CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Hydrofluoric acid, HF, is titrated with aqueous sodium hydroxide at 25 °C. The acid dissociation constant of HF at this temperature is Ka=7.2×10−4.
A student places 40.0 mL of 0.100 M HF in a beaker and adds 20.0 mL of 0.100 M NaOH. After stirring, the student records the temperature as 25 °C. Volumes are additive.
(a) Calculate the pH of the original 0.100 M HF solution before any NaOH is added.
(b) Determine the pH of the solution after the 20.0 mL sample of NaOH has been added and the reaction has gone to completion.
(c) An additional 2.0 mL of 0.100 M HCl is now introduced into the mixture from part (b). Calculate the new pH and explain, with quantitative evidence, how the presence of both HF and F− in solution influences the pH change produced by the added strong acid.
(d) Discuss how the buffer capacity of the solution in part (b) would be affected if the concentrations of HF and F− were both doubled while their ratio remained unchanged.