CBSE Class 9 Answered

Formula is Al₂ (SO₄)₃

Aluminum sulphate contains two atoms of aluminum represented by Al₂ And three sulphate units represented by (SO₄)₃. The three sulphate units consist of three atoms of sulphur (1 x 3), and twelve atoms of oxygen (4 x 3).

Total atoms are 17.

Aluminium is in third group of periodic table and so it is trivalent.Chloride ion is monovalent.Hence
Aluminium chloride formula is AlCl3
Similarly Sulphate ion is the congugate base of Sulphuric acid H2SO4, formed after loss of both the 2 protons. Thus Phosphate ion is divalent, i.e. SO4 (-2). Thus Aluminium sulphate is Al2(SO4)3