Molar MassMolar mass of magnesium sulphate (MgSO4)
The molar mass of magnesium sulphate, MgSO4, is 120.36 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 120.36 grams.
120.36 g/mol
24.305 + 32.06 + 4 × 15.999 = 120.361 · 6 atoms in the formula · also called anhydrous magnesium sulfate
About magnesium sulphate
Anhydrous magnesium sulphate is a white powder used as a drying agent for organic liquids. With seven molecules of water it is Epsom salt, which has a much higher molar mass.
Other names: anhydrous magnesium sulfate.
The working
The formula MgSO4 has 1 magnesium atom, 1 sulphur atom and 4 oxygen atoms, 6 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 20.19% |
| Sulphur (S) | 1 | 32.06 | 32.060 | 26.64% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 53.17% |
24.305 + 32.06 + 4 × 15.999 = 120.361 g/mol
With the rounded atomic masses used in many school books (Mg = 24, S = 32, O = 16) the answer is 120 g/mol. The difference from 120.36 is too small to matter in most school sums.
Percentage composition
By mass, MgSO4 is 20.19% magnesium, 26.64% sulphur and 53.17% oxygen. The largest share comes from oxygen. In a 100 g sample there are 20.2 g of magnesium, 26.6 g of sulphur and 53.2 g of oxygen.
Worked examples
Grams to moles. How many moles are in 25 g of MgSO4? Divide the mass by the molar mass: 25 ÷ 120.36 = 0.2077 mol.
Moles to grams. What is the mass of 0.1 mol of MgSO4? Multiply: 0.1 × 120.36 = 12.04 g.
Counting particles. 25 g contains 0.2077 mol × 6.022 × 1023 = 1.251 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 120.36 = 12.04 g of MgSO4 in each litre. For 250 mL, weigh 3.009 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 12.04 g |
| 0.5 mol | 60.18 g |
| 1 mol | 120.4 g |
| 2 mol | 240.7 g |
| 5 mol | 601.8 g |
| Mass | Moles |
|---|---|
| 1 g | 0.008308 mol |
| 10 g | 0.08308 mol |
| 100 g | 0.8308 mol |
| 1 kg | 8.308 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of MgSO4?
The molar mass of magnesium sulphate (MgSO4) is 120.36 g/mol, or 120.361 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of MgSO4?
100 ÷ 120.36 = 0.8308 mol.
What is the mass of one formula unit of MgSO4?
Divide the molar mass by the Avogadro constant: 120.36 ÷ 6.022 × 1023 = 1.999 × 10-22 g.
What is the percentage of oxygen in MgSO4?
Oxygen makes up 53.17% of the mass of MgSO4: 4 × 15.999 ÷ 120.361 × 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.