the following aldohexose exists in its D-configuration, the total number of stereoisomers in its
pyranose form is
CHO
CH2
CHOH
CHOH
CHOH
CH2OH

Sol. (8)
H H
CHO
H OH
H OH
H OH
H2C OH
1
2
3
4
5
6
O
CH2
H
H
OH
H
H
H
OH
H
OH
OH
1
23
4
5
Fixed in D-configuration
6
Hence total number of stereoisomers in pyranose form of D-configuration = 23
= 8
40. The periodic table consists of 18 groups. An isotope of copper, on bombardment with protons, undergoes a
nuclear reaction yielding element X as shown below. To which group, element X belongs in the periodic
table?
63 1 1 1
29 1 0 1Cu H 6 n 2 H X+ → + α + +
Sol. (8)
63 1 1 4 1 A
29 1 0 2 1 ZCu H 6 n He 2 H X+ → + + +
Mass number: 63 + 1 = 1 × 6 + 4 + 1 × 2 + A
A = 64 – 12 = 52
Atomic number: 29 + 1 = 6 × 0 + 2 + 2 × 1 + Z
Z = 30–4 = 26
A 52
Z 26X Fe=
Hence X is in group ‘8’ in the periodic table.

IITJEE 2012
SECTION I : Single Correct Answer Type
41. If
2
1
lim 4
1→∞
 + +
− − = 
+ x
x x
ax b
x
, then
(A) a = 1, b = 4 (B) a = 1, b = –4
(C) a = 2, b = – 3 (D) a = 2, b = 3
Sol. (B)
Given
2
1
lim 4
1→∞
 + +
− − =  + x
x x
ax b
x
⇒
( )
2 2
1
lim 4
1→∞
+ + − − − −
=
+x
x x ax ax bx b
x
⇒
( ) ( ) ( )
( )
2
1 1 1
lim 4
1→∞
− + − − + −
=
+x
a x a b x b
x
⇒ 1 − a = 0 and 1 − a − b = 4 ⇒ b = −4, a = 1.
42. Let P = [aij] be a 3 × 3 matrix and let Q = [bij], where bij = 2i + j
aij for 1 ≤ i, j ≤ 3. If the determinant of P is 2,
then the determinant of the matrix Q is
(A) 210
(B) 211
(C) 212
(D) 213
Sol. (D)
2 3 4
11 12 13
3 4 5
21 22 23
4 5 6
31 32 33
2 2 2
2 2 2
2 2 2
=
a a a
Q a a a
a a a
⇒
11 12 13
2 3 4
21 22 23
2 2 2
31 32 33
2 2 2 2 2 2
2 2 2
= ⋅ ⋅
a a a
Q a a a
a a a
11 12 13
9 2
21 22 23
31 32 33
2 2 2= ⋅ ⋅
a a a
Q a a a
a a a
⇒ |Q| = 212
|P|
|Q| = 213
.
43. The locus of the mid–point of the chord of contact of tangents drawn from points lying on the straight line
4x – 5y = 20 to the circle x2
+ y2
= 9 is
(A) 2 2
20( ) 36 45 0+ − + =x y x y (B) 2 2
20( ) 36 45 0+ + − =x y x y
(C) 2 2
36( ) 20 45 0+ − + =x y x y (D) 2 2
36( ) 20 45 0+ + − =x y x y
Sol. (A)
Equation of the chord bisected at P (h, k)
hx + ky = h2
+ k2
…(i)
Let any point on line be
4
, 4
5
 
