How to find molar mass by hand
- Count the atoms of each element. In Ca(OH)2, the 2 applies to everything in the bracket: 1 Ca, 2 O and 2 H.
- Multiply by the atomic mass. Ca 40 × 1 = 40, O 16 × 2 = 32, H 1 × 2 = 2.
- Add them up. 40 + 32 + 2 = 74 g/mol.
For a hydrate like CuSO4·5H2O, add the water: 159.5 + 5 × 18 = 249.5 g/mol.
Which masses to use? JEE, NEET and board papers use rounded values (H 1, C 12, N 14, O 16, Na 23, S 32, Cl 35.5, Cu 63.5) unless they say otherwise. That's the default here. Switch to exact values for lab work.
Molar masses of common compounds
| Formula | Name | g/mol (exam) |
|---|---|---|
| H2O | water | 18 |
| NaOH | sodium hydroxide | 40 |
| NaCl | sodium chloride | 58.5 |
| H2SO4 | sulphuric acid | 98 |
| HCl | hydrochloric acid | 36.5 |
| HNO3 | nitric acid | 63 |
| KMnO4 | potassium permanganate | 158 |
| K2Cr2O7 | potassium dichromate | 294 |
| CaCO3 | calcium carbonate | 100 |
| NaHCO3 | sodium bicarbonate (baking soda) | 84 |
| Na2CO3 | sodium carbonate | 106 |
| C6H12O6 | glucose | 180 |
| C12H22O11 | sucrose | 342 |
| CO2 | carbon dioxide | 44 |
| NH3 | ammonia | 17 |
| CH4 | methane | 16 |
| C2H5OH | ethanol | 46 |
| CH3COOH | acetic acid | 60 |
| KOH | potassium hydroxide | 56 |
| Ca(OH)2 | calcium hydroxide | 74 |
| Mg(OH)2 | magnesium hydroxide | 58 |
| CuSO4 | copper sulphate (anhydrous) | 159.5 |
| CuSO4·5H2O | copper sulphate pentahydrate (blue vitriol) | 249.5 |
| FeSO4·7H2O | ferrous sulphate heptahydrate (green vitriol) | 278 |
| FeSO4·(NH4)2SO4·6H2O | Mohr's salt | 392 |
| H2C2O4·2H2O | oxalic acid dihydrate | 126 |
| Na2S2O3·5H2O | sodium thiosulphate (hypo) | 248 |
| AgNO3 | silver nitrate | 170 |
| BaCl2 | barium chloride | 208 |
| KCl | potassium chloride | 74.5 |
| Al2O3 | aluminium oxide | 102 |
| Fe2O3 | iron(III) oxide | 160 |
| H3PO4 | phosphoric acid | 98 |
| NH4Cl | ammonium chloride | 53.5 |
| (NH4)2SO4 | ammonium sulphate | 132 |
| KI | potassium iodide | 166 |
| Na2SO4 | sodium sulphate | 142 |
| MgSO4·7H2O | magnesium sulphate heptahydrate (Epsom salt) | 246 |
| C6H6 | benzene | 78 |
| O2 | oxygen gas | 32 |
| N2 | nitrogen gas | 28 |
| Cl2 | chlorine gas | 71 |
| H2O2 | hydrogen peroxide | 34 |
| CO(NH2)2 | urea | 60 |
| Al2(SO4)3 | aluminium sulphate | 342 |
| CaCl2 | calcium chloride | 111 |
| NaNO3 | sodium nitrate | 85 |
| KClO3 | potassium chlorate | 122.5 |
Look up any element on the periodic table, or balance the reaction first with the equation balancer.
Questions students ask
What is the molar mass of H2SO4?
98 g/mol, from 2 × 1 + 32 + 4 × 16. With exact atomic masses it's 98.08 g/mol.
What is the difference between molar mass and molecular mass?
They have the same number. Molecular mass is the mass of one molecule in u (atomic mass units); molar mass is the mass of one mole in g/mol.
How do I convert grams to moles?
Divide the mass by the molar mass. 49 g of H2SO4 is 49 ÷ 98 = 0.5 mol, which is 0.5 × 6.022 × 10²³ = 3.011 × 10²³ molecules.
Why is chlorine 35.5 and not 35?
Natural chlorine is a mix of Cl-35 (about 75%) and Cl-37 (about 25%), so its average atomic mass is about 35.5.