Chemical formulas come in two varieties: molecular and empirical. The molecular formula tells you the exact number of each type of atom in a molecule. The empirical formula gives the simplest whole-number ratio of atoms. For example, glucose has the molecular formula C6H12O6 and the empirical formula CH2O. Both convey different information, and knowing how to find each is an essential skill in chemistry.
Finding the Empirical Formula
The process starts with either mass data or percent composition data. Convert each element's mass to moles by dividing by its atomic weight. Then divide each mole value by the smallest of the mole values to get a ratio. If the ratios are not whole numbers, multiply all of them by the smallest integer that converts them to whole numbers. For example, if a compound is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen: carbon gives 40.0/12.011 = 3.33 mol, hydrogen gives 6.7/1.008 = 6.65 mol, and oxygen gives 53.3/15.999 = 3.33 mol. Dividing by 3.33 gives C1H2O1, or CH2O. The Empirical Formula Calculator handles this entire process for you.
From Empirical to Molecular
To go from empirical formula to molecular formula, you need the molar mass of the compound. The molecular formula is always a whole-number multiple of the empirical formula. If the empirical formula mass is 30.03 g/mol (for CH2O) and the molecular mass is 180.16 g/mol, the multiplier is 180.16 / 30.03 = 6. So the molecular formula is C6H12O6. You can find the molecular mass using our Molar Mass Calculator.
Percent Composition
Percent composition tells you the mass percentage of each element in a compound. To calculate it, find the molar mass of the compound, then divide the total mass of each element by the molar mass and multiply by 100%. For H2SO4 (molar mass 98.08 g/mol): hydrogen is 2.02/98.08 = 2.06%, sulfur is 32.06/98.08 = 32.69%, and oxygen is 64.00/98.08 = 65.25%. Percent composition data is often the starting point for empirical formula calculations. The Percent Composition Calculator performs this analysis for any compound formula you enter.