I need help calculating the pH of a solution that is .0400M Na3AsO4

Question

I was trying to calculate the pH with [H30+]=sqrt(ka1*ka2)

What is the correct way to calculate this?
Thankyou

Answer 1

What you have finds the pH for a solution of NaH2AsO4.
For a solution of Na3AsO4,
write the hydrolysis equation.
AsO4^-3 + HOH ==> HAsO4^-2 + OH^-
Kb = Kw/K3 = (OH^-)^2/(AsO4^-3)
You know Kw, K3 (look up K3 for H3AsO4), and (AsO4^-3= 0.0400 M)
This gives (OH^-). Convert to pOH and then to pH. Post your work if you get stuck.