Five buffer solutions are prepared at 25 °C by mixing different proportions of a…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Five buffer solutions are prepared at 25 °C by mixing different proportions of a monoprotic weak acid, HX, with its sodium salt, NaX, each to a total volume of 100.0 mL. The concentration of HX in every mixture is maintained at 0.150 M, while the concentration of X⁻ is varied as shown in the table below. A laboratory pH probe is used to measure the pH of each buffer.
Trial
[X⁻] (M)
Measured pH
1
0.015
3.46
2
0.030
3.61
3
0.075
3.90
4
0.150
4.16
5
0.300
4.39
(a) For each trial, compute log([HX][X−]). Record your values in a clearly labeled table.
(b) On graph paper, sketch a plot of pH (vertical axis) versus log([HX][X−]) (horizontal axis). Draw the best-fit straight line through the data points.
(c) Determine the slope and y-intercept of the best-fit line. Explain what each of these two parameters represents in the context of the acid–base system studied.
(d) Use the y-intercept obtained in part (c) to estimate the pKₐ of HX at 25 °C. Discuss one systematic error that could cause the experimental pKₐ to deviate from the true value.