There's no single formula for "sulphate". It's a polyatomic ion, meaning it's a group of atoms that act as a single charged unit.

The formula for the sulphate ion is SO₄²⁻. This means it's composed of one sulfur atom (S) and four oxygen atoms (O), with a negative two charge (2-).

To form a compound, the sulphate ion needs to combine with a positively charged ion. For example:

* Sodium sulphate (Na₂SO₄) - two sodium ions (Na⁺) combine with one sulphate ion.

* Copper sulphate (CuSO₄) - one copper ion (Cu²⁺) combines with one sulphate ion.

So, when discussing "sulphate", we're actually referring to the sulphate ion (SO₄²⁻) which is a part of many different compounds.