Two buffer solutions, Buffer X and Buffer Y

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

Two buffer solutions, Buffer X and Buffer Y, are each prepared to a total volume of

1.00\,\text{L}

25\,^{\circ}\text{C}

. • Buffer X:

0.30\,\text{mol}

of formic acid (HCOOH) and

0.30\,\text{mol}

of sodium formate (HCOONa) • Buffer Y:

0.15\,\text{mol}

of formic acid (HCOOH) and

0.45\,\text{mol}

of sodium formate (HCOONa) Both buffers initially have the same volume and temperature. A student adds

2.0\,\text{mL}

0.10\,\text{M}

HCl(aq) to each buffer in separate beakers. Assume that the added volume does not appreciably change the total volume of either buffer. Answer the following questions. All explanations must include reference to the relative amounts of conjugate acid and conjugate base in each buffer. (1) Part 1: Identify which buffer, X or Y, will experience the greater numerical change in pH after the HCl addition. Justify your answer qualitatively. (2) Part 2: For the buffer you identified in part (a), describe how the ratio

[\text{base}]/[\text{acid}]

changes as a result of the HCl addition and explain how this leads to the predicted pH shift. (3) Part 3: Suppose the student instead added

2.0\,\text{mL}

0.10\,\text{M}

NaOH to fresh samples of Buffer X and Buffer Y. Predict which buffer would then experience the greater numerical pH change and justify your prediction.

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