Butyric Acid and Hydrazoic Acid

Hydrazoic acid, HN3, and butyric acid, HC4H7O2, are both weak monoprotic acids. Their pKa values (at 25oC) are 4.72 and 4.82, respectively. What is [C4H7O2-] in a solution in which the stoichiometric concentrations of HN3 and HC4H7O2 are 0.165 mol/L and 1.19x10-6 mol/L, respectively?

Give your answer with three significant figures.

Hints:

1. The stoichiometric concentration refers to the total concentration in solution (i.e. it is the total amount of acid dissolved in solution). For example, when you dissolve 0.1 mol of HA in water to make 1.0 L of solution, the stoichiometric concentration of HA is 0.1 mol/L. The equilibrium concentration of HA will be less than the stoichiometric concentration because some or all of HA ionizes.

2. In this problem, you are dealing with a mixture of two weak monoprotic acids, HA and HB. However, the stoichiometric concentration of HA is much greater than HB. Consider first the ionization of HA and then consider the implications that this has on the ionization of HB.