How to calculate it
Multiply each element's atomic mass by how many atoms there are, then add: 24 + 32 + 11 × 16 + 14 × 1 = 246 g/mol.
| Element | Atoms | Mass (u) | Total | % by mass |
|---|---|---|---|---|
| Mg | 1 | 24 | 24 | 9.86% |
| S | 1 | 32 | 32 | 13.01% |
| O | 11 | 16 | 176 | 71.40% |
| H | 14 | 1 | 14 | 5.73% |
The water of crystallisation counts: 7 × 18 = 126 g/mol of the 246 is water, which is 51.2% by mass. The anhydrous MgSO4 alone is 120 g/mol.
Useful conversions
- 1 mol of MgSO4·7H2O weighs 246 g and contains 6.022 × 10²³ formula units.
- 1 g of MgSO4·7H2O is 0.004057 mol, or 2.443 × 1021 formula units.
- One formula unit weighs 4.093 × 10−22 g.
Equivalent weight
Equivalent weight = molar mass ÷ n-factor. For magnesium sulphate heptahydrate (Epsom salt), the n-factor is 2: total positive charge 2. So the equivalent weight is 246 ÷ 2 = 123 g/eq. Use it for normality: N = M × 2.
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 |
Questions students ask
What is the molar mass of MgSO₄·7H₂O?
246 g/mol using exam atomic masses (24 + 32 + 11 × 16 + 14 × 1), or 246.47 g/mol with exact IUPAC values.
How many moles are in 1 g of MgSO₄·7H₂O?
1 ÷ 246 = 0.004057 mol.
What is the mass of one molecule of MgSO₄·7H₂O?
246.47 ÷ 6.022 × 10²³ = 4.093 × 10⁻²² g.
What is the percentage of O in MgSO₄·7H₂O?
71.40% by mass.
What is the equivalent weight of MgSO₄·7H₂O?
123 g/eq: molar mass 246 ÷ n-factor 2. Basis for the n-factor: total positive charge 2.