Here's why:

* Bisulfate is a compound containing the hydrogen sulfate ion (HSO₄⁻).

* The sulfate ion (SO₄²⁻) has a charge of -2.

* Bisulfate has one hydrogen ion (H⁺) attached to the sulfate ion, which reduces the overall charge to -1.

Therefore, the formula for bisulfate is HSO₄⁻.