Please answer this question, it is driving me mad!!?

Question

Rank these acids in order of decreasing acid strength.....

a.) H^3PO^4 Ka (H^3PO^4/H^2PO4^-) = 7.9 x 10-3

b.) NH4+ pKa (NH4+/NH3) 9.2

c.) HNO3 pKa (HNO3/NO3) = 1.5

d.) CH3COOH Ka (CH3COOH/CH3COO-) = 2.0 x 10-5

Answer 1

Data for b.) and c.) are in pKa, so convert them to Ka to make every thing consistent.

b.) pKa = 9.2, so Ka = 6.31 x 10^-10

c.) pKa = 1.5, so Ka = 3.16 x 10^-2

So c.) is strongest, a.) is second, d.) is third, and b.) is weakest