Is it possible to find a molecular formula from molar mass?

1 Answer
Aug 24, 2014

Not without more information.

Mass spectroscopists have computer programs that use the precise masses of isotopes and their natural abundances to calculate all the possible combinations of C, H, O, N, etc. in a molecule and then match them to a given molar mass.

We mere mortals lack such tools. We need more information like an empirical formula to find a molecular formula from the molar mass.


What is the molecular formula of a compound with a molar mass of 180.16 g/mol and an empirical formula of CH₂O?


Step 1. Find the molar mass of the empirical formula.

Molar mass = (12.01 + 2×1.008 + 16.00) g/mol = 30.016 g/mol

Step 2. Find the number of empirical formula units in the molecular formula.

No. of EF units in MF = #"MF molar mass"/"EF molar mass" = (180.16"g/mol")/(30.016"g/mol")# = 6.0021 ≈ 6

Step 3. Find the molecular formula.

Molecular formula = (CH₂O)₆ = C₆H₁₂O₆