Chapter 9

Solution Concentrations

and

Colligative Properties

% Solutions

solvent of mass solute of mass

solute of mass solute %

Mass is usually in grams

Make sure that you understand that the denominator is the total mass of solution.

Molarity

• Most widely used concentration unit• [HCl] means concentration of HCl in mol/L• Notice volume is total volume of solution

solution of liters

solute of moles (M) Molarity

Molality

• Molality (m) expresses concentration in terms of the mass of the solvent.

solvent of kilograms

solute of moles (m) Molality

Molarity vs Molality• Molarity and molality differ in two ways:

– Molarity tells you about moles of solute per volume of the entire solution (solute & solvent)

– Molality tells you about moles of solute per mass of the solvent

• Keep in mind that one liter of water weighs one kilogram

• So for a dilute solution, the amount of solution is about the same as the amount of solvent

• So for a dilute aqueous solution, molarity and molality are basically the same.

Mole Fraction

• Mole fraction equals the moles of a solute divided by total moles of solution

solution of moles total

a substance of moles aX

Remember Density

• Density is the measure of mass per unit volume.– Since density relates mass and volume, it is

useful if you need to convert between molarity, which deals with volume, and molality, which deals with mass.

• D = m/v units usually g/mL or g/cm3

What do you know if given 12.5 % NaCl?

12.5 g NaCl added to 87.5 g H2O

100 g solution

87.5 g solvent H2O

What can you find? (must be given density)

Moles solute; liters of solution; kg solvent

Molarity

Molality

Mole fraction

What do you know if given 3.0 M KCl?

3.0 moles KCl in 1 liter solution

What can you find? (need density again)

find grams KCl

find grams solvent

Molality

Mole fraction

% solution

What do you know if given 2.7 m NaOH?

2.7 moles NaOH in 1 kg water

What can you find? (need density)

grams NaOH

mole fraction

% solution

Molarity

Effect of temperature on solubility

• Most solids increase solubility when solution is heated…some do not

• All gases decrease solubility when solution is heated.

Like Dissolves Like

• Polar or ionic solutes dissolve in polar solvents– When ionic solutes dissolve they break up

into ions

• Nonpolar solutes dissolve in nonpolar solvents.

• Polar and nonpolar do not mix

Van’t Hoff factor (i)

• The Van’t Hoff factor tells how many ions one unit of a solute will dissociate into in solution.– C6H12O6 does not dissociated, so i = 1

– NaCl dissociates into Na+ and Cl-, so i = 2

– CaCl2 dissociates into Ca2+, Cl-, and Cl-, so i = 3

– HF partially dissociates, so 1 < i < 2

Boiling Point Elevation• When a solute is added to a solvent, the

boiling point of the solution increases (that is, bp is higher than that of the solvent)

ΔTb = ikbm

ΔTb = change in boiling point

i = Van’t Hoff factor

kb = boiling point elevation constant

m = molality of solution

Freezing Point Depression

• When a solute is added to a solvent, the freezing point of the solution decreases (that is, fp is lower than that of the solvent)

ΔTf = ikfm

ΔTf = change in freezing point

i = Van’t Hoff factor

kf = freezing point depression constant

m = molality of solution

Vapor Pressure Lowering

• Called Raoult’s Law – when a solute is added to a solution, the vapor pressure will decrease.

P = XP°

P = vapor pressure of solution

P = vapor pressure of solvent

X = mole fraction of solvent

Osmotic Pressure

• When a pure solvent and a solution are separated by a membrane that only allows solvent to pass through, the solvent will try to pass through the membrane to dilute the solution. The greater the concentration of solute in the solution, the greater the osmotic pressure.

Ώ =λ = MRTi Ώ =λ = osmotic pressure (atm)

M = molarity

R = gas constant = .0821 (L·atm)/(mol·K)

T = Kelvin temperature

i = Van’t Hoff factor

What do you need to know for test?

• Like dissolves like

• Affect of temperature on solubility

• Molarity

• Molality

• Mole fractions

• % solutions

• Colligitave properties (math)