1 STOICHIOMETRIC RELATIONSHIPS
1.3 REACTING MASSS AND VOLUMES
1.3.3
Calculations involving solutions
Solute
the substance that is dissolved in the solvent
Solvent
the substance having the larger volume in which a solute is dissolved
Solution
a homogeneous mixture that is transparent and cannot be separated by filtration
o
Each solute “particle” is isolated from the other particles by solvent particles.
o
Aqueous solutions are always transparent with no diffusion of light (Tyndall effect)
o
For aqueous solutions, the water molecules form a “sphere of hydration” around the solute.
o
Suspensions are heterogeneous mixtures that appear cloudy and can be separated by filtration
Standard Solution
ex.
A solution of accurately known concentration, prepared using a substance of
known high purity which may be dissolved in a known volume of solvent.
Preparation for a 1.000 mol dm-3 NaOH standard solution
About 300 cm3 of distilled water was placed in a 1.00000.8 dm3 volumetric flask.
40.00 g of NaOH (1.000 mole) was measured using a digital scale and added to the water.
The mixture was agitated until a transparent solution resulted with no precipitate observed.
Distilled water was added to the solution while swirling occasionally up to the etched line.
Concentration
the amount n of solute per unit volume V of solution
symbol: C or [X]
units: mol dm–3
ex. [Na2CO3] = 0.12 mol dm-3
C
n
 n  CV
V
Alternative Concentration Units

mass to volume ratio
volume
to volume ratio
m/v
%v/v
m msolute
V

% vv  solute
V Vsolution
Vsolution
units: g dm-3
solids in liquids
liquids in liquids
salt water
vinegar 5% v/v, peroxide 3% v/v



parts per million / billion
ppm / ppb
ppm 
msolute
m
 10 6 ppb  solute  10 9
msolution
msolution
for trace amounts of solutes
ions in bottled water, Hg in blood

Solubility
Solubility
a measure of the amount of solute that can dissolve in a given volume of solvent.
Depends on:
1. the temperature solubility of solids and liquids increase with temperature (more soluble)
solubility of gases in liquids decreases with temperature (ex. soda pop)
2. the nature of the solute and solvent
Solubility of Ionic Compounds in Water – General Rules to be aware of
 All nitrates and acetates are soluble.
 All sodium and potassium compounds are soluble
 All chlorides except silver and mercury are soluble
Precipitate
an insoluble solid that “falls out of solution”
 excess solute formed as a saturated solution cools
 insoluble product of a reaction of two solutions
Dilution
Dilute and Concentrated
Dilute
small amount of solute per volume
more water added or less solute
smaller concentration – lower molarity
 0.05 mol dm-3 is more dilute than 0.50 mol dm-3
Concentrated
large amount of solute per volume
more solute or less water
higher concentration – higher molarity
 5.0 mol dm-3 is more concentrated than 0.5 mol dm-3
How concentrated an acid/base is does not determined its strength.
Dilution Calculations
When a solution is diluted, water is added but the amount of solute remains the same.
Solutions are diluted to produce desired volumes with a specific concentration from a concentrated solution
so
n1 = n2
C1V1= C2V2
but n = CV
To find the volume of the concentrated solution, V1, needed to make
the desired volume, V2, of the dilute concentration, C2, rearrange and solve for V1.
C
V 1 2 V2
C1
The ratio of V2:V1 = C1:C2 is the dilution factor
ex.
V 1

To make 100.0 cm3 of 0.05000
mol dm-3 NaOH from 1.0000 mol dm-3 NaOH
0.05000 
C2
3
3
V2  
100.0cm  5.00cm
C1
 1.0000 


The dilution factor is 20