Question

The $K_a$ values of formic acid and acetic acid are respectively $1.77\times {10}^{-4}$ and $1.75\times {10}^{-5}$. The ratio of the acid strength of $0.1M$ acid is:

• A. $10$
• B. $3.178$
• C. $0.3$
• D. $0.1$
• E. $100$

Answer 1

Answer: (B)

$3.178$

The strength of acid are inversely proportional to the square root of dissociation constant.

$K_{a1}=1.77\times {10}^{-4},K_{a2}=1.75\times {10}^{-5}$

Ratio of strength of acids is $\frac{S_1}{S_2}=\sqrt{\frac{K_{a2}}{K_{a1}}}=\sqrt{\frac{1.75\times {10}^{-5}}{1.77\times {10}^{-4}}}=0.317$