CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A monoprotic weak acid, HA, is dissolved in water to create a solution in which analytical measurements show that 90 % of the acid remains in the protonated form (HA) and 10 % has dissociated to A⁻. No additional information about the acid is provided.
Answer the following questions about this system.
(a) Derive an expression that relates the numerical value of the ratio [HA][A−] to the difference between the solution pH and the pKa of the acid.
(b) Using the given ratio of species, calculate the numerical value of (pH−pKa) for the solution. Show all mathematical steps, including evaluation of any logarithms.
(c) State whether the solution pH is greater than, less than, or equal to the pKa of the acid, and quantify by how many pH units the two values differ. Explain how your calculation in part (b) supports your conclusion.