Question

Which is the stronger acid in each of the following pair `H_3PO_4` or `H_3AsO_4`?

Answer 1

`"H"_3"PO"_4` is the stronger acid.

Explanation:

For oxyacids with the same number of `"OH"` groups and the same number of `"O"` atoms, acid strength increases with increasing electronegativity of the central atom.

Since `"P"` is more electronegative than `"As"`, we predict that `"H"_3"PO"_4` should be a stronger acid than `"H"_3"AsO"_4"`.

This is confirmed by their `K_"a"` values .

`"H"_3"PO"_4"` has `K_a = 7.5 × 10^-3`, while`"H"_3"AsO"_4` has `K_a = 5.0 × 10^-3`.