Five separate 0.10 M aqueous solutions are prepared at 25 °C

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

Problem

Five separate 0.10 M aqueous solutions are prepared at 25 °C, each containing one of the species listed below.

Solution label	Species present	$pK$ value
1	$\text{HF}$	$pK_a = 3.17$
2	$\text{CN}^-$	$pK_b = 4.79$
3	$\text{CH}_3\text{COONa}$ (provides $\text{CH}_3\text{COO}^-$ )	$pK_b = 9.25$
4	$\text{NH}_4^+$	$pK_a = 9.25$
5	$\text{HClO}$	$pK_a = 7.54$

Assume

K_w = 1.0 \times 10^{-14}

at 25 °C. (a) For Solutions 2 and 3, determine which conjugate acid is stronger and explain how the numerical data support your conclusion. (b) Using the information provided, rank all five solutions from lowest pH (most acidic) to highest pH (most basic). Show all reasoning, including any conversions or comparisons you perform. (c) Identify the pair of solutions whose pH values are expected to be closest to each other and justify your choice using quantitative evidence.

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