Molar Mass Calculator (Formula Weight and Mass Percent)
Count the atoms of each element in one formula unit, for example C6H12O6 is 6 carbon, 12 hydrogen and 6 oxygen. Elements you do not need stay at 0.
Updated
Report a problem
Formula
molar mass = Σ (atom count × atomic weight) and mass percent of an element = (count × atomic weight) ÷ molar mass × 100.
Atomic weights used (g/mol, IUPAC conventional values): C 12.011, H 1.008, N 14.007, O 15.999, S 32.06, P 30.973762, Cl 35.45, Na 22.98976928, K 39.0983, Ca 40.078, Fe 55.845.
Worked example
Glucose, C6H12O6:
- Carbon: 6 × 12.011 = 72.066
- Hydrogen: 12 × 1.008 = 12.096
- Oxygen: 6 × 15.999 = 95.994
- Molar mass: 72.066 + 12.096 + 95.994 = 180.156 g/mol
- Mass percent: C 40.00 %, H 6.71 %, O 53.28 %
Frequently asked questions
How do you calculate molar mass?
Multiply each element's atomic weight by its atom count and add the results. Glucose C6H12O6: 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol.
Which atomic weights does this calculator use?
IUPAC conventional standard atomic weights: C 12.011, H 1.008, N 14.007, O 15.999, S 32.06, P 30.973762, Cl 35.45, Na 22.98976928, K 39.0983, Ca 40.078 and Fe 55.845 g/mol. Small differences from other tables come from rounding.
How do I find the mass percent of an element in a compound?
Divide the element's total mass in the formula by the molar mass and multiply by 100. For water, hydrogen is 2 × 1.008 = 2.016 out of 18.015, which is 11.19 %, and oxygen is 88.81 %.
How do I handle brackets such as Ca(OH)2?
Multiply the atoms inside the brackets by the number outside, then add them up. Ca(OH)2 has 1 Ca, 2 O and 2 H, so it gives 40.078 + 2 × 15.999 + 2 × 1.008 = 74.092 g/mol. Only the eleven listed elements are supported.