Mohr's salt is a pale green crystalline solid which is soluble in water. it is a 'double sulfate' which contains two cations, one of which is fe2+.the identity of the second cation was determined by heating solid mohr's salt with solid sodium hydroxide and a colourless gas was evolved. the gas readily dissolved in watdr giving an alkaline solution. a grey green solid residue was also formed which wasd insoluble in water. whate are the identities of the gas and solid residue? a. gas: h2 residue: feso4 b. nh3 na2so4 c. nh3 fe(oh)2 d. so2 fe(oh)2