Question
The $$pH$$ of a 0.1 molar solution of the acid $$HQ$$ is 3. The value
of theionizalion constant, $${K_a}$$ the acid is :
A.
$$3 \times {10^{ - 1}}$$
B.
$$1 \times {10^{ - 3}}$$
C.
$$1 \times {10^{ - 5}}$$
D.
$$1 \times {10^{ - 7}}$$
Answer :
$$1 \times {10^{ - 5}}$$
Solution :
$$\eqalign{
& {H^ + } = C\alpha ;\,\alpha = \frac{{\left[ {{H^ + }} \right]}}{C}\,\,\,or\,\,\alpha = \frac{{{{10}^{ - 3}}}}{{0.1}} = {10^{ - 2}} \cr
& Ka = C{\alpha ^2} = 0.1 \times {10^{ - 2}} \times {10^{ - 2}} = {10^{ - 5}} \cr} $$