At equilibrium, [H 3 O + ] = [OH - ] = 1.0 × 10 -7 in a sample of pure water. An aqueous solution in which [H 3 O + ] = [OH - ] is referred to as a neutral solution . However, the addition of an external acid or base will shift the relative amounts of these two ions. Adding an acid will increase the amount of H 3 O + . As a consequence, the amount of OH - will need to decrease in order to reestablish equilibrium (at which point the equilibrium expression for K w will once again have the correct value). In an acidic solution , [H 3 O + ] > [OH - ]. Similarly, in a basic solution , the amount of OH - will increase and the amount of H 3 O + will decrease, so [H 3 O + ] < [OH - ].
Leila is learning about solutions in her chemistry class. She has two samples of pure water, and wants to find out what will happen when she adds an acid to one water sample and a base to the other water sample. Leila adds an external acid to water sample A and an external base to water sample B.
In which sample will the amount of OH - need to decrease in order to reach equilibrium, sample A or sample B?