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At 298 K, a certain buffer solution contains equal concentrations of $X^-$ and HX, $K_b$ for $X^-$ is $10^{-10}$. What is the pH of this buffer solution?
A
2
B
10
C
4
D
6
Detailed Solution
$K_a \times K_b = 10^{-14} \Rightarrow K_a(HX) = \frac{10^{-14}}{10^{-10}} = 10^{-4}$, so $pK_a = 4$
$pH = pK_a + \log \frac{[X^-]}{[HX]}$
$= 4 + \log 1 = 4$