At 298 K the dissociation of acetic acid in water is represented by CH _3 COOH (aq) ;…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
At 298 K the dissociation of acetic acid in water is represented by
CH3COOH(aq)⇌H+(aq)+CH3COO−(aq),Ka=1.8×10−5
A solution is prepared by dissolving 0.100 mol of acetic acid in enough water to make 1.00 L of solution. After equilibrium is reached, the solution is subjected to one of the following separate disturbances.
• Scenario A: Solid NaCH3COO is added to give a final acetate concentration of 0.010 M.
• Scenario B: 0.010 mol of HCl(g) is bubbled into the solution and fully dissociates.
• Scenario C: Pure water is allowed to evaporate until the total volume is 0.500 L.
Assume temperature remains 298 K and activity coefficients are unity.
(a) Calculate the initial pH of the 0.100 M acetic acid solution before any disturbance occurs, showing any approximations made.
(b) For each scenario, write the expression for the reaction quotient Q immediately after the disturbance using concentrations present just before the disturbance; then compute its numerical value.
(c) For each scenario, determine whether the system will shift toward products, toward reactant, or negligibly in order to re-establish equilibrium, and predict qualitatively whether the final pH will be higher, lower, or unchanged compared with the initial pH calculated in part (a).
(d) Rank the three scenarios from the one that leads to the largest increase in pH to the one that leads to the largest decrease in pH at the new equilibrium. If any scenarios produce essentially the same pH, indicate this clearly.
(e) Provide a quantitative justification of the ranking in part (d) by comparing the magnitude of ∣log(Q/Ka)∣ for each scenario and discussing how the common-ion effect and dilution influence the position of equilibrium.