A laboratory technician prepares of a buffer solution (Buffer D) by dissolving…

CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases

Problem

A laboratory technician prepares

200\,\text{mL}

of a buffer solution (Buffer D) by dissolving ammonium chloride and ammonia so that

[\text{NH}_4^+]=0.25\,\text{M}

and

[\text{NH}_3]=0.25\,\text{M}

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

. The base-dissociation constant of ammonia is

K_b=1.8\times10^{-5}

. The technician then carries out the two separate operations described below. Assume temperature remains constant at

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

. Operation 1: Pure water is added until the total volume becomes

400\,\text{mL}

. Operation 2: Instead of adding water,

0.0020\,\text{mol}

\text{HCl}(aq)

is added directly to the original

200\,\text{mL}

of Buffer D. (a) Calculate the pH of Buffer D before either operation is performed. (b) After Operation 1 is completed, determine the new pH of the diluted buffer. Show all reasoning. (c) For Operation 2, calculate the pH of the solution after the acid addition. Then calculate what the pH would be if the same

0.0020\,\text{mol}

\text{HCl}(aq)

were added after Operation 1 (that is, to the

400\,\text{mL}

diluted buffer). (d) Based on your results from parts (b) and (c), discuss how dilution affects the buffer’s capacity to resist added strong acid.

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