Silver arsenate (Ag3AsO4) is only slightly soluble in water. The compound arseni

Question

Silver arsenate (Ag3AsO4) is only slightly soluble in water. The compound arsenic acid (H3AsO4) behaves in much the same manner as phosphoric acid. Which of the following represents a mass balance equation for the solution at pH 8.0?

A. 1/3[Ag+] = [AsO43- ] + [HAsO42-] + [H2AsO4-] + [H3AsO4]

B. [Ag+] = [AsO43-] + [HAsO42-] + [H2AsO4-] + [H3AsO4]

C. [Ag+] + [H+] = 3[AsO43-] + 2 [HAsO42-] + [H2AsO4-] + [H3AsO4] + [OH-]

D. 3[Ag+]= [AsO43-] + [HAsO42-] + [H2AsO4-] + [H3AsO4]

Please try to explain how you get your answer.

Explanation / Answer

Ag3AsO4 ==> 3Ag^+ + AsO4^3-

AsO4^3- will take up proton from water and undergo folllowing equilibrium

AsO4^3- + H^+ ==> HAsO4^2-

HAsO4^2- + H^+ ==> H2AsO4^1-

H2AsO4^1- + H^+ ==> H3AsO4

Answer is : A

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