Molar mass of gypsum (CaSO₄·2H₂O)

The molar mass of gypsum (CaSO₄·2H₂O) is 172.164 g/mol, summed from the IUPAC standard atomic weights of Ca, S, O x 6, H x 4.

Mass of sample

Amount of substance

mol

Molar mass

172.164g/mol

= Exact Molar mass of gypsum

Moles in the sample mass: 0.058 084 152 mol
Mass of that amount of substance: 172.164
Molecules in the sample mass: 3.49791e+22

Show the working

Mass of sample = 10
Amount of substance = 1
Molar mass = 40.078 + 32.06 + 15.999*6 + 1.008*4 = 172.164
Moles in the sample mass = 10 / 172.164 = 0.058 084 2
Mass of that amount of substance = 1 * 172.164 = 172.164
Molecules in the sample mass = 0.058 084 2 * NA = 3.49791e+22

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

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The formula

Molar mass = sum over elements of (atomic weight x number of atoms)
For CaSO₄·2H₂O: 40.078 + 32.06 + 15.999 x 6 + 1.008 x 4 = 172.164 g/mol
Moles = mass / molar mass; mass = moles x molar mass
Molecules = moles x 6.02214076 x 10^23 (the Avogadro constant, exact by definition)

Molar mass in grams per mole is numerically equal to the molecular mass in daltons, so this figure doubles as the molecular weight.

Atomic weights are the IUPAC 2021 standard values. Several elements have a range rather than a single value because isotope ratios vary between sources; the conventional single value is used here.

Composition by mass

Element	Atoms	Atomic weight	Contribution (g/mol)	Share of mass
Oxygen (O)	6	15.999	95.994	55.757 %
Calcium (Ca)	1	40.078	40.078	23.279 %
Sulfur (S)	1	32.06	32.06	18.622 %
Hydrogen (H)	4	1.008	4.032	2.342 %

Contributions add to the molar mass of 172.164 g/mol. Atomic weights are the IUPAC 2021 standard values.

Frequently asked

What is the molar mass of gypsum?

172.164 g/mol, for the formula CaSO₄·2H₂O.

How do you convert grams of gypsum to moles?

Divide the mass in grams by 172.164 g/mol. One gram of gypsum is 0.00580842 mol.

How many grams are in one mole of gypsum?

172.164 g, by definition of the molar mass.

Is molar mass the same as molecular weight?

Numerically yes, for the purposes of this calculation. Molecular weight is a dimensionless ratio and molar mass carries units of g/mol, but the digits are identical.

Where the numbers come from

IUPAC 2021 standard atomic weights; Avogadro constant fixed by the 2019 SI.