Physiological experiments on an isolated enzyme require a buffered saline solution at…

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

Problem

Physiological experiments on an isolated enzyme require a buffered saline solution at pH

7.40

and

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

. Four potential weak acid–conjugate base pairs are listed below along with their acid dissociation constants at

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

Candidate	Weak Acid Formula	$K_a$
A	HA $_1$	$4.5\times10^{-8}$
B	HA $_2$	$1.8\times10^{-7}$
C	HA $_3$	$6.3\times10^{-8}$
D	HA $_4$	$9.2\times10^{-8}$

A total formal buffer concentration of

0.15\,\text{M}

is desired, and the working volume will be

2.00\,\text{L}

. Neglect activity effects and assume temperature remains fixed. (a) From the data provided, determine which candidate buffer pair (A, B, C, or D) is optimal for maintaining the required pH. Explain your reasoning. (b) For the buffer you selected in part (a), calculate the ratio

[\text{A}^-]/[\text{HA}]

that will establish an initial pH of

7.40

. (c) Using the total formal concentration constraint, compute the individual molar concentrations of the conjugate base and conjugate acid that must be combined to prepare

2.00\,\text{L}

of the buffer. (d) After preparation,

1.0\,\text{mL}

1.0\,\text{M}

HCl is accidentally introduced into the buffer solution. Assume the volume change is negligible. Calculate the new pH of the solution, showing all steps in your reasoning.

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