232 ❯ STEP 4. Review the Knowledge You Need to Score High
The initial amounts—concentrations or pressures—are normally zero for the products,
and a measured or calculated value for the reactants. Once equilibrium has been estab-
lished, the amount of at least one of the substances is determined. Based on the change
in this one substance and the stoichiometry, the amounts of the other materials may be
calculated.
Measurements may include the pressure, the mass (to be converted to moles), the
volume (to be used in calculations), and the pH (to be converted into either the hydrogen
ion or hydroxide ion concentration). Some experiments measure the color intensity (with
a spectrophotometer), which may be converted to a concentration.
Do not make the mistake of “measuring” a change. Changes are never measured; they
are always calculated.Common Mistakes to Avoid
- Be sure to check the units and significant figures of your final answer.
- When writing equilibrium constant expressions, use products over reactants. Each
concentration is raised to the power of the coefficient in the balanced chemical
equation. - In converting from Kc to Kp be sure to use the ideal gas constant, R, whose units are
consistent with the units of the partial pressures of the gases. - Remember, in working Le Cha^telier problems, pressure effects are important only for
gases that are involved in the equilibrium. - Be sure, when working weak-base problems, to use Kb and not Ka.
- In titration problems, make sure you compensate for dilution when mixing two solu-
tions together. - A Ka expression must have [H+] in the numerator, and a Kb expression must have
[OH-] in the numerator.
TIP