Loading subjects
Aqueous Equilibria · Part 2 of 4
6 exam-style questions with model answers, plus 8 quick multi-choice questions — every question on this part of the standard, grouped by the 2 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
State what two components a buffer solution must contain, and write equations showing how a buffer made from CH3COOH and CH3COONa reacts with added H3O+ and with added OH−.
Explain why a solution containing only ethanoic acid is not a buffer, even though it contains a weak acid and some ethanoate ions from its own dissociation.
Buffer X contains 0.50 mol L−1 CH3COOH and 0.50 mol L−1 CH3COONa. Buffer Y contains 0.050 mol L−1 of each. Compare their pH values and their ability to resist pH change, and justify why a chemist choosing a buffer must consider both the ratio and the concentrations of its components.
A buffer contains 0.150 mol of methanoic acid, HCOOH, and 0.250 mol of sodium methanoate in 1.00 L of solution. Calculate its pH. Ka(HCOOH) = 1.8 × 10−4
A buffer contains 0.200 mol of CH3COOH and 0.200 mol of CH3COONa in 1.00 L. Calculate the pH before and after 0.050 mol of solid NaOH is added, assuming no volume change. Ka = 1.74 × 10−5
A student needs a buffer of pH 9.00. They have available ethanoic acid (pKa 4.76), hydrofluoric acid (pKa 3.17) and ammonium chloride (pKa of NH4+ = 9.25). Justify which system they should choose, calculate the ratio of the two components required, and explain why the other two acids would be unsuitable even though a calculation could be performed with them.