Ionic EquilibriumMedium
Question
The dissociation constant of formic acid is 0.00024. The hydrogen ion concentration in 0.002 M-HCOOH solution is nearly
Options
A.6.93 × 10−4 M
B.4.8 × 10−7 M
C.5.8 × 10−4 M
D.1.4 × 10−4 M
Solution
$\alpha = \sqrt{\frac{K_{a}}{C}} = \sqrt{\frac{0.00024}{0.002}} = 0.346\text{ or 34.6\%}$
Hence, α cannot be neglected.
$K_{a} = \frac{\alpha^{2}.C}{1 - \alpha} \Rightarrow 0.00024 = \frac{\alpha^{2} \times 0.002}{1 - \alpha} \Rightarrow \alpha = 0.29 $$$\text{Now, }\left\lbrack H^{+} \right\rbrack = C\alpha = 0.002 \times 0.29 = 5.8 \times 10^{- 4}$$
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