First slide

Ionization constants of weak bases, PH of weak bases

Question

The dissociation constant of H₂S and HS^- are respectively 10^-7 and 10^-13 . The pH of 0.1M aqueous solution of H₂S will be

Moderate

A

4
B

3
C

5
D

2.5

Solution

Let us consider a solution of H₂S(g)
Following equilibria gets established in the given solution.
At equilibrium
$\underset{C_{o} - x}{H_{2} S (} aq) ⇌ \underset{x - y}{{HS}^{-}} (aq) + \underset{x + y}{H^{+} (} aq)$
${\underset{x - y}{HS}}^{-} (aq) ⇌ \underset{y}{S^{- 2}} (aq) + \underset{y + x}{H^{+}} (aq)$
$K_{a_{1}} = 10^{- 7} =$ $\frac{(x) (x + y)}{C_{0} - x} \to 1$
$K_{a_{2}} = 10^{- 13} =$ $\frac{(x + y) (y)}{x - y} \to 2$
K_a1is very small and K_a2is even smaller. Also the difference in the vlaues of K_a1and K_a2is huge.
C₀ - x ~ C₀
x - y ~ x; x >>> y
x + y ~ x
(1) ⇒ 10^-7 = $\frac{x \times x}{C_{0}} \to (1)$
⇒ x² = 10^-8 ⇒ x = 10^-4
(2) ⇒ 10^-7 = $\frac{x \times y}{x} = y$
[H⁺] = x + y ~ x = 10^-4
P^H = -log[H⁺] = 4

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