Chemists use equilibrium constants, or Keq values, to express whether a reversible chemical reactions favours the formation of products or reactants. Acetic acid, or CH3COOH, for example, undergoes a partial reaction with water according to CH3COOH + H2O <---> CH3COO- + H3O+. An equilibrium constant greater than one would indicate that most of the acetic acid molecules convert to CH3COO-. An equilibrium constant less than one would indicate that most of the acetic acid molecules remain intact. When specifically applied to acids, chemists refer to equilibrium constants as acid-dissociation constants, or Ka values. These numbers are typically very small. The value for acetic acid, for example, is 0.000018, or 1.8 x 10^-5. Consequently, chemists sometimes express these quantities as pKa values, where pKa = -log Ka. In this case, the pKa of acetic acid becomes pKa = -log (0.000018) = 4.74.
Refer to a table of pKa values and locate the pKa of the compound under investigation. For example, the pKa of benzoic acid, a common food preservative, is about 4.20.
Multiply the pKa value by negative one to invert its sign. In the case of benzoic acid, 4.20 x (-1) = -4.20.
Calculate the Ka or Keq value by using a calculator to raise 10 to the power of the negative pKa. Continuing the previous example, if benzoic acid exhibits a pKa of 4.20, then its Ka = 10^(-4.20) = 6.31 x 10^-5, or 0.0000631. And because benzoic acid is acidic, its Keq and Ka values are numerically identical.
Mathematically, raising 10 to an exponential power is referred to as the "antilog" and the button for this operation is usually marked "10^x" on most scientific calculators.