Chapter 14 — Solutions
1
Overview of Chapter 14
Today’s questions to consider:
• Solutions
• Concentrations:
Molarity
Molality
Mole fraction
• Colligative Properties
Freezing point depression
Boiling point elevation
• How do we calculate concentrations?
• Why does a soda fizz when we open the lid?
• How do you calculate mole fraction?
• Osmosis
Concentration Units
Today’s Topics
• Calculating concentrations
Molarity,
Molarity, M
Molarity
M of solute =
Molality
Mole fraction
Molality, m
!
• Colligative Properties
mol solute
1 L solution (total volume)
m of solute =
• Raoult’s Law
mol solute
1 kg solvent
!
Concentration Units
Concentration Units
Mole Fraction, X
Molality (m):
Molarity (M):
0.100 m K2CrO4 =
0.100 M K2CrO4 =
19.4g K2CrO4
+
1.00 kg H2O
19.4g K2CrO4
in
1.00 L total volume
For a mixture of A, B, and C:
X A = mol fraction A =
mol A
mol A + mol B + mol C
WEIGHT % = grams solute per 100 g solution
Chapter 14 — Solutions
Concentration Calculations
If we make dissolve 62.1 g (1.00 mol)
of ethylene glycol in 250. g of H2O,
calculate the
mol fraction
molality
2
Concentration Calculations
Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g
of H2O. Calculate X, m, and % of glycol.
250. g H2O x (1 mol H2O/18.0 g H2O)
= 13.9 mol H2O
Calculate mole fraction X:
weight %
X glycol =
of ethylene glycol.
1.00 mol glycol
1.00 mol glycol + 13.9 mol H2O
Xglycol = 0.0672
Concentration Calculations
Dissolving Gases & Henry’s Law
Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g
of H2O. Calculate X, m, and % of glycol.
Calculate molality:
conc (molality) =
1.00 mol glycol
= 4.00 molal
0.250 kg H2O
Gas solubility (in mol/L) = kH • Pgas
Calculate weight %:
%glycol =
62.1 g
x 100% = 19.9%
62.1 g + 250. g
Scuba Diving
kH for O2 = 1.66 x 10-6 M / mmHg
When Pgas drops, solubility drops.
Colligative Properties
On adding a solute to a solvent, the props.
of the solvent are modified.
• Vapor pressure decreases
• Melting point
decreases
• Boiling point
increases
• Osmosis is possible (osmotic pressure)
They depend only on the NUMBER of solute
particles relative to solvent particles, not
on the KIND of solute particles.