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A solution contains 8.28×10-3 M magnesium nitrate and 1.47×10-2 M calcium acetate. Solid ammonium fluoride is...

A solution contains 8.28×10-3 M magnesium nitrate and 1.47×10-2 M calcium acetate. Solid ammonium fluoride is added slowly to this mixture. What is the concentration of calcium ion when magnesium ion begins to precipitate?

Expert Solution

When Qs = Ks the solution will be saturated and there will be precipitation.

Given that the solution has components that ionize completely,

Mg(NO₃)₂-> Mg⁺² + 2 NO₃ and [Mg⁺²]=8.28×10-3 M

Ca(CH₃COO)₂-> Ca⁺² +2CH₃COO and [Ca⁺²]=1.47×10-2 M

The precipitation reactions are:

Ca⁺² + 2F^- <-> CaF₂ ks= 3.9x10^-11

Mg⁺² + 2F^-<-> MgF₂ ks= 6.4x10^-9

We have to find wich component (Ca or Mg) precipitate first, therefore Qs > Ks:

Qs= [Ca][F]² > Ks ------> [F]=

Qs= [Mg][F]² > Ks ------> [F]=

Given that the [F] needed in order to precipitate Calcium is less than the [F] in order to precipitate Mgnesium, the CaF2 will be formed first.

In order to make Mg precipitate, a [F] of at least 8.92x10^-4 M is needed. However, when the [F] reaches that point, some CaF2 has already been formed.

Therefore,

Qs= [Ca][F]² = Ks = 3.9x10^-11 --------> Ca = = 4.9x10^-5 M has already precipitated.

Given an initial Calcium concentration of 1.47x10^-2 and a Calcium precipitate of 4.9x10^-5,

the concentration of the calcium ion when magnesium ion begins to precipitate is 0.0146 M.

queen_honey_blossom answered 1 year ago

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