Ionic EquilibriumMedium

Question

Which of the following mixture gives a buffer solution with $pH = 9.25$ ?

Given : ${pK}_{b}\left( {NH}_{4}OH \right) = 4.75$

Options

A.$0.2M{NH}_{4}OH(0.4\text{ }L) + 0.1MHCl(1\text{ }L)$
B.$0.2M{NH}_{4}OH(0.5\text{ }L) + 0.1MHCl(0.5\text{ }L)$
C.$0.5M{NH}_{4}OH(0.2\text{ }L) + 0.2MHCl(0.5\text{ }L)$
D.$0.4M{NH}_{4}OH(1\text{ }L) + 0.1MHCl(1\text{ }L)$

Solution

$pOH = {pK}_{b} + log\frac{\text{~Salt~}}{\text{~Base~}}$

$$4.75 = 4.75 + log\frac{\text{~Salt~}}{\text{~Base~}} $$Milimoles of [Salt] = milimoles of [Base]

Option (D) :

$$\begin{matrix} & {NH}_{4}OH + HCl \rightarrow {NH}_{4}Cl + H_{2}O \\ & 0.2M,0.5\text{ }L\ 0.1M,0.5\text{ }L \\ & 100\text{~}\text{mmole}\text{~}\ 50\text{~}\text{mmole}\text{~} \\ & 50\text{~}\text{mmole}\text{~}\ - \ 50\text{~}\text{mmole}\text{~} \\ & \text{~}\text{Milimoles of}\text{~}{NH}_{4}OH = \text{~}\text{milimoles of}\text{~}{NH}_{4}Cl \end{matrix}$$

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