5 Steps to a 5 AP Chemistry 2019

Solutions and Colligative Properties ❮ 181

An unsaturated solution has less than the maximum amount of solute dissolved.

Sometimes, if the temperature, purity of the solute and solvent, and other factors

are just right, you might be able to dissolve more than the maximum amount of

solute, resulting in a supersaturated solution. Supersaturated solutions are unsta-

ble, and sooner or later separation of the excess solute will occur, until a saturated

solution and separated solute remain.

The formation of a solution depends on many factors, such as the nature of

the solvent, the nature of the solute, the temperature, and the pressure. Some

of these factors were addressed in the Reactions and Periodicity chapter. In

general, the solubility of a solid or liquid will increase with temperature and

be unaffected by pressure changes. The solubility of a gas will decrease with

increasing temperature and will increase with increasing partial pressure of the

gas (Henry’s law).

Keywords and Equations

p = osmotic pressure

i = van’t Hoff factor

Kf = molal freezing-point depression constant

Kb = molal boiling-point elevation constant

Kf for H 2 O = 1.86 K kg mol-^1

Kb for H 2 O = 0.512 K kg mol-^1

DTf = iKf molality

DTb= iKb molality

p = iMRT

molarity, M = moles solute per liter solution

molality, m = moles solute per kilogram solvent

Concentration Units

There are many ways of expressing the relative amounts of solute(s) and solvent in a solu-

tion. The terms saturated, unsaturated, and supersaturated give a qualitative measure, as do

the terms dilute and concentrated. The term dilute refers to a solution that has a relatively

small amount of solute in comparison to the amount of solvent. Concentrated, on the

other hand, refers to a solution that has a relatively large amount of solute in comparison

to the solvent. However, these terms are very subjective. If you dissolve 0.1 g of sucrose

per liter of water, that solution would probably be considered dilute; 100 g of sucrose per

liter would probably be considered concentrated. But, what about 25 g per liter––dilute

or concentrated? In order to communicate effectively, chemists use quantitative ways of

expressing the concentration of solutions. Several concentration units are useful, including

percentage, molarity, and molality.

Percentage

One common way of expressing the relative amount of solute and solvent is through

percentage, amount-per-hundred. Percentage can be expressed in three ways:

mass percent

mass/volume percent

volume/volume percent

KEY IDEA

ENRICHMENT