A biochemical assay must be carried out at pH 7.40 ± 0.05
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A biochemical assay must be carried out at pH 7.40 ± 0.05. The laboratory has HEPES free acid (pK_a = 7.55 at 25 °C, M_w = 238 g·mol⁻¹) and a large supply of 1.0 M NaOH. You are asked to prepare 500 mL of a HEPES buffer that can neutralize up to 2.0 mmol of added HCl while keeping the pH within the required range.
(a) Calculate the mole ratio of conjugate base (HEPES⁻) to acid (HEPESH) required to set the initial pH at 7.40.
(b) Determine the minimum total concentration (in mol·L⁻¹) of the conjugate pair necessary in 500 mL of solution so that addition of 2.0 mmol HCl will change the pH by no more than 0.05 unit. Show your reasoning.
(c) Using your results from part (b), compute the exact masses of HEPES free acid and the volume of 1.0 M NaOH you must combine to prepare the buffer.
(d) After preparation, 2.0 mmol of HCl is added to a 500 mL aliquot of the buffer. Predict the final pH and comment on whether the specification of pH 7.40 ± 0.05 is still met.