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Molar mass of magnesium sulphate heptahydrate (MgSO4·7H2O)

The molar mass of magnesium sulphate heptahydrate, MgSO4·7H2O, is 246.47 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 246.47 grams.

246.47 g/mol

24.305 + 32.06 + 11 × 15.999 + 14 × 1.0080 = 246.466 · 27 atoms in the formula · also called Epsom salt

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About magnesium sulphate heptahydrate

Epsom salt is magnesium sulphate with seven molecules of water. It is used in bath salts and as a magnesium fertiliser.

Other names: Epsom salt.

The working

The formula MgSO4·7H2O has 1 magnesium atom, 1 sulphur atom, 11 oxygen atoms and 14 hydrogen atoms, 27 atoms in all. Multiply each count by the atomic weight of the element and add:

ElementAtomsAtomic weightTotalShare by mass
Magnesium (Mg)124.30524.3059.86%
Sulphur (S)132.0632.06013.01%
Oxygen (O)1115.999175.98971.40%
Hydrogen (H)141.008014.1125.73%

24.305 + 32.06 + 11 × 15.999 + 14 × 1.0080 = 246.466 g/mol

With the rounded atomic masses used in many school books (Mg = 24, S = 32, O = 16, H = 1) the answer is 246 g/mol. The difference from 246.47 is too small to matter in most school sums.

The water in the crystal

The part after the dot is water of crystallisation: 7 molecules of water for each MgSO4. That water has a mass of 7 × 18.015 = 126.11 g/mol, which is 51.2% of the crystal. Without it, the anhydrous part MgSO4 has a molar mass of 120.36 g/mol. Heating drives the water off, so a sample loses up to 51.2% of its mass. How this is used to find the number of waters is shown on the hydrates page.

Percentage composition

By mass, MgSO4·7H2O is 9.86% magnesium, 13.01% sulphur, 71.40% oxygen and 5.73% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 9.9 g of magnesium, 13.0 g of sulphur, 71.4 g of oxygen and 5.7 g of hydrogen.

Notice that hydrogen has more atoms in the formula (14) than oxygen (11), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.

Worked examples

Grams to moles. How many moles are in 250 g of MgSO4·7H2O? Divide the mass by the molar mass: 250 ÷ 246.47 = 1.014 mol.

Moles to grams. What is the mass of 0.2 mol of MgSO4·7H2O? Multiply: 0.2 × 246.47 = 49.29 g.

Counting particles. 250 g contains 1.014 mol × 6.022 × 1023 = 6.108 × 1023 formula units.

Making a solution. A 0.1 mol/L solution contains 0.1 × 246.47 = 24.65 g of MgSO4·7H2O in each litre. For 250 mL, weigh 6.162 g and make it up to the mark with water.

Quick conversion tables

AmountMass
0.1 mol24.65 g
0.5 mol123.2 g
1 mol246.5 g
2 mol492.9 g
5 mol1232 g
MassMoles
1 g0.004057 mol
10 g0.04057 mol
100 g0.4057 mol
1 kg4.057 mol

For other amounts use the grams and moles converter.

Questions people ask

What is the molar mass of MgSO4.7H2O?

The molar mass of magnesium sulphate heptahydrate (MgSO4.7H2O) is 246.47 g/mol, or 246.466 g/mol before rounding, using IUPAC abridged atomic weights.

How many moles are in 100 g of MgSO4.7H2O?

100 ÷ 246.47 = 0.4057 mol.

What is the mass of one formula unit of MgSO4.7H2O?

Divide the molar mass by the Avogadro constant: 246.47 ÷ 6.022 × 1023 = 4.093 × 10-22 g.

What is the percentage of oxygen in MgSO4.7H2O?

Oxygen makes up 71.40% of the mass of MgSO4.7H2O: 11 × 15.999 ÷ 246.466 × 100.

Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.

More hydrates

All hydrates · All compounds