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Title: Calculating the pH of a polyprotic acid? Post by: smitty2 on Oct 23, 2012 Find the pH and the concentration of H2SO3, HSO3-, and SO3(2-) if you have a 0.050 M H2SO3 solution, a 0.050 M NaHSO3 solution, and a 0.050 M Na2SO3 solution.
Title: Re: Calculating the pH of a polyprotic acid? Post by: cloveb on Apr 29, 2021 H2SO3 <---> HSO3- + H+ <----> SO3-- + H+
ka1 = 1.3E-2 ka2 = 6.3E-8 If you have a 0.050 M H2SO3, 0.050 M HSO3-, and 0.050 M SO3-- in solution: H2SO3 <---> HSO3- + H+ [H2SO3] = 0.050-X [HSO3-] = 0.050+x ka = [HSO3-][H+]/[H2SO3] 1.3E-2 = (0.050+x)x / (0.050-x) x = [H+] = 0.00902 M [H2SO3] = 0.050 - 0.00902 = 0.04098 M [HSO3-] = 0.050 + 0.00902 = 0.05902 M HSO3- <----> H+ + SO3- [HSO3-] = 0.05902 - x [SO3--] = 0.050 + x [H+] = 0.00902 + x ka = [H+][SO3-] / [HSO3-] 6.3E-8 = (0.00902+x)(0.050+x)/(0.05902-x) x = We will just assume that x is so small, that it is has a negligible effect on the concentration and pH. [SO3-] = 0.050 M [H+] = 0.00902 M [HSO3-] = 0.05902 M [H2SO3] = 0.04098 M pH = -log[H+] = -log[0.00902] = 2.04 |