[Diamonds]

Calculate the hydronium ion concentration in 50.0 mL of 0.10M NaH₂AsO₄.
a) 2.4 E-2
b) 1.6 E-3
c) 1.0 E-4
d) 2.5 E-5
It also gives Ka₁, Ka₂, and Ka₃.

So basically the reactions I have are (A means AsO₄^3-):

H3A    H₂A^- + H⁺     Ka1= 6.0 E-3
H₂A^- + H₂O    H₃A + OH^- Kb1 = 1.66 E-12

H₂A^-    H⁺ + HA^2- Ka2 = 1.1 E-7
HA^2- + H₂O  H₂A^- + OH^- Kb2 = 9.09 E-8

HA^2-    A^3- + H⁺ Ka3 = 3.0 E-12
A^3- + H₂O    HA^2- + OH^- Kb3 = 3.33 E-3

NaH₂AsO₄ (H₂A^-) comes up in more than 1 equation, so which one do I use to figure out the pH or [H⁺] ?

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt