a 8.6 *10 to the -3 power M solution of H3PO4 has a PH of 2.30 what is the Ka for H3PO4?

Answer 1

Convert pH to (H^+). That's approximately 5E-3.
............H3PO4 ==> H^+ + H2PO4^-
I.........0.0086 M....0.......0
C...........-x........x........x
E.....0.0086-x........x........x
The problem tells you x = (H^+) = (approximately) 5E-3
Plug that in for x (into the Ka expression) and solve for Ka.