Aluminum Sulfat / Tawas is a specific chemical compound. The specific compound is hydrated potassium aluminum sulfate (potassium alum) with the formula KAl (SO4) 2 · 12H2O. More broadly, alum is a double sulfate salt, with the formula AM (SO4) 2 · 12H2O, where A is a monovalent cation like potassium or ammonium and M is a trivalent metal ion such as aluminum or chromium (III).