Which barium salt will precipitate first? A solution containing a mixture of 0.0

Question

Which barium salt will precipitate first?

A solution containing a mixture of 0.0436 M potassium chromate (K_2CrO_4) and 0.0592 M sodium oxalate (Na_2C_2O_4) was titrated with a solution of barium chloride (BaCI2) for the purpose of separating CrO_4^2- and C_2O_4^2+ by precipitation with the Ba^2+ cation. Answer the following questions regarding this system. The solubility product constants (K_sp) for BaCrO_4 and BaC_2O_4 are 2.10 times 10^-10 and 1.30 times l0^-6, respectively. Which barium salt will precipitate first? What concentration of Ba^2+ must be present for BaCrO_4 to begin precipitating? What concentration of Ba^2+ is required to reduce oxalate to 10% of its original concentration? What is the ratio of oxalate to chromate ([C_2O_4^2-]/[CrO_4^2-]) when the Ba^2+ concentration is 0.0090 M?

Explanation / Answer

a) Since Ksp of BaCrO4 is lower, means that this compound is less soluble, therefore, BaCrO4 will precipitate first.

b) The reaction will be:

BaCrO4 <-------> Ba2+ + CrO42-

Ksp = [Ba2+] [CrO42-]

[Ba2+] = Ksp / [CrO42-]

[Ba2+] = 2.1x10-10 / 0.0436 = 4.82x10-9 M

c) BaC2O4 <--------> Ba2+ + C2O42-

Ksp = [Ba2+] * 10/100[C2O42-]

[Ba2+] = Ksp / 0.10[C2O42-]

[Ba2+] = 1.3x10-6 / 0.10 * 0.0592

[Ba2+] = 2.2x10-4 M

d) ratio = Ksp BaC2O4 / Ksp BaCrO4

ratio = 1.3x10-6 / 2.1x10-10

ratio = 6190.48 = 6.19x103

Hope this helps

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