Question

A solution contains 0.50M NH3 and 1.50MNH4Cl. What is the maximum concentration ogMG^2+ that can be present in such a solution (without) precipitating Mg(OH)2?

Answer 1

Answer on Question #56434 – Chemistry - Inorganic Chemistry

A solution contains 0.50 M NH₃ and 1.50 M NH₄Cl. What is the maximum concentration of Mg²⁺ that can be present in such a solution (without) precipitating Mg(OH)₂?

Answer:

NH₄Cl ionizes completely. So that C(NH₄)=1.50 M.

NH₃ is a weak base. It reacts with water as:

NH₃ + H₂O = NH₄OH

NH₄OH dissociates according to:

NH₃ + H₂O = NH₄OH

1 0.5 0 0

2 -x x x

3 0.5-x x x

K_b = [NH₄⁺][OH⁻]/[NH₃]

[OH⁻] = (K_b·[NH₃])/ [NH₄⁺]

K_b(NH₄OH) = 1.85·10⁻⁵

[OH⁻] = x

[NH₄⁺] already exists in the solution from NH₄Cl. So that: C([NH₄⁺]) = 1.5+x. But x here can be ignored.

[NH₃] = 0.5-x, where x can be ignored

[OH⁻] = (1.85·10⁻⁵·0.5)/1.5 = 6.16·10⁻⁶

K_sp(Mg(OH)₂) = [Mg²⁺][OH⁻]

K_sp(Mg(OH)₂) = 5.6·10⁻¹²

[Mg²⁺] = (5.6·10⁻¹²)/(6.16·10⁻⁶) = 9.1·10⁻⁷

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