The table below lists the first dissociation constant of a diprotic acid and the…

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

Problem

The table below lists the first dissociation constant of a diprotic acid and the dissociation constants of two monoprotic acids, all at 25 °C.

Substance	Relevant equilibrium	$pK$ value
Acid P	$\text{H}_2\text{A} \rightleftharpoons \text{H}^+ + \text{HA}^-$	$pK_{a1}=4.30$
Acid Q	$\text{HB} \rightleftharpoons \text{H}^+ + \text{B}^-$	$pK_a=4.85$
Acid R	$\text{HC} \rightleftharpoons \text{H}^+ + \text{C}^-$	$pK_a=3.96$

Assume solutions are 0.10 M and that temperature is 25 °C. (a) Rank the three acids in order of their ability to donate the FIRST proton (strongest to weakest acid for the first dissociation step). (b) The diprotic Acid P has a second dissociation constant

K_{a2}\ll K_{a1}

. Explain qualitatively how the large difference between

K_{a1}

and

K_{a2}

affects the pH of a 0.10 M solution of Acid P in comparison with a 0.10 M solution of monoprotic Acid Q. (c) For the 0.10 M solutions of Acids P, Q, and R, rank the percent ionization of the first proton from greatest to least and justify your ranking using the data provided.

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