An environmental chemistry lab must design an ammonia/ammonium buffer (NH₃/NH₄⁺) to…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
An environmental chemistry lab must design an ammonia/ammonium buffer (NH₃/NH₄⁺) to treat wastewater that could receive intermittent discharges of either acid or base. The buffer should be able to neutralize 0.0080 mol of added HCl and 0.0080 mol of added NaOH without the pH moving outside the range 9.10 ± 0.05 at 25 °C. The final buffer volume is 2.00 L. Commercially available solutions are 1.50 M NH₃(aq) and 1.50 M NH₄Cl(aq). The base-dissociation constant of NH₃ at 25 °C is K_b = 1.8 × 10⁻⁵.
(a) Determine the ratio [NH₃][NH₄⁺] that produces an initial pH of 9.10.
(b) Using the result from part (a), calculate the minimum total concentration of buffering species (NH₃ + NH₄⁺) that ensures the buffer can neutralize either the specified amount of acid or base without the pH leaving the allowable range.
(c) Compute the volumes (in mL) of the 1.50 M NH₃ solution and the 1.50 M NH₄Cl solution required to prepare 2.00 L of the buffer satisfying the design criteria.
(d) The lab manager suggests preparing only 1.00 L of buffer at double the concentrations calculated in part (b). Analyze whether this alternative meets the same pH and capacity requirements, and justify your conclusion quantitatively.