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Molar mass of calcium sulphate dihydrate (CaSO4·2H2O)

The molar mass of calcium sulphate dihydrate, CaSO4·2H2O, is 172.16 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 172.16 grams.

172.16 g/mol

40.078 + 32.06 + 6 × 15.999 + 4 × 1.0080 = 172.164 · 12 atoms in the formula · also called gypsum

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About calcium sulphate dihydrate

Gypsum is the mineral from which plaster of Paris is made by gentle heating, and it is added to cement to slow its setting.

Other names: gypsum.

The working

The formula CaSO4·2H2O has 1 calcium atom, 1 sulphur atom, 6 oxygen atoms and 4 hydrogen atoms, 12 atoms in all. Multiply each count by the atomic weight of the element and add:

ElementAtomsAtomic weightTotalShare by mass
Calcium (Ca)140.07840.07823.28%
Sulphur (S)132.0632.06018.62%
Oxygen (O)615.99995.99455.76%
Hydrogen (H)41.00804.0322.34%

40.078 + 32.06 + 6 × 15.999 + 4 × 1.0080 = 172.164 g/mol

With the rounded atomic masses used in many school books (Ca = 40, S = 32, O = 16, H = 1) the answer is 172 g/mol. The difference from 172.16 is too small to matter in most school sums.

The water in the crystal

The part after the dot is water of crystallisation: 2 molecules of water for each CaSO4. That water has a mass of 2 × 18.015 = 36.03 g/mol, which is 20.9% of the crystal. Without it, the anhydrous part CaSO4 has a molar mass of 136.13 g/mol. Heating drives the water off, so a sample loses up to 20.9% of its mass. How this is used to find the number of waters is shown on the hydrates page.

Percentage composition

By mass, CaSO4·2H2O is 23.28% calcium, 18.62% sulphur, 55.76% oxygen and 2.34% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 23.3 g of calcium, 18.6 g of sulphur, 55.8 g of oxygen and 2.3 g of hydrogen.

Worked examples

Grams to moles. How many moles are in 40 g of CaSO4·2H2O? Divide the mass by the molar mass: 40 ÷ 172.16 = 0.2323 mol.

Moles to grams. What is the mass of 3 mol of CaSO4·2H2O? Multiply: 3 × 172.16 = 516.5 g.

Counting particles. 40 g contains 0.2323 mol × 6.022 × 1023 = 1.399 × 1023 formula units.

Quick conversion tables

AmountMass
0.1 mol17.22 g
0.5 mol86.08 g
1 mol172.2 g
2 mol344.3 g
5 mol860.8 g
MassMoles
1 g0.005808 mol
10 g0.05808 mol
100 g0.5808 mol
1 kg5.808 mol

For other amounts use the grams and moles converter.

Questions people ask

What is the molar mass of CaSO4.2H2O?

The molar mass of calcium sulphate dihydrate (CaSO4.2H2O) is 172.16 g/mol, or 172.164 g/mol before rounding, using IUPAC abridged atomic weights.

How many moles are in 100 g of CaSO4.2H2O?

100 ÷ 172.16 = 0.5808 mol.

What is the mass of one formula unit of CaSO4.2H2O?

Divide the molar mass by the Avogadro constant: 172.16 ÷ 6.022 × 1023 = 2.859 × 10-22 g.

What is the percentage of oxygen in CaSO4.2H2O?

Oxygen makes up 55.76% of the mass of CaSO4.2H2O: 6 × 15.999 ÷ 172.164 × 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