Preparing Solutions
Standard Solutions
Solutions can be prepared two ways:
i) solids added to liquids OR
ii) liquids added to liquids.
Standard Solution: A solution for which the
precise concentration is known. (Prepared in
labs and industry.)
We will be preparing a standard solution by
accurately massing a solute then carefully and
gradually adding a precise amount of water.
Solution Preparation
Stock Solution: a solution, usually concentrated
or saturated, that is in stock or on the shelf and
available for use.
Dilution: the process of decreasing the
concentration of a solution, usually by adding
more solvent.
We will be preparing a standard solution by
accurately measuring a volume of stock or
standard solution then carefully and
gradually adding a precise amount of water.
Required Equipment:
• Volumetric flask – comes in
different sizes and has one precise
marking, unlike an Erlenmeyer flask
that has approximate volume
markings.
• Precise electronic balance – to
mass the solid that will be put into a
volumetric flask to make a standard
solution
• Pipette – for adding a very
accurate amount of stock
solution to a volumetric flask
and then diluting the initial
solution to get a new standard
solution.
Pipettes
Preparing a Standard Solution - Technique
1) Mass the solute on wax weighing paper, or plastic
container & place the solute in a VERY CLEAN beaker
2) Rinse off the weighing paper or plastic container with
distilled water to ensure all the massed solute
molecules end up in the beaker
3) Add some distilled water & stir the solute
MAKE SURE all the solute is DISSOLVED!
4) Once dissolved transfer the solution into a
volumetric flask using a funnel and ENSURE
NOT A DROP is spilled or lost
5) Rinse out the beaker 3 times with distilled
water starting from the top and transfer
the wash into volumetric flask
Technique
6) Rinse out the funnel & rinse off the tip when removed
7) Add distilled water until 2-3 centimeters BELOW the mark,
put a stopper on & invert the flask several times to mix it well
8) Looking at the line on the volumetric flask at eye level,
add the distilled water DROP by DROP up to the line until
the bottom of the meniscus is sitting on the line.
You can use a dropper or
small pipette to do this.
9) The solution must be stored with
a stopper to avoid impurities.
BEFORE using the solution, it will need to be shaken or
inverted several times to ensure all the solute is dissolved
since it has been sitting still for a while.
http://www.youtube.com/watch?v=cckAwavEKA0
Calculations Starting with Mass
What is the mass of NaOH pellets needed to
make 750 mL of a 3.00 M solution?
Given:
V = 750 mL = 0.750 L
M or c = 3.00 M = 3.00 mol/L
1st find n = ?
n = c x V (or M x V)
2nd find m = ?
Calculations Starting with Mass
What is the mass of NaOH pellets needed to
make 750 mL of a 3.00 M solution?
Given:
V = 750 mL = 0.750 L
M or c = 3.00 M = 3.00 mol/L
1st find n = ?
n = c x V (or M x V)
2nd find m = ?
m = n x Molar mass
n=
c
x
V
= # mol
n = 3.00 mol/L x 0.750 L = 2.25 mol
m = 2.25 mol x 40.0 g/mol = 90.0 g
Dilution Calculations
When determining the volume needed of a
stock solution to make a new solution, just
remember that original volume of solution has
the same number of moles as the final
solution, only added H2O…
moles solute BEFORE
= moles solute AFTER
the dilution
n before = n after
the dilution
Dilution Calculations
moles solute BEFORE
= moles solute AFTER
the dilution
n= CxV
n=MxV
n before = n after
C1 x V1 = C2 x V2
Mi x Vi = Mf x Vf
the dilution
OR
Dilution Calculations: Example
Water is added to 0.200 L (Vi ) of a 2.40 mol/L
(Mi) NH3(aq) cleaning solution, until the final
volume is 1.000 L (Vf). Find the molar
concentration of the final, diluted solution.
Dilution Calculations: Example
Water is added to 0.200 L (Vi ) of a 2.40 mol/L (Mi)
NH3(aq) cleaning solution, until the final volume is
1.000 L (Vf). Find the molar concentration of the
final, diluted solution.
Only one
equation
needed
n before =
Ci x Vi
Mi x Vi
=
=
n after
Cf x Vf
Mf x Vf
OR
2.40 mol/L x 0.200 L = Mf x 1.000 L
Mf = 2.40 mol/L x 0.200 L
1.000 L
Mf = 0.480 mol/L
Dilution Calculations: Example
Water is added to 0.200 L (Vi ) of a 2.40 mol/L
(Mi) NH3(aq) cleaning solution, until the final
volume is 1.000 L (Vf). Find the molar
concentration of the final, diluted solution.
NEED: Mf = ?
Mi x Vi = Mf x Vf
Mf = Mi x Vi
Vf
Mf = 2.40 mol/L x 0.200 L
1.000 L
Mf = 0.480 mol/L
Preparing a Standard Solution by Dilution
1) Using calculations, determine the amount of
stock solution required to make the new solution
and choose an appropriately-sized pipette.
Example, the stock solution is 2.00 mol/L and the
diluted solution needs to be 0.040 mol/L in a
1.00 L volumetric flask. Vi = Mf x Vf
Mi
= 0.040 M x 1.00 L
2.00 M
= 0.020 L (20 mL)
2) Choose a 20 mL pipette or a graduated pipette
that holds 20 mL of solution
Preparing a Standard Solution by Dilution
3) Using the bulb, fill the pipette with solution a few
centimeters above the marked amount.
Quickly replace the bulb with your thumb
4) Carefully, slide or loosen your thumb until you
see the level of the solution lowering at a slow
rate. Stop at the marked line.
You may have to try a few times since using a bulb
and pipette effectively takes practice.
5) Once the bottom of the meniscus is at the line,
press firmly with your thumb to hold that level
and carry the pipette to the volumetric flask.
Release the solution into the flask.
6) Fill the volumetric flask carefully as explained in
technique section of preparing a Standard
Solution from Mass
http://www.youtube.com/watch?v=3EOiCrtvUUM
HOMEWORK
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Read page 300-307
pg 302 # 1,2,4,5a
pg 306 # 6-8
pg 306-307 Questions # 2 – 5, 8 (skip d :)
READ OVER Activity 6.5.1 pg 301 &
Activity 6.5.2 pg 305