Formula for Mohr's salt is (NH4)2Fe(SO4)2.6H2O. ∴ Number of water molecule in Mohr' salt =6. Formula for potash alum is K2SO4Al2(SO4)3.24H2O. ∴ Number of water molecules in potash alum is 12. ∴ Ratio of water molecules in Mohr's salt and potash alum is