A biochemistry laboratory requires an acetate buffer that can neutralize up to of…

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

Problem

A biochemistry laboratory requires an acetate buffer that can neutralize up to

0.025\,\text{mol}

of added strong acid without the pH falling below

4.50

. The buffer will be prepared at

25\,\text{°C}

using

1.00\,\text{L}

of solution made from available

0.500\,\text{M}

solutions of acetic acid (CH₃COOH) and sodium acetate (CH₃COONa). The acid-dissociation constant for acetic acid at

25\,\text{°C}

K_a = 1.8 \times 10^{-5}

. Assume that volume changes on mixing are negligible. (a) Determine the minimum number of moles of acetate ion that must be present in the buffer so that it can completely neutralize the specified amount of added strong acid. (b) Using your result from part (a), calculate the minimum number of moles of acetic acid that must also be present so that the initial pH of the buffer is exactly

4.50

. (c) Calculate the volumes (in mL) of the

0.500\,\text{M}

acetic acid solution and the

0.500\,\text{M}

sodium acetate solution that must be combined to prepare

1.00\,\text{L}

of the required buffer. (d) A student proposes doubling the concentrations of both components (while maintaining the ratio determined in parts (a)–(c)) but keeping the final volume at

1.00\,\text{L}

. Predict and justify the effect of this change on (i) the initial pH and (ii) the buffer’s capacity to neutralize added strong acid.

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