What is the pH of a solution that results from mixing together equal volumes of a 0.070 M solution of ammonia and a 0.035 M solution of hydrochloric acid? (pKa of NH4+ = 9.25)

Answer 1

NH₃ + HCl ==> NH4Cl ... balanced molecular equation

NH₃ + H⁺ ==> NH₄⁺ ... balanced net ionic equation

Let us assume we have 0.50 L of each reactant:

moles NH₃ = 0.50 L x 0.070 mol / L = 0.035 mols NH3

moles HCl = 0.50 L x 0.035 mol / L = 0.0175 mols HCl

This produces a buffer since all the HCl is used up forming NH₄⁺, and we still have NH₃ left over.

This can be visualized in the following ICE table:

NH₃ + H⁺ ==> NH₄⁺

0.035........0.0175.......0..........Initial

-0.0175..-0.0175...+0.0175....Change

0.0175........0...........0.0175....Equilibrium

The pH for a basic buffer such as this can be calculated from the Henderson Hasselbalch equation:

pOH = pKb + log [conj.acid] / [base]

pKb + pKa = 14

pKb = 14 - pKa = 14 - 9.25

pKb = 4.75

Solving for pOH and then for pH, we have...

pOH = 4.75 + log [0.0175] / [0.0175]

pOH = 4.75

pH = 14 - pOH

pH = 14 - 4.75

pH = 9.25