Question

question 20 (4 points)
which of the following is the weakest acid?
a) hf (ka = 6.8 x 10-4)
b) hcio (ka = 3.0 x 10-8)
c) hno2 (ka = 4.5 x 10-4)
d) hcn (ka = 4.9 x 10-10)

Explanation:

Step1: Recall acid strength and Ka relation

The acid dissociation constant ($K_a$) indicates acid strength: smaller $K_a$ means weaker acid (less dissociation).

Step2: Compare Ka values

• HF: $K_a = 6.8 \times 10^{-4}$
• HClO: $K_a = 3.0 \times 10^{-8}$
• HNO₂: $K_a = 4.5 \times 10^{-4}$
• HCN: $K_a = 4.9 \times 10^{-10}$

Order of $K_a$ (smallest to largest): $4.9 \times 10^{-10}$ (HCN) < $3.0 \times 10^{-8}$ (HClO) < $6.8 \times 10^{-4}$ (HF) < $4.5 \times 10^{-4}$ (HNO₂)? Wait, no—wait, $4.9 \times 10^{-10}$ is smaller than $3.0 \times 10^{-8}$ (since $10^{-10} < 10^{-8}$). So HCN has the smallest $K_a$, hence the weakest acid.

Answer:

d) HCN ($K_a = 4.9 \times 10^{-10}$)