Question

Which solution has the lowest pH?
1.0 M HNO2
1.0 M CH3COOH
1.0 M HCOOH
1.0 M HNO3
1.0 M HPO4–

Answer 1

-Basically, organic acids are weaker than inorganic acids.
- The more the protons lost in a solution, the lower the pH.
I would go for the last alternative, because phosphoric acids tend to be stronger than nitric or nitrous acids.

Answer 2

I'd go with the nitric acid. The phosphoric acid has already lost a proton and its second value of Ka is much smaller.