Potash alum is adouble salt . Explain this statement using the chemical formula of the alum

Answer 1

An alum, when I was in school years and years ago, was defined as the double salt consisting of the sulfate of a monovalent metal (Na, K, etc) along with the sulfate of a trivalent metal (Al, Cr, etc) plus 24 molecules of water of hydration per formula unit of the salt. K2SO4 +Al2(SO4)3.24 H2O or K2Al2(SO4)4.24H2O. The more modern definition is to simplify that to
KAl(SO4)2.12H2O. So potash alum is the combination of K2SO4 and Al2(SO4)3 compounds into one compound.