A solution is prepared by dissolving 0.080 mol of sodium hydrogen carbonate (NaHCO₃)…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A solution is prepared by dissolving 0.080 mol of sodium hydrogen carbonate (NaHCO₃) and 0.120 mol of sodium carbonate (Na₂CO₃) in water to make a final volume of 1.00 L (pKₐ₂ for carbonic acid = 10.33).
The solution is to be titrated with 0.50 M HCl in 0.20 mL increments while monitoring pH.
(1) Determine which conjugate-acid–base component is in excess and calculate the millimole difference between the two species before any acid is added.
(2) Predict the initial direction (increase or decrease) of pH drift upon the first 0.20 mL addition of 0.50 M HCl, and justify your prediction using buffer-capacity considerations.
(3) Sketch a qualitative graph of pH (vertical axis) versus cumulative volume of HCl added (horizontal axis) for additions up to 2.0 mL. Clearly label the initial pH, indicate the trend in slope, and mark the point at which the buffer is expected to break.
(4) Provide a chemical explanation for the change in slope you indicated in part (c), relating your reasoning to the evolving component ratio as titration proceeds.