Question

Which of the following solutions would have the highest pH? Assume that they are all 0.10 M in acid at 25C. The acid is followed by its Ka value.

1) HF, 3.5 × 10-4

2) HCHO2, 1.8 × 10-4

3) HNO2, 4.6 × 10-4

4) HCN, 4.9 × 10-10

5) HClO2, 1.1 × 10-2

Answer 1

Answer: 4) HCN, 4.9 × 10-10

Explanation:

K_a, the acid ionization constant, is the equilibrium constant for chemical reactions involving weak acids in aqueous solution. The numerical value of K_a is used to predict the extent of acid dissociation. A large K_a value indicates a stronger acid (more of the acid dissociates) and small K_a value indicates a weaker acid (less of the acid dissociates).

As HCN has lowest K_a value, it indicates a weaker acid (less of the acid dissociates). Thus HCN solutions has highest pH than other solutions because less acid dissociates into the solution compared to other chemical solutions which has high K_a values.