In: Chemistry
Arsenic acid is a triprotic acid in water, ionizing in the following sequential steps: H3AsO4 + H2O <==> H2AsO4- + H3O+ Ka 5.5 x 10^-3 H2AsO4- + H2O <==> HAsO42- + H3O+ Ka 1.7 x 10^-7 HAsO42- + H2O <==> AsO43- + H3O+ Ka 5.1 x 10^-12 What is the approximate pH of a 0.80 M H3AsO4 solution?
Ka = 5.5 x 10^-3
H3AsO4 + H2O <--------------------> H2AsO4- + H3O+
0.80 0 0 -----------------> initial
0.80-x x x ---------------------> equilibrium
Ka1 = x^2 / 0.8- x
5.5 x 10^-3 = x^2 / 0.8- x
x^2 + 5.5 x 10^-3 x - 4.4 x 10^-3 = 0
x = 0.0636
x = [H3O+] = 0.0636 M
pH = -log[H3O+] = -log (0.0636)
pH = 1.2