Use the rule of 13 - the formula of a compound is a multipleĀ nĀ of 13 (the molar mass ofĀ CH) plus a remainderĀ r.Ā ForĀ O, addĀ OĀ and removeĀ CHā and forĀ N, addĀ NĀ and remove oneĀ CHā.
84Ā Ć· 13 = 6r6, the integer equals the number of carbon atoms and the integerĀ plusĀ the remainder equals the number of hydrogen atoms,Ā so possible formula is CāHāā.
M(CāHāā) = 6Ā Ā· 12,00 + 12Ā Ā· 1,008 = 84,096.
Add one oxygen atom to molecular formula: Cā
HāO.
M(Cā
HāO) = 5Ā Ā· 12,00 + 8Ā Ā· 1,008 + 15,995 = 84,059, not.
Add one nitrogen atom to molecular formula: Cā
HāāN.
M(Cā
HāāN) = 5Ā Ā· 12,00 + 10Ā Ā· 1,008 + 14,003 = 84,083, not.
If we add nitrogen and oxygen mass will be lower than 84,095, so molecular formula is CāHāā.