A laboratory prepares a very dilute solution of hydrofluoric acid, HF ( )
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A laboratory prepares a very dilute solution of hydrofluoric acid, HF (Ka=6.6×10−4). Exactly 35.0 mL of 0.0500 M HF is titrated with 0.0500 M KOH to the equivalence point. The total volume of the mixture at equivalence may be assumed to be the sum of the individual volumes.
(a) Determine the molar concentration of fluoride ions, F⁻, in the titration vessel at the equivalence point.
(b) Assess the validity of the approximation x≪C for solving the hydrolysis equilibrium of F⁻ by calculating the ratio Cx that would result from using the approximation.
(c) Without assuming x≪C, solve the equilibrium expression exactly (using the quadratic formula where necessary) to obtain the pH of the solution at the equivalence point. Give the pH to two decimal places.
(d) A chemist proposes using bromothymol blue (transition range 6.0 – 7.6) as the indicator for this titration. Evaluate the suitability of this choice based on your calculated equivalence-point pH.