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Effect on Kc

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Question

Calculate $[⊝ O H]$ which would be produced by each equilibrium concentration of $N H_{3}$ in part (a). Predict whether $Z n (O H)_{2}$ or $Z n (O H)_{4}^{2 -}$ would form in prefemce to $Z n (N H_{3})_{4}^{2 +}$ upon addition of sufficient $N H_{3}$ to produce the equilibrium concentration calculated in part (a).

A

3.33 \times 10^{- 4} M

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B

2.5 \times 10^{- 4} M

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C

4.5 \times 10^{- 4} M

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D

9.0 \times 10^{- 4} M

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Solution

The correct option is D $9.0 \times 10^{- 4} M$
The concentration of $⊝ O H$ in equilibrium with the $N H_{3}$ concentration of part (a) is given by:

$N H_{3} + H_{2} O ⇌ \oplus N H_{4} + ⊝ O H$ ,

(Let $x = [\oplus N H_{4}] = [⊝ O H])$

$K_{b} = \frac{[\oplus N H_{4}] [⊝ O H]}{[N H_{3}]}$

$1.8 \times 10^{- 5} = \frac{x^{2}}{4.5 \times 10^{- 2}}$

$⟹ x^{2} = 8.1 \times 10^{- 7} ⟹ x = [⊝ O H] = 9.0 \times 10^{- 4} M$

$[⊝ O H] (9.0 \times 10^{- 4} M)$ from the $4.5 \times 10^{- 2} M N H_{3}$ is sufficient to precipitate $Z n (O H)_{2}$ [part (b)] but not sufficient to form $Z n (O H_{4}^{2 -})$ [part (c)]

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