α α − 
 
Equation of the chord of contact is
PART - III: MATHEMATICS

⇒
4
4 9
5
 
α + α − = 
 
x y …(ii)
Comparing (i) and (ii)
2 2
4 94
5
+
= =
α α −
h k h k
α =
20h
4h 5k−
Now,
( ) 2 2h 4h 5k h k
20h 9
− +
=
20 (h2
+ k2
) = 9 (4h − 5k)
20 (x2
+ y2
) − 36x + 45y = 0.
P(h, k)
x y
1
5 4
− =
x
y
x
2
+y
2
= 9
(α,
4
5
α−4)
O
44. The total number of ways in which 5 balls of different colours can be distributed among 3 persons so that
each person gets at least one ball is
(A) 75 (B) 150
(C) 210 (D) 243
Sol. (B)
Number of ways
= 35
− 3
C1.25
+ 3
C215
= 243 − 96 + 3 = 150.
45. The integral
2
9
2
sec
(sec tan )+
∫
x
dx
x x
equals (for some arbitrary constant K)
(A) { }2
11
2
1 1 1
(sec tan )
11 7(sec tan )
− − + +
+
x x K
x x
(B) { }2
11
2
1 1 1
(sec tan )
11 7(sec tan )
− + +
+
x x K
x x
(C) { }2
11
2
1 1 1
(sec tan )
11 7(sec tan )
− + + +
+
x x K
x x
(D) { }2
11
2
1 1 1
(sec tan )
11 7(sec tan )
+ + +
+
x x K
x x
Sol. (C)
2
9
2
sec
I
(sec tan )
=
+
∫
x
dx
x x
Let sec x + tan x = t
⇒ sec x − tan x = 1/t
Now (sec x tan x + sec2
x) dx = dt
sec x (sec x + tan x) dx = dt
sec x dx =
dt
t
,
1 1
2
 
+ 
 
t
t
= sec x
I = 9
2
1
1
2
 
+ 
 
∫
t
dtt
tt
=
9 13
2 2
1
( )
2
− −
+∫ t t dt
=
9 131 1
2 21
9 132 1 1
2 2
− + − + 
 +
 
− + − +  
t t

=
7 11
2 21
7 112
2 2
− − 
 +
 
− −  
t t
=
7 11
2 2
1 1
7 11
− −
− −t t
= 7 11
22
1 1 1 1
7 11
− −
tt
=
( )
( )
2
2
11 11/2
2
1 1 1 1 1
sec tan
11 7 11 7sec tan
   
− + = − + + +      + 
t
x x k
x xt
46. The point P is the intersection of the straight line joining the points Q(2, 3, 5) and R(1, –1, 4) with the plane
5x – 4y – z = 1. If S is the foot of the perpendicular drawn from the point T(2, 1, 4) to QR, then the length
of the line segment PS is
(A)
1
2
(B) 2
(C) 2 (D) 2 2
Sol. (A)
D. R. of QR is 1, 4, 1
Coordinate of P ≡
4 1 13
, ,
3 3 3
 
 
 
D. R. of PT is 2, 2, –1
Angle between QR and PT is 45º
And PT = 1
⇒ PS = TS =
1
2
T(2, 1, 4)
Q(2, 3, 5)
P
4 1 13
, ,
3 3 3
 
 
 
R(1, –1, 4)
θ
S
47. Let
2
cos , 0
( )
, ,
0, 0
 π
≠

= 
∈
 =
x x
x
f x
x
x
then f is
(A) differentiable both at x = 0 and at x = 2
(B) differentiable at x = 0 but not differentiable at x = 2
(C) not differentiable at x = 0 but differentiable at x = 2
(D) differentiable neither at x = 0 nor at x = 2
Sol. (B)
( )
( ) ( )
0
0 0
0 lim
→
+ −
′ =
h
f h f
f
h
=
2
0
cos 0
lim
→
π
−
h
h
h
h
=
0
lim cos 0
→
π 
= 
 h
h
h
so, f(x) is differentiable at x = 0
( ) ( ) ( )
0
2 2
2 lim+
→
+ −
′ =
h
f h f
f
h
=
( )2
0
2 cos 0
2
lim
→
π
+ −
+
h
h
h
h

