Question

• Design a buffer of pH 4.0 that resist the addition of 75 mmol HCl. Calculate the moles of weak acid and conjugate base in solution before and after addition of HCl

Answer 1

0.67 M Lactic Acid 100 mL

0.95 M Lactate100 mL

+ 75 mmol HCl

Resulting pH = 4.00

67 mmol Lactic Acid before (0.67 M x 100 mL = 67 mmol)

95 mmol Lactate (conjugate base) before (0.95 x 100 mL = 95 mmol)

HCl reacts with Lactate to produce Lactic Acid

95 mmol – 75 mmol = 20 mmol Lactate left

67 mmol + 75 mmol = 142 mmol Lactic Acid left

This buffer resists the pH change because there is enough lactate present to consume the added HCl and convert it to Lactic acid instead of letting the HCl add more H+ ions to the solution, which would decrease the pH significantly.