Ka for an Acid (HA) is5×10^-6 .
What is the pH of 0.2 M solution of HA.
Find also the molar concentration of À
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Answer:
Secondary School Chemistry 5 points
If the Ka of a weak monoprotic acid is 4* 10-6 and
its concentration is 0.2 M then pH of solution is
(1) 305
(2) 3.72
(3) 3.99
(4)4.62
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shubham0204
shubham0204 Expert
Answer:
See below.
Explanation:
We are given,
\begin{aligned}K_{a}=4\times 10^{-6}\\ C=0.2\end{aligned}
For a weak acid, we know,
\begin{aligned}K_{a}=C\alpha ^{2}\\ \Rightarrow \alpha =\left( \dfrac {K_{a}}{c}\right) ^{\dfrac {1}{2}}\end{aligned}
Calculating the degree of dissociation ( alpha ),
\begin{aligned}\alpha ^{2}=\dfrac {4\times 10^{-6}}{0.2}=2\times 10^{-5}\\ \Rightarrow \alpha =0.447\times 10^{-2}\end{aligned}
Calculating the concentration of H+ ions,
\begin{aligned}\left[ H^{+}\right] =C\alpha =0.2\times 0.477\times 10^{-2}\\ =0.0954\times 10^{-2}\\ =9.54\times 10^{-4}\end{aligned}
Calculating pH,
\begin{aligned}pH=-\log \left[ H^{+}\right] \\ =-\log \left( 9.54\times 10^{-4}\right) \\ =-\left( 0.98-4\right) \\ =3.02\end{aligned}
The correct option should be 3.05 ( nearest answer ).