Question

A 0.2 molar solution of formic acid is 3.2% ionized, its ionization constant is:

• A. $9.6\times {10}^{-3}$
• B. $2.1\times {10}^{-4}$
• C. $1.25\times {10}^{-6}$
• D. $2.1\times {10}^{-8}$

Answer 1

Answer: (B)

$2.1\times {10}^{-4}$

$HCOOH+H_{2}O\to H_3{O}^{+}+COO{H}^{-}$

$C$ $O$ $O$

$C-C\alpha$ $C\alpha$ $C\alpha$

We know

$K_a=\frac{C^2{\alpha }^2}{C(1-\alpha )}$

We are given $C=0.2M$

$\alpha =3.2\%=0.032$

$K_a=\frac{0.2\times (0.032{)}^2}{(1-0.032)}$

$K_a=2.1\times {10}^{-4}$