The formula of Ammonium sulphate is:

The formula of Ammonium sulphate is (NH₄)₂SO₄.
Ammonium sulphate (NH₄)₂SO₄ is a salt composed of positively charged ammonium cation, i.e. NH₄⁺, and negatively charged sulphate anion, i.e. SO₄^-2, which carries a formal charge of -2. Both of these ions combine, and by cross multiplication of the formal charge, we obtain the chemical formula of the salt, i.e. (NH₄)₂SO₄.
 NH₄⁺ + SO₄^-2→ (NH₄)₂SO₄