Write the molecular formula for a compound with the possible elements C, H, N and O that exhibits a molecular ion at M+ = 84.095

I understand the concept of adding together the specific mass C=12.000, H=1.008, N=14.003 and O=15.995 but I need help with a systematic approach to solve for these problems when that is the only information that is given.

Respuesta :

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 C₆H₁₂.
M(C₆H₁₂) = 6Ā Ā· 12,00 + 12Ā Ā· 1,008 = 84,096.
Add one oxygen atom to molecular formula: Cā‚…Hā‚ˆO.
M(Cā‚…Hā‚ˆO) = 5Ā Ā· 12,00 + 8Ā Ā· 1,008 + 15,995 = 84,059, not.
Add one nitrogen atom to molecular formula: Cā‚…H₁₀N.
M(Cā‚…H₁₀N) = 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 C₆H₁₂.