If the pH of a solution of magnesium chloride gets too high, magnesium hydroxide (insoluble metal hydroxide) could precipitate from solution. If you prepare a buffer solution, it is possible to prevent the precipitation of Magnesium Hydroxide by controling the pH. Which of the following buffers solutions would not result in the precipitation of Magnesium Hydroxide from a solution that is 0.0173 M in Magnesium chloride . Check all that apply.

Buffers would not result in precipitation are:

*1.0M HNO2 with 1.0M NaNO2 (Ka = 4.5x10⁻⁴; pKa = 3.347)

*5.6M C6H5OH with 1.0M KC6H5O (Ka = 1.3x10⁻¹⁰; pKa = 9.89)

*2.0M HCN with 1.0M NaCN (Ka = 4.0x10⁻¹⁰; pKa = 9.40)

Explanation:

The Ksp of magnesium hydroxide, Mg(OH)₂, is:

Mg(OH)₂(s) ⇄ Mg²⁺(aq) + 2 OH⁻(aq)

Ksp = [Mg²⁺] [OH⁻]² = 1.2x10⁻¹¹

The precipitation will occurs when the product [Mg²⁺] [OH⁻]² > Ksp. If [Mg²⁺] is 0.0173M:

[0.0173M] [OH⁻]² > 1.2x10⁻¹¹

[OH⁻]² > 6.936x10⁻¹⁰

[OH⁻] > 2.63x10⁻⁵M

pOH (-log [OH]) is 4.58, and pH (pH = 14-pOH) is:

pH = 9.42, if pH > 9.42, the precipitation will occurs

For the following options, using H-H equation, pH is:

*1.0M HNO2 with 1.0M NaNO2 (Ka = 4.5x10⁻⁴; pKa = 3.347)

pH = 3.347 + log (1.0M / 1.0M)

pH = 3.347. As pH <  9.42; the buffer solution will not result in precipitation.

*2.0M C6H5OH with 1.32M KC6H5O (Ka = 1.3x10⁻¹⁰; pKa = 9.89)

pH = 9.89 + log (1.32M / 2.0M)

pH = 9.71. As pH >  9.42; the buffer solution will result in precipitation.

*1.0M HCN with 2.0M NaCN (Ka = 4.0x10⁻¹⁰; pKa = 9.40)

pH = 9.40 + log (2.0M / 1.0M)

pH = 9.70. As pH >  9.42; the buffer solution will result in precipitation.

*5.6M C6H5OH with 1.0M KC6H5O (Ka = 1.3x10⁻¹⁰; pKa = 9.89)

pH = 9.89 + log (1.0M / 5.6M)

pH = 9.14. As pH <  9.42; the buffer solution will not result in precipitation.

*2.0M HCN with 1.0M NaCN (Ka = 4.0x10⁻¹⁰; pKa = 9.40)

pH = 9.40 + log (1.0M / 2.0M)

pH = 9.10. As pH <  9.42; the buffer solution will not result in precipitation.

*1.0M C6H5OH with 1.0M KC6H5O (Ka = 1.3x10⁻¹⁰; pKa = 9.89)

pH = 9.89 + log (1.0M / 1.0M)

pH = 9.89. As pH >  9.42; the buffer solution will result in precipitation.

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