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

1 Answer
Ernest Z.
Jul 31, 2015

#"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#.

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