Question

A solution is prepared by dissolving 0.045 mol of 4-nitrobenzoic acid (K_a = 3.61×10^-4) and 0.030 mol of sodium 4-nitrobenzoate in water to a total solution volume of 1.00 L.
First, calculate a pH for the solution by assuming that the concentrations of 4−(NO₂)C₆H₄CO₂H and 4−(NO₂)C₆H₄CO₂⁻ are equal to the formal concentrations of the substances dissolved. Next, calculate the pH for the solution based on the equilibrium concentrations of 4−(NO₂)C₆H₄CO₂H and 4−(NO₂)C₆H₄CO₂⁻. Use the Henderson-Hasselbalch approximation to calculate the pH of a solution prepared by dissolving in water, to a final volume of 1.00 L, 0.160 mol of 4-nitrobenzoic acid, 0.040 mol of sodium 4-nitrobenzoate, 0.060 mol of HNO₃ and 0.060 mol of Ca(OH)₂.