Question

The $p^H$ of a buffer solution is $5$. It consists of a weak acid $HA$ and it's conjugate base $A^{-}$. $P^{Ka}$ of $HA$ is $4.699$. In which of the following conc. $HA$ and $A^{-}$ are present in the solution?

• A. $100$ ml of $0.1$ M $HA$ and $200$ ml of $0.1$ M $A^{-}$
• B. $200$ ml of $0.1$ M $HA$ and $100$ ml of $0.1$ M $A^{-}$
• C. $100$ ml of $0.1$ M $HA$ and $200$ ml of $0.2$ M $A^{-}$
• D. $200$ ml of $0.1$ M $HA$ and $100$ ml of $0.2$ M $A^{-}$

Answer 1

The correct option is A $100$ ml of $0.1$ M $HA$ and $200$ ml of $0.1$ M $A^{-}$

The expression for the pH of the acidic buffer solution is as given below.

$pH=p{K}_a+log\frac{Salt]}{\left[Acid\right]}$

$p{K}_a=4.699$, $pH=5$

Substitute values in the above equation.
$5=4.699+\log \frac{\left[Salt\right]}{\left[Acid\right]}$

Hence, $\log \frac{\left[Salt\right]}{\left[Acid\right]}=0.3010$ or $\frac{\left[Salt\right]}{\left[Acid\right]}=2$

In option A, $\left[Salt\right]=0.2\times 0.1=0.02$ M and $\left[Acid\right]=0.1\times 0.1=0.01$ M

Hence, $\frac{\left[Salt\right]}{\left[Acid\right]}=\frac{0.2}{0.1}=2$.

Hence, the correct option is $\text{A}$