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

Jul 31, 2015

${\text{H"_3"PO}}_{4}$ is the stronger acid.

#### Explanation:

For oxyacids with the same number of $\text{OH}$ groups and the same number of $\text{O}$ atoms, acid strength increases with increasing electronegativity of the central atom.

Since $\text{P}$ is more electronegative than $\text{As}$, we predict that ${\text{H"_3"PO}}_{4}$ should be a stronger acid than $\text{H"_3"AsO"_4}$.

This is confirmed by their ${K}_{\text{a}}$ values .

$\text{H"_3"PO"_4}$ has K_a = 7.5 × 10^-3, while${\text{H"_3"AsO}}_{4}$ has K_a = 5.0 × 10^-3.