=
( )2
0
2 cos
2
lim
→
π 
+  
+ 
h
h
h
h
( )
2
0
(2 )
2 lim sin
2 2
+
→
+ π π ′ = − 
+ h
h
f
h h
=
2
0
(2 )
lim sin
2(2 )→
 + π⋅
 + h
h h
h h
=
2
0
(2 )
lim sin
2(2 ) 2(2 )
2(2 )
→
+ π π
×
π + +
+
h
h h
h h h
h
= π
Again, ( ) 0
(2 ) (2)
2 lim−
→
− −
′ =
−h
f h f
f
h
=
( )2
0
2 cos
2
lim
→
π 
−  
− 
−h
h
h
h
=
( )2
0
2 cos
2
lim
→
π 
− −  
− 
−h
h
h
h
=
2
0
(2 ) sin
2 2
lim
→
π π 
− − − 
h
h
h
h
=
2
0
(2 )
lim sin
2(2 )→
 − −π
⋅  − h
h h
h h
=
2
0
(2 )
lim sin
2(2 ) 2(2 )
2(2 )
→
− π π
− ⋅ × = −π
π − −
−
h
h h
h h h
h
48. Let z be a complex number such that the imaginary part of z is nonzero and a = z2
+ z + 1 is real. Then a
cannot take the value
(A) –1 (B)
1
3
(C)
1
2
(D)
3
4
Sol. (D)
Given equation is z2
+ z + 1 − a = 0
Clearly this equation do not have real roots if
D < 0
⇒ 1 − 4(1 − a) < 0
⇒ 4a < 3
a <
3
4
.
49. The ellipse
2 2
1 : 1
9 4
+ =
x y
E is inscribed in a rectangle R whose sides are parallel to the coordinate axes.
Another ellipse E2 passing through the point (0, 4) circumscribes the rectangle R. The eccentricity of the
ellipse E2 is
(A)
2
2
(B)
3
2

(C)
1
2
(D)
3
4
Sol. (C)
Equation of ellipse is (y + 2) (y − 2) + λ(x + 3) (x − 3) = 0
It passes through (0, 4) ⇒ λ =
4
3
Equation of ellipse is
2 2
1
12 16
+ =
x y
e =
1
2
.
y
(0, 4)
x
y = 2
y = −2 (3, −2)(−3, −2)
(−3, 2)
x = −3
x = 3
Alternate
Let the ellipse be
2 2
2 2
x y
1
a b
+ = as it is passing through (0, 4) and (3, 2).
So, b2
= 16 and 2
9 4
1
16a
+ =
⇒ a2
= 12
So, 12 = 16 (1 − e2
)
⇒ e = 1/2.
50. The function f : [0, 3] → [1, 29], defined by f(x) = 2x3
– 15x2
+ 36x + 1, is
(A) one–one and onto (B) onto but not one–one
(C) one–one but not onto (D) neither one–one nor onto
Sol. (B)
f (x) = 2x3
− 15x2
+ 36x + 1
f′(x) = 6x2
− 30x + 36
= 6 (x2
− 5x + 6)
= 6 (x − 2) (x − 3)
f (x) is increasing in [0, 2] and decreasing in [2, 3]
f (x) is many one
f (0) = 1
f (2) = 29
f (3) = 28
Range is [1, 29]
Hence, f (x) is many-one-onto
SECTION II : Multiple Correct Answer(s) Type
51. Tangents are drawn to the hyperbola
2 2
1
9 4
− =
x y
, parallel to the straight line 2x − y = 1. The points of
contact of the tangents on the hyperbola are
(A)
9 1
,
2 2 2
 
 
 
(B)
9 1
,
2 2 2
 
− − 
 
(C) ( )3 3, 2 2− (D) ( )3 3, 2 2−
Sol. (A, B)
Slope of tangent = 2

The tangents are y = 2x ± 9 4 4× −
i.e., 2x − y = ± 4 2
⇒ 1
2 2 4 2
− =
x y
and 1
2 2 4 2
− + =
x y
Comparing it with 1 1
1
9 4
− =
xx yy
We get point of contact as
9 1
,
2 2 2
 
