Molecular Formula Calculator
Determine the molecular formula from an empirical formula and molar mass.
Molecular formula
C6H12O6
Empirical formula
CH2O
Empirical mass
30.026g/mol
Multiplier (n)
×6
Calculated mass
180.156g/mol
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What it does
The molecular formula calculator finds the actual molecular formula of a compound from its empirical formula and its experimentally determined molar mass. It is used by students, analytical chemists, and researchers who have mass spectrometry or elemental analysis data and need to convert an empirical formula into the true molecular formula.
How it works
The molecular formula is a whole-number multiple of the empirical formula. First compute the empirical formula mass by summing the atomic masses of the atoms in the empirical formula. Then the multiplier n equals the measured molar mass divided by the empirical formula mass: n = M / empirical mass. The molecular formula is the empirical formula multiplied by n, where n must be a near-integer (1, 2, 3, ...). For example, for CH2O the empirical mass is about 30.03 g/mol, and if the measured molar mass is 180.16 g/mol then n = 180.16 / 30.03 = 6, giving C6H12O6.
Worked example
An unknown compound has the empirical formula CH2O and a measured molar mass of 180.16 g/mol. The empirical formula mass is 12.01 + 2(1.008) + 16.00 = 30.03 g/mol. The multiplier is n = 180.16 / 30.03 = 5.998, which rounds to 6. Multiplying each subscript by 6 gives the molecular formula C6H12O6, which is glucose. The calculated mass of C6H12O6 is 180.16 g/mol, matching the measurement.
When to use it
Use it after combustion analysis when you have an empirical formula and need the molecular formula of an unknown compound. Use it to confirm the identity of a synthesized molecule against the molar mass from mass spectrometry. Use it in teaching labs to show students the difference between empirical and molecular formulas.
FAQ
- How do I find the molecular formula from the empirical formula and molar mass?
- Calculate the empirical formula mass, divide the measured molar mass by it to get the multiplier n, then multiply every subscript in the empirical formula by n. The result is the molecular formula.
- What is the difference between an empirical formula and a molecular formula?
- The empirical formula gives the simplest whole-number atom ratio, while the molecular formula gives the actual number of each atom in a molecule. The molecular formula is always a whole-number multiple of the empirical formula.
- Why must the multiplier n be a whole number?
- A molecule contains whole atoms, so the ratio between its true molar mass and the empirical formula mass must be an integer. Small differences from rounding atomic masses are normal, so n is taken as the nearest whole number.
- Can one empirical formula correspond to several molecular formulas?
- Yes. For example CH2O is the empirical formula of formaldehyde (CH2O), acetic acid (C2H4O2), glucose (C6H12O6), and many sugars. Only the molar mass distinguishes them.