Ionic EquilibriumHard

Question

Two buffers, X and Y of pH 4.0 and 6.0, respectively, are prepared from acid HA and the salt NaA. Both the buffers are 0.50 M in HA. What would be the pH of the solution obtained by mixing equal volumes of the two buffers? The value of Ka for HA = 1.0 × 10–5 (log 5.05 = 0.7).

Options

A.5.0
B.4.3
C.4.7
D.5.7

Solution

$4.0 = 5.0 + \log\frac{C_{1}}{0.5} \Rightarrow C_{1} = 0.05\text{ M}$

$6.0 = 5.0 + \log\frac{C_{2}}{0.5} \Rightarrow C_{2} = 5.0\text{ M}$

Now, final PH = $5.0 + \log\frac{V \times 0.05 + V \times 5.0}{V \times 0.5 + V \times 0.5} = 5.7$

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