 
 
and
9 1
,
2 2 2
 
− − 
 
Alternate:
Equation of tangent at P (θ) is
sec tan
x y 1
3 2
θ θ   
− =   
   
⇒ Slope =
2sec
2
3tan
θ
=
θ
⇒
1
sin
3
θ =
⇒ points are
9 1
,
2 2 2
 
 
 
and
9 1
,
2 2 2
 
− − 
 
.
52. Let θ, ϕ ∈ [0, 2π] be such that ( ) 2
2cos 1 sin sin tan cot cos 1
2 2
θ θ 
θ − ϕ = θ + ϕ − 
 
, ( )tan 2 0π − θ > and
3
1 sin
2
− < θ < − . Then ϕ cannot satisfy
(A) 0
2
π
< ϕ < (B)
4
2 3
π π
< ϕ <
(C)
4 3
3 2
π π
< ϕ < (D)
3
2
2
π
< ϕ < π
Sol. (A, C, D)
2 cos θ (1 – sin ϕ) =
2
2sin
cos 1 2sin cos 1
sin
θ
ϕ − = θ ϕ −
θ
2 cos θ – 2 cos θ sin ϕ = 2 sin θ cos ϕ – 1
2 cos θ + 1 = 2 sin (θ + ϕ)
tan(2 ) 0π − θ > ⇒ tan θ < 0 and
3
1 sin
2
− < θ < −
⇒
3 5
,
2 3
π π 
θ∈ 
 
1
2
< sin (θ + ϕ) < 1
⇒
5
2 2
6 6
π π
π + < θ+ ϕ < + π
max min
5
2 2
6 6
π π
π + − θ < ϕ < π + − θ
4
2 3
π π
< ϕ < .
53. If y(x) satisfies the differential equation y′ − ytanx = 2x secx and y(0) = 0, then
(A)
2
4 8 2
π π 
= 
 
y (B)
2
4 18
π π ′ = 
 
y

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(C)
2
3 9
π π 
= 
 
y (D)
2
4 2
3 3 3 3
π π π ′ = + 
 
y
Sol. (A, D)
tan 2 sec− =
dy
y x x x
dx
( )cos sin 2+ − =
dy
x x y x
dx
( )cos 2=
d
y x x
dx
y (x) cos x = x2
+ c, where c = 0 since y (0) = 0
when x =
4
π
,
2
y
4 8 2
π π 
= 
 
, when x =
2
2
, y
3 3 9
π π π 
= 
 
when x = , y
4 4
π π 
′ 
 
=
2
8 2 2
π π
+
when x = , y
3 3
π π 
′ = 
 
2
2 4
33 3
π π
+
54. A ship is fitted with three engines E1, E2 and E3. The engines function independently of each other with
respective probabilities
1 1
,
2 4
and
1
4
. For the ship to be operational at least two of its engines must
function. Let X denote the event that the ship is operational and let X1, X2 and X3 denote respectively the
events that the engines E1, E2 and E3 are functioning. Which of the following is(are) true ?
(A) 1
3
16
  =
 
c
P X X
(B) P [Exactly two engines of the ship are functioning | X] =
7
8
(C) 2
5
16
  = P X X (D) 1
7
16
  = P X X
Sol. (B, D)
1
1
( )
2
=P X , 2
1
( )
4
=P X , 3
1
( )
4
=P X
P(X) = 1 2 3 1 2 3 1 2 3 1 2 3( ) ( ) ( ) ( )∩ ∩ + ∩ ∩ + ∩ ∩ + ∩ ∩C C C
P X X X P X X X P X X X P X X X =
1
4
(A) 1( / )C
P X X = 1( )
( )
∩ C
P X X
P X
=
1
132
1 8
4
=
(B) P [exactly two engines of the ship are functioning | X] =
7
732
1 8
4
=
(C)
2
 
 
 
X
P
X
=
5
532
1 8
4
=
(D)
1
 
 
 
X
P
X
=
7
732
1 16
2
=

55. Let S be the area of the region enclosed by y =
2
−x
e , y = 0, x = 0 and x = 1. Then
(A) S ≥
1
e
(B) S ≥ 1 −
1
e
(C) S ≤
1 1
1
4
 
