Two separate samples are prepared by dissolving the same monoprotic weak acid HA in…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Two separate samples are prepared by dissolving the same monoprotic weak acid HA in pure water.
• Sample I is made by dissolving enough HA to give an initial concentration of 0.10M.
• Sample II is made by dissolving enough HA to give an initial concentration of 1.0×10−4M.
The acid-dissociation constant of HA at the temperature of the experiment is 1.8×10−5.
Answer the following questions about the validity of the common approximation that the equilibrium concentration of HA remains essentially equal to the initial concentration.
(a) State the specific assumption that is made about the concentration of HA when that approximation is applied in weak-acid pH calculations.
(b) Without performing any detailed pH calculations, predict whether the approximation will introduce an error greater than 5 % in Sample I, in Sample II, or in both samples. Indicate your choice(s).
(c) Justify your prediction in part (b) by discussing percent ionization and the relative magnitudes of the equilibrium and initial concentrations of the acid in each sample.