Compound B forms hydrated crystals with Al2​(SO4​)3​.Also, B is formed with univalent metal on heating with sulphur. Hence, compound B must has the molecular formula M2​SO4​ and compound A must be an oxide of M which reacts with sulphur to give metal sulphate asA+S⟶M2​SO4​∵0.321 g sulphur gives 1.743 g of M2​SO4​∴32.1 g S (one mole) will give 174.3 gM2​SO4​Therefore, molar mass of M2​SO4​=174.3 g⇒174.3=2× Atomic weight of M+32.1+64⇒ Atomic weight of M=39, metal is potassium (K)K2​SO4​ on treatment with aqueous Al2​(SO4​)3​ gives potash-alum.K2​SO4​+Al2​(SO4​)3​+24H2​O⟶K2​SO4​Al2​(SO4​)3​⋅24H2​OBIf the metal oxide A has molecular formula MOx​, two moles of it combine with one mole of sulphur to give one mole of metal sulphate as2KOx​+S⟶K2​SO4​⇒x=2, i.c. A is KO2​.