Quick answer: Glucose, C6H12O6, has a molar mass of 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol, and 53.28% of it is oxygen.
How it's calculated
Molar mass is the sum of the atomic weights of all atoms in the formula: M = Σ (number of atoms × atomic weight). Its unit is g/mol, and numerically it equals the mass of one molecule in daltons (u).
Groups in brackets are multiplied by the number after them: in Ca(OH)2 there are 2 O and 2 H. A dot joins a hydrate: CuSO4·5H2O adds five water molecules. You can type the dot as · or as a full stop.
Atomic weights are the IUPAC/CIAAW standard values (abridged to five significant figures, conventional values for elements such as H, C and Cl whose weight varies slightly in nature). Mass percent of an element = its mass in the formula ÷ the molar mass × 100.
Worked examples
- H2O: 2 × 1.008 + 15.999 = 18.015 g/mol
- Ca(OH)2: 40.078 + 2 × 15.999 + 2 × 1.008 = 74.092 g/mol
- CuSO4·5H2O: 63.546 + 32.06 + 9 × 15.999 + 10 × 1.008 = 249.677 g/mol
Molar masses of common compounds
| Compound | Formula | g/mol |
|---|---|---|
| Water | H2O | 18.015 |
| Carbon dioxide | CO2 | 44.009 |
| Sodium chloride (table salt) | NaCl | 58.44 |
| Glucose | C6H12O6 | 180.156 |
| Sucrose (sugar) | C12H22O11 | 342.297 |
| Sulfuric acid | H2SO4 | 98.072 |
| Sodium hydroxide | NaOH | 39.997 |
| Calcium carbonate | CaCO3 | 100.086 |
| Calcium hydroxide | Ca(OH)2 | 74.092 |
| Methane | CH4 | 16.043 |
| Ethanol | C2H5OH | 46.069 |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.677 |
Frequently asked questions
Is molecular weight the same as molar mass?
Numerically yes. Molar mass is in g/mol; relative molecular mass (molecular weight) is the same number without a unit, and the mass of one molecule in daltons is that number too.
What is the molar mass of water?
18.015 g/mol: two hydrogen atoms (2 × 1.008) and one oxygen (15.999).
Why do I get an error for 'co2'?
Element symbols are case-sensitive. Write CO2: 'co' is not a symbol, and 'Co' would be cobalt.
What is the molar mass of CO₂?
44.01 g/mol. One carbon at 12.011 and two oxygens at 15.999 each.
What is the molar mass of table salt (NaCl)?
58.44 g/mol: sodium at 22.99 plus chlorine at 35.45.
What is mass percent?
How much of the molar mass comes from one element. In water, oxygen makes up 88.8% of the mass and hydrogen the other 11.2%.