Molecular Formula Calculator
Scale an empirical formula up to its molecular formula using a measured molar mass.
What is Molecular Formula?
An empirical formula tells you the ratio of atoms; a molecular formula tells you how many there really are in one molecule. The two are related by a single whole number, and that number comes from one measurement: the compound's molar mass, usually found by mass spectrometry or freezing-point depression.
How it works
Divide the measured molar mass by the empirical formula's mass to get n, then multiply every subscript by n. If the division does not land near a whole number the tool says so — a ratio of 2.37 means either the empirical formula or the molar mass is wrong, and quietly rounding to 2 would hide that.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
CH2O has a molar mass of 180.16 g/mol. What is the molecular formula?
C6H12O6. The empirical mass of CH2O is 30.026 g/mol, and 180.16 ÷ 30.026 = 6 — load the example to see the multiplier alongside the formula.
What if my ratio is not a whole number?
The tool reports the ratio rather than rounding it, because a non-integer result means the input is wrong: usually a molar mass from a different compound, or a mass spectrum peak misread as the molecular ion.