CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
An unknown monoprotic weak acid, HZ, is being characterized at 25 °C. A chemist prepares 1.000 L of an aqueous solution containing 0.120 M HZ (uncertainty ±0.003 M) and 0.080 M NaZ (uncertainty ±0.002 M). The measured pH of the solution is 4.75 (instrumental uncertainty ±0.02 pH unit).
(a) Determine the value of Kₐ and pKₐ for HZ using the experimental data provided.
(b) Propagate the stated concentration and pH uncertainties to estimate the uncertainty in the calculated pKₐ. Report pKₐ with its uncertainty to two significant figures.
(c) A reference source lists Kₐ for HZ as 1.7×10−5. Based on your result from part (a) and its associated uncertainty from part (b), state whether the experimental value agrees with the reference value at the 95 % confidence level. Justify your answer quantitatively.
(d) A separate experiment adds 0.0050 mol of HCl (strong acid, negligible volume change) directly to the original 1.000 L buffer solution. Predict the new pH of the mixture. Clearly outline the reasoning used to arrive at your answer.