Question

Arrange the acids
I) hydrogen selenate ion (HSeO−
4
),
pKa = 1.92;
II) phosphoric acid (H3PO4), pKa = 2.12;
III) selenous acid (H2SeO3), pKa = 2.46;
IV) phosphorous acid (H3PO3),
pKa1 = 2.00;
in increasing order of strengths.
1. I, III, II, IV
2. None of these
3. I, III, IV, II
4. II, IV, III, I
5. III, II, IV, I
6. IV, II, III, I
7. I, II, IV, III
8. Cannot be determined
9. I, IV, II, III
10. III, IV, II, I

Answer 1

pKa goes from weak acid to strong acid as the number goes down; i.e., pKa 2 is a stronger acid than pKa 5 which in turn is stronger than a pKa of 9.
You can arrange them with that information.