Molecular Formula from Empirical Formula
Determine a molecular-formula candidate from an empirical formula and measured molar mass, with an integer multiplier and explicit mass-fit tolerance.
Use an empirical atom ratio and a measured molar mass to calculate a compatible molecular-formula candidate. The calculator tests the nearest positive integer multiple and reports its mass discrepancy, so an inconsistent measurement is not silently turned into a plausible-looking formula.
How to use this tool
- Enter the empirical formula in its lowest whole-number ratio.
- Enter the measured molar mass in g/mol.
- Set the acceptable relative mass difference, from zero to five percent.
- Read the integer multiplier, candidate formula and mass-fit result.
Formula
n ≈ measured molar mass / empirical formula mass; molecular subscripts = n × empirical subscripts
n must be a positive whole number. The displayed discrepancy is 100 × |predicted mass − measured mass| / measured mass. A candidate is returned only when that discrepancy is within your chosen tolerance.
Example
CH2O has empirical formula mass 12.011 + 2 × 1.008 + 15.999 = 30.026 g/mol. A measured mass of 180.156 g/mol gives n = 6. Multiplying every subscript by six gives C6H12O6, with a predicted mass of 180.156 g/mol.
Frequently asked questions
Why must the input be an empirical formula?
It defines the smallest atom ratio. H2O2 is rejected because both counts share a factor of two; enter HO instead. This avoids hiding a reduction step inside the integer-multiple calculation.
What if the mass ratio is not an integer?
A small difference may reflect measurement or rounding. A difference outside the selected tolerance produces an error. Recheck the analytical data instead of increasing tolerance merely to force a result.
Does the candidate identify a compound?
No. Isomers can share the same molecular formula. The calculation does not establish bonding, chemical stability or even whether a discrete molecule exists; independent experimental evidence remains necessary.
What limits apply?
The reference uses CIAAW 2024 nominal atomic weights. Hydrates, isotope labels and charges are unsupported here. Multipliers are capped at 10,000 and expanded formulas at one million atoms. Acceptance tolerance is a user-selected fit criterion, not propagated measurement uncertainty.
Sources
Last updated: 2026-10-30