Question

Formic acid is $4.5$% dissociated in a $0.1N$ solution at ${20}^{o}C$. The ionisation of water is:

• A. $21\times {10}^{-4}$
• B. $21$
• C. $0.21\times {10}^{-4}$
• D. $2.1\times {10}^{-4}$

Answer 1

Answer: (D)

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

When formic acid is dissolved in water, it partially dissociates (4.5%).

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

We assume $\alpha \sim 0\Rightarrow 1-\alpha \sim 1$

$\Rightarrow K_a=\frac{C{\alpha }^2}{1}=0.1\times 0.045\times 0.045=2.02\times {10}^{-4}$