CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 50.0 mL sample of 0.150 M phosphoric acid, H3PO4(aq), is titrated with 0.200 M potassium hydroxide, KOH(aq), in water at 25 °C. Assume that only the first proton of phosphoric acid is transferred in the initial, fast proton-transfer step.
Answer the following.
(a) Determine the volume of KOH (in mL) required to reach the point at which exactly one-third of the acid molecules have donated a proton.
(b) Write the balanced net-ionic equation that represents the proton-transfer reaction responsible for the change calculated in part (a).
(c) Identify and label the conjugate acid–base pairs in the equation from part (b).
(d) After the addition described in part (a), write the expression for the reaction quotient Q for the proton-transfer step, and explain qualitatively (no calculation required) whether the reaction will proceed predominantly forward or backward to reach equilibrium.