Question

Consider these acids $$ \begin{array}{ccccc} \hline \text { Acid } & \text { A } & \text { B } & \text { C } & \text { D } \\\ \text { pK }_{\text {a }} & 3.7 & 9.2 & 7.4 & 1.6 \\ \hline \end{array} $$ (a) Arrange the acids in order of decreasing acid strength from strongest to weakest. (b) Which acid has the largest \(K_{\mathrm{a}}\) value?

Short answer

Question: Arrange the four acids A, B, C, and D with pKa values 3.7, 9.2, 7.4, and 1.6, respectively, in order of decreasing acid strength and identify the acid with the largest Ka value. Answer: The order of decreasing acid strength is D > A > C > B, and Acid D has the largest Ka value.

Step-by-step solution

Find the order of decreasing acid strength

To find the order of decreasing acid strength, compare the pKa values of the acids. Lower pKa values correspond to stronger acids, as pKa is the negative logarithm of Ka: $$ pK_{a} = - \log_{10} (K_{a}) $$ So, we'll first order acids by their pKa from lowest to highest. $$ \text{Acid D: } 1.6, \text{ Acid A: } 3.7, \text{ Acid C: } 7.4, \text{ Acid B: } 9.2 $$ Thus, the order of decreasing acid strength (from strongest to weakest) is D > A > C > B.

Determine the acid with the largest Ka value

Since we already have the acids ordered by their pKa values, we can determine the one with the largest Ka value. As mentioned earlier, lower pKa values correspond to larger Ka values. Therefore, the acid with the largest Ka value is Acid D (pKa = 1.6).