An analytical method for quantifying atmospheric ammonia uses a sampling solution…

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

Problem

An analytical method for quantifying atmospheric ammonia uses a sampling solution that must remain at pH

9.00\pm0.05

while exposed to ambient temperatures between

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

and

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

. Two candidate buffer systems are under consideration. Their

pK_a

values and temperature coefficients are provided below:

Buffer System	$pK_a$ at $20\,^{\circ}\!\text{C}$	$\displaystyle \frac{dpK_a}{dT}$ (per $^{\circ}\!\text{C}$ )
System Q	9.06	$+0.011$
System R	8.93	$-0.005$

Total buffer component concentration will be fixed at

0.080\,\text{M}

. Assume the base form and acid form will initially be present in the ratio that achieves pH

9.00

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

. (a) For each buffer system, derive an expression for the pH as a function of temperature

T

(in

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

) over the range

20\le T\le22

, assuming the concentrations of the conjugate acid and conjugate base remain constant. (b) Using your expressions from part (a), determine the maximum and minimum pH values each buffer would reach within the stated temperature range. (c) Identify which buffer system (Q or R) better satisfies the required pH tolerance of

9.00\pm0.05

across the temperature window, and defend your choice quantitatively. (d) For the buffer you selected in part (c), calculate the conjugate-base-to-conjugate-acid concentration ratio that should be used in the initial preparation at

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

Related theory

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