A chemistry student investigates how the validity of a common approximation depends…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A chemistry student investigates how the validity of a common approximation depends on the initial concentration of a monoprotic weak acid HX whose acid-dissociation constant is 2.0 × 10⁻⁶ at 25 °C. The student prepares four solutions, each at the same temperature, with initial concentrations shown in the table.
Initial concentration C₀ (M): 1.0, 0.10, 0.010, 0.0010
(a) For the solution with C₀ = 1.0 M, calculate the pH using the approximation that the equilibrium concentration of HX remains equal to 1.0 M. Then determine the percent ionization for this solution to evaluate the correctness of the approximation.
(b) Repeat the calculations in part (a) for the solution with C₀ = 0.10 M.
(c) Without carrying out full equilibrium calculations for the remaining two solutions, predict whether the approximation will yield an error larger than 5 % when C₀ = 0.010 M and when C₀ = 0.0010 M. Support your prediction with quantitative or semi-quantitative reasoning.
(d) Describe qualitatively how percent ionization of HX is expected to vary as C₀ decreases over the range given, and explain how this trend relates to the reliability of the approximation.