Molar MassMolar mass of calcium sulphate (CaSO4)
The molar mass of calcium sulphate, CaSO4, is 136.13 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 136.13 grams.
136.13 g/mol
40.078 + 32.06 + 4 × 15.999 = 136.134 · 6 atoms in the formula · also called anhydrite
About calcium sulphate
Anhydrous calcium sulphate occurs as the mineral anhydrite. With two molecules of water it is gypsum, and with half a molecule it is plaster of Paris.
Other names: anhydrite.
The working
The formula CaSO4 has 1 calcium 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 |
|---|---|---|---|---|
| Calcium (Ca) | 1 | 40.078 | 40.078 | 29.44% |
| Sulphur (S) | 1 | 32.06 | 32.060 | 23.55% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 47.01% |
40.078 + 32.06 + 4 × 15.999 = 136.134 g/mol
With the rounded atomic masses used in many school books (Ca = 40, S = 32, O = 16) the answer is 136 g/mol. The difference from 136.13 is too small to matter in most school sums.
Percentage composition
By mass, CaSO4 is 29.44% calcium, 23.55% sulphur and 47.01% oxygen. The largest share comes from oxygen. In a 100 g sample there are 29.4 g of calcium, 23.6 g of sulphur and 47.0 g of oxygen.
Worked examples
Grams to moles. How many moles are in 25 g of CaSO4? Divide the mass by the molar mass: 25 ÷ 136.13 = 0.1836 mol.
Moles to grams. What is the mass of 0.1 mol of CaSO4? Multiply: 0.1 × 136.13 = 13.61 g.
Counting particles. 25 g contains 0.1836 mol × 6.022 × 1023 = 1.106 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 13.61 g |
| 0.5 mol | 68.07 g |
| 1 mol | 136.1 g |
| 2 mol | 272.3 g |
| 5 mol | 680.7 g |
| Mass | Moles |
|---|---|
| 1 g | 0.007346 mol |
| 10 g | 0.07346 mol |
| 100 g | 0.7346 mol |
| 1 kg | 7.346 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of CaSO4?
The molar mass of calcium sulphate (CaSO4) is 136.13 g/mol, or 136.134 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of CaSO4?
100 ÷ 136.13 = 0.7346 mol.
What is the mass of one formula unit of CaSO4?
Divide the molar mass by the Avogadro constant: 136.13 ÷ 6.022 × 1023 = 2.261 × 10-22 g.
What is the percentage of oxygen in CaSO4?
Oxygen makes up 47.01% of the mass of CaSO4: 4 × 15.999 ÷ 136.134 × 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.