The molar mass of gypsum, CaSO4·2H2O, is 172.164 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 172.164 g, and a gram of it is 3.498e+21 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Gypsum is 40.078 (Ca) + 32.06 (S) + 6 × 15.999 (O) + 4 × 1.008 (H), which is 172.164 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 6 of the 12 atoms in a formula unit are oxygen, and they account for 95.994 g of the 172.164 g, or 55.76% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of gypsum is 172.164 mg and one micromole is 172.164 µg, so a balance reading to a milligram places you within 5.808 µmol. A single formula unit weighs 2.8589e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Calcium sulfate dihydrate: plaster, drywall and the brewing salt that hardens water. Heating it to 150 degrees C drives off water and makes plaster of Paris. At 172.164 g/mol it is heavier than 24 of the 34 salts covered here, and that ranking matters more than it looks: silver nitrate has a molar mass of 169.8722 g/mol, so a gram of it contains 1.35% more formula units than a gram of gypsum. Weigh by mass and you are not weighing equal amounts of substance.
Watch which form you have. This page is the hydrate, CaSO4·2H2O, at 172.164 g/mol; the anhydrous salt is CaSO4 at 136.134 g/mol. The 36.03 g of water per mole is 20.93% of the weight, so weighing the hydrate as though it were anhydrous puts you 20.93% short on the compound itself. Suppliers sell both, and the label is the only way to tell them apart by eye.
The formula
m- The mass of gypsum in grams
172.164- The molar mass of gypsum in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of gypsum in grams.
- It is divided by the molar mass, 172.164 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of gypsum as a molecule count
0.5 g divided by 172.164 g/mol is 0.00290421 mol, and multiplying by the Avogadro constant gives 1.749e+21 formula units of gypsum. The same mass is 2.9042 mmol, which is the figure a reaction is actually planned in.
5 g of gypsum as a molecule count
5 g divided by 172.164 g/mol is 0.0290421 mol, and multiplying by the Avogadro constant gives 1.749e+22 formula units of gypsum. The same mass is 29.042 mmol, which is the figure a reaction is actually planned in.
50 g of gypsum as a molecule count
50 g divided by 172.164 g/mol is 0.290421 mol, and multiplying by the Avogadro constant gives 1.749e+23 formula units of gypsum. The same mass is 290.42 mmol, which is the figure a reaction is actually planned in.
How to read your score
Frequently asked questions
What is the molar mass of gypsum (CaSO4·2H2O)?
It is 172.164 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 40.078 (Ca) + 32.06 (S) + 6 × 15.999 (O) + 4 × 1.008 (H). One mole of gypsum therefore weighs 172.164 g, and a gram of it is 5.8084 mmol.
What percentage of gypsum is oxygen?
55.76% by mass. Each formula unit contains 6 oxygen atoms contributing 95.994 g of the 172.164 g total, so 100 g of gypsum contains 55.76 g of oxygen and a kilogram contains 557.57 g of it.
How many molecules are in one gram of gypsum?
About 3.498e+21. One gram is 0.00580842 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 172.164.
How much does a single molecule of gypsum weigh?
2.8589e-22 g, which is 172.164 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for gypsum it is 172.164. Molar mass carries grams per mole: 172.164 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why gypsum comes out at 172.164 g/mol rather than 172.
Molar mass, composition and mole equivalents of gypsum
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 6 | 15.999 | 95.994 | 55.76% |
| Calcium (Ca) | 1 | 40.078 | 40.078 | 23.28% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 18.62% |
| Hydrogen (H) | 4 | 1.008 | 4.032 | 2.342% |
| Total — one mole of gypsum | 172.164 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 172.164 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000172164 | 0.172164 | 6.022e+17 |
| 10 µmol | 0.00172164 | 1.72164 | 6.022e+18 |
| 100 µmol | 0.0172164 | 17.2164 | 6.022e+19 |
| 1 mmol | 0.172164 | 172.164 | 6.022e+20 |
| 10 mmol | 1.72164 | 1721.64 | 6.022e+21 |
| 50 mmol | 8.6082 | 8608.2 | 3.011e+22 |
| 100 mmol | 17.2164 | 17,216.4 | 6.022e+22 |
| 250 mmol | 43.041 | 43,041 | 1.506e+23 |
| 500 mmol | 86.082 | 86,082 | 3.011e+23 |
| 750 mmol | 129.123 | 129,123 | 4.517e+23 |
| 1 mol | 172.164 | 172,164 | 6.022e+23 |
| 2 mol | 344.328 | 344,328 | 1.204e+24 |
| 5 mol | 860.82 | 860,820 | 3.011e+24 |
| 10 mol | 1721.64 | 1721640.000000 | 6.022e+24 |
Mass is the amount multiplied by 172.164 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Trisodium phosphate | Na3PO4 | 163.9391 | 6.0998 |
| Potassium iodide | KI | 166.0028 | 6.024 |
| Silver nitrate | AgNO3 | 169.8722 | 5.8868 |
| Gypsum (this page) | CaSO4·2H2O | 172.164 | 5.8084 |
| Potassium sulfate | K2SO4 | 174.2526 | 5.7388 |
| Epsom salt | MgSO4·7H2O | 246.466 | 4.0574 |
| Sodium thiosulfate pentahydrate | Na2S2O3·5H2O | 248.1715 | 4.0295 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.