+ 
 e
(D) S ≤
1 1 1
1
2 2
 
+ − 
 e
Sol. (A, B, D)
S >
1
e
(As area of rectangle OCDS = 1/e)
Since
2
x x
e e− −
≥ ∀ x ∈ [0, 1]
⇒
1
0
1
1−  
> = − 
 ∫
x
S e dx
e
Area of rectangle OAPQ + Area of rectangle QBRS > S
( )
1 1 1
1 1
2 2
  
< + −   
  
S
e
.
Since
1 1
1
4
 
+ 
 e
< 1 −
1
e
Hence, (C) is incorrect.
A(0, 1) P
R
SQO
2
x
y e−
= 1 1
B ,
2 e
 
 
 
1
1,
e
 
 
 DC
SECTION III : Integer Answer Type
This section contains 5 questions. The answer to each question is single digit integer, ranging from 0 to 9 (both
inclusive).
56. If ,
rr
a b and
r
c are unit vectors satisfying
2 2 2
9− + − + − =
r rr r r r
a b b c c a , then 2 5 5+ +
rr r
a b c is
Sol. (3)
As,
2 2 2 22 2 2
3 − + − + − = + + − + + 
 
r r r rr r r r r r r r
a b b c c a a b c a b c
⇒ 3 × 3 −
2
+ +
rr r
a b c = 9
⇒ 0+ + =
rr r
a b c ⇒ 0+ + =
rr r
a b c
⇒ + = −
r r r
b c a
⇒ ( )2 5 3 3 3+ + = − = =
rr r r r
a b c a a .
57. Let f : IR → IR be defined as f(x) = 2
1+ −x x . The total number of points at which f attains either a local
maximum or a local minimum is
Sol. (5)
( )
2
2
1
(2 )
1
−
′ = + ⋅
−
xx
f x x
x x
=
2 1 , 1
(2 1) , 1 0
1 2 , 0 1
2 1 , 1
− < −
− + − < <

− < <
 + >
x x
x x
x x
x x
−ve +ve −ve +ve −ve +ve
−1 0 11/2−1/2

So, f′(x) changes sign at points
x = −1,
1 1
, 0, ,1
2 2
−
so, total number of points of local maximum or minimum is 5.
58. Let S be the focus of the parabola y2
= 8x and let PQ be the common chord of the circle x2
+ y2
− 2x − 4y =
0 and the given parabola. The area of the triangle PQS is
Sol. (4)
The parabola is x = 2t2
, y = 4t
Solving it with the circle we get :
4t4
+ 16t2
− 4t2
− 16t = 0
⇒ t4
+ 3t2
− 4t = 0 ⇒ t = 0, 1
so, the points P and Q are (0, 0) and (2, 4) which are also diametrically opposite points on the circle. The
focus is S ≡ (2, 0).
The area of ∆PQS =
1
2 4 4
2
× × = .
59. Let p(x) be a real polynomial of least degree which has a local maximum at x = 1 and a local minimum at x
= 3. If p(1) = 6 and p(3) = 2, then p′(0) is
Sol. (9)
Let p′(x) = k(x − 1) (x − 3)
⇒ p(x) =
3
2
2 3
3
 
− + +  
 
x
k x x c
Now, p(1) = 6 ⇒
4
3
k + c = 6
also, p(3) = 2 ⇒ c = 2
so, k = 3, so, p′(0) = 3k = 9.
60. The value of 3/2
1 1 1 1
6 log 4 4 4
3 2 3 2 3 2 3 2
 
 + − − − ⋅⋅⋅
 
 
is
Sol. (4)
Let
1 1 1
4 4 4
3 2 3 2 3 2
− − − ⋅⋅⋅ = y
So, 21
4
3 2
− =y y (y > 0)
⇒ 2 1
4 0
3 2
+ − =y y ⇒
8
3 2
=y
so, the required value is 3/2
1 8
6 log
3 2 3 2
 
+ × 
 
= 3/2
4
6 log
9
+ = 6 − 2 = 4.

IIT JEE 2012 Solved Paper by Prabhat Gaurav

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IIT JEE 2012 Solved Paper by Prabhat Gaurav