To which of the solutions, addition of water would not affect the pH ?

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$100 m L o f 0.2 M C H_{3} C O O H + 100 m L o f 0.1 M N a O H$

$100 m L o f 0.2 M C H_{3} C O O H + 100 m L o f 0.2 M K O H$

$200 m L o f 0.2 M C H_{3} C O O H + 100 m L o f 0.1 M N a O H$

$100 m L o f 0.2 M C H_{3} C O O H + 200 m L o f 0.2 M K O H$

answer is A, C.

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Detailed Solution

Addition of $H_{2} O$ will not change $p H$ only for buffer solution.(Buffer is formed if $W_{A}$ and $W_{B}$ is left after neutralization)
1. Acid buffer: $C H_{3} C O O H = 100 \times 0.2 = 20 m m o l K O H = 100 \times 0.1 = 10 m m o l$
10m. mol Of $C H_{3} C O O H$ will react with 10m. mol of $K O H$ to form 10m. mol of $C H_{3} C O O K$ and 10m. mol of $C H_{3} C O O H$ is left
2.    $m m o l o f C H_{3} C O O H = 100 \times 0.2 = 20$
   $\begin{array}{l} m m o l o f N a O H = 100 \times 0.2 = 20 \\ (S a l t o f W_{A} / S_{B} i s f o r m e d) \end{array}$
3. $m m o l o f C H_{3} C O O H = 200 \times 0.2 = 40$
   $\begin{array}{l} m m o l o f N a O H = 100 \times 0.1 = 10 \\ m m o l o f C H_{3} C O O H l e f t = 40 - = 30 \\ m m o l o f C H_{3} C O O N a f o r m e d = 10 \\ (B u f f e r i s f o r m e d) \end{array}$
4. $m m o l o f C H_{3} C O O H = 100 \times 0.2 = 20$
$\begin{array}{l} m m o l o f N a O H = 200 \times 0.2 = 40 \\ m m o l o f N a O H l e f t = 40 - 20 = 20 \\ S_{B} i s l e f t \end{array}$