188 Step 4. Review the Knowledge You Need to Score High
close track of the units may simplify the problem. Experiment 4 in the Experimental chap-
ter uses these concepts.
Concentration problems are concerned with the definitions of the various units. It is
possible to calculate the mass and/or volume of the solvent and solute by taking the differ-
ence between the final and initial measurements. The density, if not given, is calculated, not
measured. It is important to recognize the difference between the values that must be meas-
ured and those that can be calculated. Moles are also calculated, not measured.
Do not forget that nearly all the concentration units use the total for the solution in
the denominator. For these units it is important to remember to combine the quantities
for the solvent and all solutes present. Molal concentrations are exceptions. Molality uses
only the kilograms of solvent in the denominator. Do not make the mistake of using the
entire solution in the denominator for molal concentrations.
Colligative properties may involve changes in the melting or boiling points. Changes
cannot be measured; only “before” and “after” values can be measured. In an experiment,
ΔTis not measured. The freezing, or boiling, point of the solution is measured and com-
pared to that of the pure solvent. The difference is then calculated.
The vapor pressure of a solution and the osmotic pressure are measurable quantities.
Many errors are associated with electrolytes. The van’t Hoff factor is often forgotten.
The van’t Hoff factor is a calculated value, not a measured value. As a calculated value, it
may or may not be a whole number.Common Mistakes to Avoid
1.When dealing with percentage solutions, be sure you know what type of percentage
(mass, mass/volume, volume/volume) is being used.
2.Percentage solutions use amount of solute per hundred parts of solution.
3.In molarity problems, be sure to use liters of solution.
4.Molality problems deal with moles of solute per kilogram of solvent.
5.In colligative property problems, be sure to incorporate the van’t Hoff factor for elec-
trolytes.
6.In freezing-point depression and boiling-point elevation problems, be sure to use the
molality of the solution.
7.In freezing-point depression and boiling-point elevation problems, to find the actual
freezing/boiling point, calculate the ΔT(change in temperature), then subtract that
amount from the solvent’s freezing point, or add it to the solvent’s boiling point.
8.Make sure your units cancel, leaving you with the units desired in your final answer.
9.Round off your final numerical answers to the correct number of significant figures.
10.Remember, most molecular compounds––compounds containing only nonmetals––do
not ionize in solution. Acids are the most common exceptions.TIP