Carbon-12 scale

FORMULAE AND CHEMICAL EQUATIONS

INTRODUCTION

RELATIVE ATOMIC RELATIVE MOLECULAR
MASS (RAM) MASS (RMM)

EXERCISE CARBON-12 SCALE EXERCISE

INTRODUCTION

 Mass of one atom is small. For
example the mass of one atom
cuprum 1.06 x10-22 gram.

 We can’t weigh one atom directly
but we can compare the mass of
one atom with another mass of
one atom by using the relative
mass scale.

RELATIVE ATOMIC MASS (RAM)

Definition

Then, relative atomic mass of
element:
= mass of one element
1/12 x mass of one atom C-12

Example

DEFINITION OF RELATIVE
ATOMIC MASS (RAM)

 RELATIVE ATOMIC MASS (RAM)
of elements is how much time one
atom of element more heavy than
1/12 mass of one atom C-12 with
the mass of one atom C-12 is
12.000.

EXAMPLE:

 The mass of one atom potassium is 39
time more bigger than 1/12 x mass of
one atom C-12.

So, relative atomic mass for one atom
potassium is 39.

K = 39

RELATIVE MOLECULAR MASS (RMM)

Definition

Relative molecular mass of
compound=
Mass of one molecule of compound
1/12 x mass of one atom C-12

Example

DEFINITION OF RELATIVE
MOLECULAR MASS (RMM)

 RELATIVE MOLECULAR MASS
(RMM) is how much time the
mass of one molecule is
compound is more bigger than
1/12 mass of one atom C-12 with
the mass of one atom C-12 is
12.000.

EXAMPLE:

 Relative molecular mass for water, H2O:

= 2 x (relative atomic mass H) + 1 x
(relative atomic mass O)
= ( 2 x 1) + ( 1 x 16)
=18

So, relative molecular mass for H2O is 18.

(RAM: H = 1; O = 16)

EXAMPLE:

 Relative molecular mass for HCl.

= 1 x (relative atomic mass H) + 1 x
(relative atomic mass Cl)
= (1 x 1) + (1 x 35.5)
= 36.5

So, relative molecular mass for HCl is
36.5.

(RAM : H=1 ; Cl=35.5 )

INTRODUCTION OF CARBON-12

 At the beginning, all chemists
agreed to use atom hydrogen as a
standard in determined RAM and
RMM with one atom hydrogen is
given as 1 units.

 Then, chemists found that this
standard is not suitable
because:

 In 1860, chemists agreed to use
atom oxygen as a standard. This
is because:

 A problem occurred when there
are two standard of relative atomic
mass.

 Finally, in 1961 all scientists
agreed to use isotope 126C as a
standard. This is because:

WHY ATOM HYDROGEN IS NOT
SUITABLE AS A STANDARD:

 Hydrogen has isotopes, which is 11H, 21H
(deuterium) and 31H (tritium). So,
hydrogen is not 1 units.
 Hydrogen existed as gases and its
masses was very difficult to determine.
 There are a lot of elements that is not
react with hydrogen.

WHY ATOM OXYGEN USED AS A
STANDARD:

 Atom oxygen can react with many
elements.
 Most of the compounds produced oxide
is existed as solid.
 The mass of this compound was easy to
determine.

WHY CARBON-12 IS USED AS A
STANDARD FOR DETERMINING
RAM AND RMM

 Carbon existed is many kinds of
materials and easy to get.
 Carbon occurred as a solid in room
temperature compare with oxygen gas.
This element doesn’t need a stopper to
keep it.
 The existing of other isotope such as 126
C,136 C and 146 C are too little.

WHAT IS THE RELATIVE ATOMIC
MASS OF THESE ELEMENTS??
 C =?  Na = ?
 O =?  Al = ?
 N =?  Fe = ?
 F =?  Ar = ?
 Br =?  Mn = ?

 C = 12
 O = 16
 N = 14
 F = 19
 Br = 80
 Na = 23
 Al = 27
 Fe = 56
 Ar = 40
 Mn= 55

WHAT ARE THE RELATIVE
MOLECULAR MASSES OF THESE
MOLECULES??
 NaCl ?  H2S ?
 H2O ?  CaCl2 ?
 HCl ?  K2O ?
 CuO ?  NaOH ?
 MgCl2 ?
 CO2 ?
(RAM: H=1; C=12; O=16; Na=23; Mg=24;
S=32; Cl=35.5; K=39; Ca=40; Cu=64)

 NaCl = 58.5
 H2O = 18
 HCl =36.5
 CuO =80
 MgCl2 =95
 H2S =34
 CaCl2 =111
 K2O =94
 NaOH =40
 CO2 =44

Carbon-12 scale

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Carbon-12 scale