At 25 ∘C the reaction CaCrO4(s)←−→Ca2+(aq)+CrO2−4(aq) has an equilibrium constant Kc=7.1×10−4.?

What is the equilibrium concentration of Ca2+ in a saturated solution of CaCrO4?

Answer 1

In a saturated solution, an additional solute will not dissolve : kc = [Ca 2+][ CrO4 2-]
CaCrO4 is a solid, it’s omitted from the equilibrium expression. If x is concentrations of Ca2+ and CrO4 2- at the EQ : x^2 = 7.1 x 10^-4 => x = 2.7 x 10^-2
At EQ : [ Ca 2+] = [ CrO4 2- ] = 2.7 x 10^-2

Answer 2

[Ca2+][CrO4^2-] = 7.1 x 10^-4
Let x = molar solubility of CaCrO4
[Ca2+] = x ; [CrO4^2-] =x

x(x) = 7.1 x 10^-4
x = 2.67 x 10^-2
Answer with 2 significant figures [Ca2+] = [CrO4^2-] = 2.7 10^-2

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