Three separate aqueous solutions (Buffers A–C) are prepared at with the component…

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

Problem

Three separate

100\,\text{mL}

aqueous solutions (Buffers A–C) are prepared at

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

with the component concentrations shown. Buffer A:

[\text{CH}_3\text{COOH}]=0.50\,\text{M}

[\text{CH}_3\text{COO}^-]=0.50\,\text{M}

Buffer B:

[\text{CH}_3\text{COOH}]=0.10\,\text{M}

[\text{CH}_3\text{COO}^-]=0.10\,\text{M}

Buffer C:

[\text{CH}_3\text{COOH}]=0.50\,\text{M}

[\text{CH}_3\text{COO}^-]=0.10\,\text{M}

For acetic acid,

K_a=1.8\times10^{-5}

. Strong acids and bases are assumed to dissociate completely, and all volume changes produced by titrant additions may be neglected unless otherwise specified. (a) Calculate the initial pH of each buffer solution. (b) A

0.0050\,\text{mol}

aliquot of

\text{HCl}(aq)

is added to each buffer. Determine the pH of the resulting solution in each case and rank the buffers from greatest to least resistance toward the added acid. Justify your ranking quantitatively. (c) Independently, a

0.0050\,\text{mol}

aliquot of

\text{NaOH}(aq)

is added to a fresh

100\,\text{mL}

sample of each buffer. Determine the pH of the resulting solution in each case and rank the buffers from greatest to least resistance toward the added base. Justify your ranking quantitatively. (d) For Buffer A only, calculate the minimum number of moles of

\text{NaOH}(aq)

that must be added (with negligible volume change) to raise its pH by exactly

+1.00

unit from the initial value. Show all reasoning and calculations.

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