5 Steps to a 5 AP Chemistry

KEY IDEA

Anunsaturated solutionhas less than the maximum amount of solute dis-

solved. 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 maxi-

mum amount of solute, resulting in a supersaturated solution. Supersaturated

solutions are unstable, 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 chap-

ter. 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

Kffor H 2 O=1.86 K kg mol−^1

Kbfor H 2 O=0.512 K kg mol−^1

ΔTf=iKfmolality

ΔTb=iKbmolality

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 diluterefers to a solution that has a rela-

tively 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 compar-

ison 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 quantita-

tive 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 per-

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

mass percent

mass/volume percent

volume/volume percent

180  Step 4. Review the Knowledge You Need to Score High

KEY IDEA