🔬 Molecular Formula Calculator
Find the molecular formula from empirical formula and molar mass.
Molecular Formula
How to Use This Calculator
To find the molecular formula, you need two things: the empirical formula (the simplest atom ratio) and the actual molar mass of the compound in g/mol. The molar mass typically comes from mass spectrometry, freezing point depression, or it is given in the problem. The calculator finds the multiplier n and applies it to each subscript.
Enter the empirical formula in standard notation: capital letters for elements, numbers for subscripts. Examples: CH2O, C2H5, NO2, C3H4O.
Enter the actual molar mass in g/mol. This is the experimentally determined molar mass of the whole molecule, not the empirical unit.
Click Calculate. The tool computes the empirical formula mass, divides it into the molar mass to get n, and multiplies each subscript by n.
Read off the molecular formula. The result panel also shows the empirical unit mass and the multiplier so you can trace the calculation.
The Formula
n must come out to a whole number (1, 2, 3, 4, and so on) because a molecule can only contain whole atoms. If n is not close to an integer, either the molar mass or the empirical formula contains an error. The tool flags this situation with a warning so you can recheck your inputs.
Worked Examples
Where This Calculation Comes Up
The molecular formula problem is the natural second step after finding an empirical formula from combustion analysis. In a typical exam question, you burn a sample of an unknown organic compound, measure the CO₂ and H₂O produced, calculate the empirical formula (say CH₂), then use a separately measured molar mass (say 56 g/mol) to find n = 56/14.03 = 4, giving C₄H₈. Identifying C₄H₈ as cyclobutane or one of the butene isomers then requires additional spectroscopic data, but the molecular formula is the essential first step.
Mass spectrometry gives you the molar mass directly from the molecular ion peak (M⁺ at the highest m/z value in the spectrum). Combined with combustion analysis, it is the classic pair of techniques used to characterise new compounds. In undergraduate labs, you might also measure molar mass by freezing point depression or osmometry, then use this calculator to convert the empirical formula from percent composition data into the complete molecular formula. Both approaches end at the same calculation: n = M/M_emp.
Frequently Asked Questions
How does molecular formula differ from empirical?
The molecular formula shows the actual number of each atom; empirical is the simplest ratio. Glucose C₆H₁₂O₆ (molecular) vs CH₂O (empirical).
How do you find the molecular formula?
n = molar mass / empirical formula mass. Then multiply each subscript in empirical formula by n.
What if n is not a whole number?
Rounding errors can occur. If n is very close to a whole number (within 0.05), it is rounded. A value like 1.5 suggests an error in the empirical formula or molar mass.
Example: empirical CH₂O, molar mass 180 g/mol?
Empirical mass = 12.011+2×1.008+15.999 = 30.026. n = 180/30.026 ≈ 6. Molecular formula: C₆H₁₂O₆.
Can molecular and empirical formulas be the same?
Yes! For compounds like H₂O, NaCl, and CO₂, the simplest ratio IS the molecular formula (n=1).