Question

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

Answer 1

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.

Example

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

Solution

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