Find the [H+].?

2018-03-31 1:15 am

A chemist mixes HClO with water. The concentration is originally 0.24 M HClO, but after reacting with the water, the concentration is something less than 0.24 M. The Ka = 2.8 x 10–8. Find the [H+].

A) 6.72 x 10–9 M

B) 2.80 10–8 M

C) 8.20 x 10–5 M

D) 3.42 x 10–4 M

Thanks so much for your time! Any help is greatly appreciated!

回答 (1)

胡雪8°

2018-03-31 1:41 am

✔ 最佳答案

Consider the dissociation of HClO :
____________ HClO(aq) __ ⇌ __ ClO⁻(aq) __ + __ H⁺(aq) ___ Ka = 2.8 × 10⁻⁸
Initial: ______ 0.24 M ________ 0 M __________ 0 M
Change: ______ -y M ________ +y M _________ +y M
At eqm: ___ (0.24 - y) M ______ y M __________ y M

As Ka is very small, the dissociation of HClO is to a very small extent.
It is assume that 0.24 ≫ y and thus [HClO] at equilibrium = (0.24 - y) M ≈ 0.24 M

At equilibrium
Ka = [ClO⁻] [H⁺] / [HClO]
2.8 × 10⁻⁸ = y² / 0.24
y = √{0.24 × (2.8 × 10⁻⁸)} = 8.20 × 10⁻⁵
Hence, [H⁺] = y M = 8.20 × 10⁻⁵ M

The answer: C) 8.20 × 10⁻⁵ M

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