Calculate the mass of ammonium chloride that must be added to 500.0 mL of 0.32 M NH₃ to prepare a pH 8.50 buffer; K_b for NH₃ is 1.8 × 10^-5.

Strategy We need to calculate the amount of the acid (NH₄⁺ ions from NH₄Cl) that we must add to prepare an ammonium/ammonia buffer of pH 8.50. It will be easiest if we use the Henderson–Hasselbalch equation in terms of the number of moles of acid and base.

We know the pH, the value for K_b, and the volume and concentration of the base. To solve for n_a, we will also need to know Ka (and pK_a), but we can calculate these values from K_b.