Question

A solution contains 5.10×10^-3 M sodium phosphate and 1.29×10^-2 M ammonium carbonate.  
Solid calcium nitrate is added slowly to this mixture.

A. What is the formula of the substance that precipitates first?

formula =

B. What is the concentration of calcium ion when this precipitation first begins?  
[Ca²⁺] =  M

Answer 1

(A) It is possible to form two kinds of salt precipitation: calcium phosphate (Ca3(PO4)2) and calcium carbonate (CaCO3).

Ksp of Ca3(PO4)2 = 2.07×10 ^ -33

Ksp of calcium carbonate = 5×10 ^ -9

Therefore, the Ksp of Ca3(PO4)2 is smaller, so it will precipitate first.

Therefore, the formula = Ca3 (PO4) 2

(B) Ca3(PO4)2(s)<=> 3Ca2 +(aq)+ 2PO4 ^ 3-(aq)

Ksp = [3Ca2 +] ^ 3 [2PO4 ^ 3-] ^ 2

2.07×10 ^ -33 = [3Ca2 +] ^ 3 (2×5.10×10 ^ -3) ^ 2

[Ca2 +] = 9×10 ^ -11M

Answer 2

A.

After mixing sodium phosphate and calcium nitrate, Ca3(PO4)2 precipitates are formed, with a Ksp value of 1 x 10-25.

Similarly, after mixing ammonium carbonate and calcium nitrate, CaCO3 precipitates are formed, with a Ksp value of 8.7 x 10-9.

Because the Ksp value of Ca3(PO4)2 is less than that of CaCO3, Ca3(PO4)2 will form precipitation first.

Therefore, the molecular formula of the first precipitated substance is Ca3(PO4)2

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B.

The partial dissociation of Ca3(PO4)2 is:

Ca3(PO4)2(s)<-------------------> 3Ca2 +(aq)+ 2PO42-(aq)

The expression of the solubility product, that is, the Ksp of Ca3 (PO4) 2 (equal to the product of the molar concentration of the product at the equilibrium stage during the reaction process increases to increase its stoichiometric coefficient).

Ksp = [Ca2 +] 3. [PO43-] 2

[Ca2 +] 3 = Ksp / [PO43-] 2

[Ca2 +] 3 = 1 x 10-25 /(5.10×10-3) 2

[Ca2 +] 3 = 3.84 x 10-21

[Ca2 +] = (3.84 x 10-21) 1/3

[Ca2 +] = 1.57 x 10-7 M

Therefore, [Ca2 +] = 1.57 x 10-7 M