A laboratory technician prepares 1.00 L of a buffer by mixing 0.300 mol of lactic…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A laboratory technician prepares 1.00 L of a buffer by mixing 0.300 mol of lactic acid (HC₃H₅O₃, Ka=1.4×10−4) with 0.250 mol of sodium lactate (NaC₃H₅O₃). The temperature is 25 °C and the volume change on mixing is negligible.
The technician then adds 0.0100 mol of solid NaOH to the buffer with stirring, again assuming negligible volume change.
(a) Calculate the initial pH of the buffer before any NaOH is added.
(b) Determine the pH of the solution after the NaOH has completely dissolved and reacted. Show all calculations.
(c) For comparison, calculate the pH that would result if the same 0.0100 mol of NaOH were added to 1.00 L of pure water.
(d) Using your answers to parts (b) and (c), explain in chemical terms why the buffer resists the pH change far more effectively than pure water.