sch3u_u4_lesson_3_diluting_solutions.ppt

Diluting Aqueous Solutions
(page 300-304)
Lesson 3
December 12th
and maybe part of the 13th
.

 Solutions can be prepared two ways; solids
can be added to liquids or liquids can be
added to liquids.

 Standard Solution
 A solution with precisely known
concentrations that have been prepared in
labs and industry.
 Preparing standard solutions takes skill and
precision equipment to measure the mass
(electronic balance) and volume
(volumetric flask and/or volumetric pipet).

Preparation of Standard
Solution
 Dilution is a common scientific procedure
that begins with a stock or standard solution
which is added to a solvent to decrease the
concentration to the desired value.
 The concentration is inversely related to
the solution’s volume.

C1 =
Concentration
of solution 1
v1 = Volume of
Solution 1
C2 =
Concentration
of solution 2
v2 = Volume of
Solution 2
As long as three of the values are known, the forth
can always be calculated.

sch3u_u4_lesson_3_diluting_solutions.ppt

Rules to rearranging
equations
 RULE #1: you can add, subtract, multiply and
divide by anything, as long as you do the
same thing to both sides of the equals sign.
 In an equation, the equals sign acts like the
fulcrum of a balance: if you add 5 of
something to one side of the balance, you
have to add the same amount to the other
side to keep the balance steady

Rule 1 example
 Find b in the equation
 y = mx + b
 Subract mx from each side of the
equation
 Y – mx = mx – mx + b
 Y – mx = b

 RULE #2: to move or cancel a quantity or
variable on one side of the equation,
perform the "opposite" operation with it
on both sides of the equation.
 For example if you had g-1=w and
wanted to isolate g, add 1 to both sides
(g-1+1 = w+1). Simplify (because (-
1+1)=0) and end up with g = w+1.

More complex example
 This equation is used to calculate the
density of icebergs
 We will isolate ρobject which is the density
of the iceberg itself.

 Start by isolating the equation in brackets
 Divide both sides by Htotal
 =
 =

 Next we will isolate the faction that contains
ρobject
 Subtract 1 on each side
 =

 Multiply both sides by ρfluid to get rid of
the fraction
 Cancel out ρfluid

 We will now remove the negative sign
by multiplying by -1
 =

 Rearrange the equation a little
 And we are done

Example 1
 How can we prepare of 100 mL of 0.040 mol
/L K2Cr2O7 from 0.200 mol/L K2Cr2O7?
 c1= 0.040 mol / L
 v1 = 100 mL
 c2 = 0.200 mol / L
 v2 = ?

 It asked for us to prepare 100ml.
 Volume of water = 100 mL – 20 mL = 80 ml
 Therefore, we would need 20 mL of 0.200 M
of K2Cr2O7 and 80mL of water to form 100
mL of 0.040 M K2Cr2O7.

Example 2
 How can a student prepare 123mL of 1 %
Normal Saline solution from a 3% saline
solution?
 v1 = 41 mL
 C1 = 3 % NS
 v2 = 41 mL + 82 mL = 123mL
 C2=?

 = 1 %
 Therefore, the student made a 1% NS
solution when they added 82 mL to 41 mL of
3 % NS.

Preparation of Standard
Solution

Preparing a Standard Solution
from a Solid Reagent
1. Calculate the required mass of the solute
from the volume and concentration of the
solution.
2. Obtain the required solute in a clean, dry
beaker or weighing boat.

3. Dissolve the solid in pure water
using less than one half of the
final solution volume.
4. Transfer the solution and all of
the water used to rinse the
equipment into a clean volumetric
flask. (All of the equipment should
be rinsed two to three times.)

5. Add pure water, using a medicine
dropper for the final few millimetres
until the bottom of the meniscus
reaches the calibration line.
6. Stopper the flask and mix the solution
slowly by inverting the flask.

Preparing a Standard
Solution by Dilution
1. Calculate the volume of the concentrated
reagent required.
2. Add approximately one half of the final
volume of pure water to the volumetric flask.
3. Measure the required volume of stock
solution using a pipet.

Pipet technique
 Rinse the pipet with small volumes of distilled
water.
 A clean pipet has no visible residue or liquid drops
clinging to the inside wall.
 Attach the bulb to the pipet and place the pipet
around 1 cm from the bottom of the container.
Make sure that it does not touch the sides of
the container.

 Draw the solution into the pipet. If using a
bulb pipet that must be removed, draw the
liquid past the calibration line.

 Wipe all solution from the outside of the
pipet using a paper towel.
 While holding the piper vertically, touch the
pipet tip to the inside wall of a clean
receiving container. Release the contents
into the container.
 Depending on the type of pipet, the flow of fluid may need
to be stopped at a calibration line at the bottom.

 Finish by holding the pipet tip at a 45 degree
angle. Do not shake the pipet as most are
calibrated to have a small volume in the tip.

Preparing a Standard
Solution by Dilution
4. Transfer the stock solution slowly into the
volumetric flask while mixing.
5. Add pure water and then use a medicine
dropper for the final few millilitres until the
bottom of the meniscus rests on the
calibration line.
6. Stopper the flask and mix the solution by
slowly inverting the flask several times.

 When carrying out a dilution with an
acid. Always add acid to water.
 Never add water to the acid.
 When acid and water mix a great deal of
heat is released. Acid added to water gives a
large heat sink for the heat to dissipate in. If
you add water to acid, the heat generated
will cause the water to boil, and may splatter
you with the hot acidic solution.

Home work + Practice
 Page 306 # 6 – 8, P 306 #3-5, 8ab, 9.
 Practice using the tools with proper
technique.
 Lab Tomorrow
 Read 360 - 374

sch3u_u4_lesson_3_diluting_solutions.ppt

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