A biochemical assay must be performed at and pH

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

Problem

A biochemical assay must be performed at

37\,^{\circ}\!\text{C}

and pH

6.20

. Three candidate weak acids, each available with its conjugate base, have the temperature-dependent

pK_a

values shown in the table.

Weak Acid	$pK_a$ at $25\,^{\circ}\!\text{C}$	$pK_a$ at $37\,^{\circ}\!\text{C}$
Acid M	6.10	6.07
Acid N	5.79	5.70
Acid P	6.60	6.55

Assume that the assay solution must contain a total buffer component concentration of

0.050\,\text{M}

and that ionic strength constraints are negligible. (a) Choose the weak acid–conjugate base pair that best meets the assay’s pH requirement at

37\,^{\circ}\!\text{C}

. Justify your selection quantitatively. (b) For the buffer system selected in part (a), calculate the numerical ratio of conjugate base concentration to conjugate acid concentration required to achieve pH

6.20

37\,^{\circ}\!\text{C}

. (c) If the assay undergoes a brief temperature drop to

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

while the buffer component concentrations remain unchanged, determine the resulting pH of the solution. Clearly outline your reasoning.

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