Clair Sawyer (Chemistry for Environmental Engineering). Chapter 4 (Equilibrium C

Question

Clair Sawyer (Chemistry for Environmental Engineering). Chapter 4 (Equilibrium Chemistry).

Arsenic acid (H3AsO4) is a triprotic acid with pKA1 = 2.22, pKA2 = 6.98, and pKA3 = 11.53. KH2AsO4 is added to water to give a total concentration of 0.001 M. Write the mass balance for this solution. Write the charge balance for this solution. Write the proton condition for this solution. Using the algebraic approximation method, calculate the equilibrium pH. Determine the equilibrium pH using a spreadsheet.

Explanation / Answer

solution: H3AsO4 H2AsO4- + H+ 2.2 x 10^-3 = x^2 / 0.68-x x = [H2AsO4-] = [H+] = 0.056 M [H3AsO4]= 0.68 - 0.056 = 0.62 M H2AsO4- H+ + HAsO42- 7 x 10^-8 = ( 0.056+x)( x) / 0.056-x x = [HAsO42-] = 8 x 10^-8 M [H+] = 0.056 + 7 x 10^-8 = 0.056 M HAsO42- H+ + AsO43- 2.22 x 10^-10 = (0.056+x)(x) / 7 x 10^-8-x x = [AsO43-] = 8.6 x 10^-16 M

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