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Equilibrium: Common Mistakes to Avoid for AP Chemistry

By — McGraw-Hill Professional
Updated on Mar 31, 2011

Experiments Strategy

Equilibrium experiments such as 10, 11, and 19, directly or indirectly involve filling a table like the following:

      Reactants and Products
      Initial amount
      Change
      Equilibrium amount

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 established, 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

  1. Be sure to check the units and significant figures of your final answer.
  2. When writing equilibrium constant expressions, use products over reactants. Each concentration is raised to the power of the coefficient in the balanced chemical equation.
  3. 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.
  4. Remember, in working Le Châtelier problems, pressure effects are important only for gases that are involved in the equilibrium.
  5. Be sure, when working weak-base problems, to use Kb and not Ka.
  6. In titration problems, make sure you compensate for dilution when mixing two solutions together.
  7. A Ka expression must have [H+] in the numerator, and a Kb expression must have [OH] in the numerator.
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