The molar mass of sodium thiosulfate pentahydrate, Na2S2O3·5H2O, is 248.17154 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 248.1715 g, and a gram of it is 2.427e+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. Sodium thiosulfate pentahydrate is 2 × 22.9898 (Na) + 2 × 32.06 (S) + 8 × 15.999 (O) + 10 × 1.008 (H), which is 248.17154 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: 8 of the 22 atoms in a formula unit are oxygen, and they account for 127.992 g of the 248.1715 g, or 51.57% 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 sodium thiosulfate pentahydrate is 248.172 mg and one micromole is 248.172 µg, so a balance reading to a milligram places you within 4.029 µmol. A single formula unit weighs 4.1210e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Photographic fixer, chlorine neutraliser and the titrant for iodine. The pentahydrate is the form sold, and it melts in warm weather. At 248.1715 g/mol it is heavier than 27 of the 34 salts covered here, and that ranking matters more than it looks: Epsom salt has a molar mass of 246.466 g/mol, so a gram of it contains 0.692% more formula units than a gram of sodium thiosulfate pentahydrate. Weigh by mass and you are not weighing equal amounts of substance.
Watch which form you have. This page is the hydrate, Na2S2O3·5H2O, at 248.1715 g/mol; the anhydrous salt is Na2S2O3 at 158.0965 g/mol. The 90.075 g of water per mole is 36.3% of the weight, so weighing the hydrate as though it were anhydrous puts you 36.3% 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 sodium thiosulfate pentahydrate in grams
248.17154- The molar mass of sodium thiosulfate pentahydrate 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 sodium thiosulfate pentahydrate in grams.
- It is divided by the molar mass, 248.17154 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 sodium thiosulfate pentahydrate as a molecule count
0.5 g divided by 248.1715 g/mol is 0.00201474 mol, and multiplying by the Avogadro constant gives 1.213e+21 formula units of sodium thiosulfate pentahydrate. The same mass is 2.0147 mmol, which is the figure a reaction is actually planned in.
5 g of sodium thiosulfate pentahydrate as a molecule count
5 g divided by 248.1715 g/mol is 0.0201474 mol, and multiplying by the Avogadro constant gives 1.213e+22 formula units of sodium thiosulfate pentahydrate. The same mass is 20.147 mmol, which is the figure a reaction is actually planned in.
50 g of sodium thiosulfate pentahydrate as a molecule count
50 g divided by 248.1715 g/mol is 0.201474 mol, and multiplying by the Avogadro constant gives 1.213e+23 formula units of sodium thiosulfate pentahydrate. The same mass is 201.47 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 sodium thiosulfate pentahydrate (Na2S2O3·5H2O)?
It is 248.17154 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 22.9898 (Na) + 2 × 32.06 (S) + 8 × 15.999 (O) + 10 × 1.008 (H). One mole of sodium thiosulfate pentahydrate therefore weighs 248.1715 g, and a gram of it is 4.0295 mmol.
What percentage of sodium thiosulfate pentahydrate is oxygen?
51.57% by mass. Each formula unit contains 8 oxygen atoms contributing 127.992 g of the 248.1715 g total, so 100 g of sodium thiosulfate pentahydrate contains 51.57 g of oxygen and a kilogram contains 515.74 g of it.
How many molecules are in one gram of sodium thiosulfate pentahydrate?
About 2.427e+21. One gram is 0.00402947 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 248.1715.
How much does a single molecule of sodium thiosulfate pentahydrate weigh?
4.1210e-22 g, which is 248.17154 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 sodium thiosulfate pentahydrate it is 248.17154. Molar mass carries grams per mole: 248.17154 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 sodium thiosulfate pentahydrate comes out at 248.17154 g/mol rather than 248.
Molar mass, composition and mole equivalents of sodium thiosulfate pentahydrate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 8 | 15.999 | 127.992 | 51.57% |
| Sulfur (S) | 2 | 32.06 | 64.12 | 25.84% |
| Sodium (Na) | 2 | 22.98977 | 45.97954 | 18.53% |
| Hydrogen (H) | 10 | 1.008 | 10.08 | 4.062% |
| Total — one mole of sodium thiosulfate pentahydrate | 248.17154 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 248.17154 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000248172 | 0.248172 | 6.022e+17 |
| 10 µmol | 0.00248172 | 2.48172 | 6.022e+18 |
| 100 µmol | 0.0248172 | 24.8172 | 6.022e+19 |
| 1 mmol | 0.248172 | 248.172 | 6.022e+20 |
| 10 mmol | 2.48172 | 2481.72 | 6.022e+21 |
| 50 mmol | 12.4086 | 12,408.6 | 3.011e+22 |
| 100 mmol | 24.8172 | 24,817.2 | 6.022e+22 |
| 250 mmol | 62.0429 | 62,042.9 | 1.506e+23 |
| 500 mmol | 124.086 | 124,086 | 3.011e+23 |
| 750 mmol | 186.129 | 186,129 | 4.517e+23 |
| 1 mol | 248.172 | 248,172 | 6.022e+23 |
| 2 mol | 496.343 | 496,343 | 1.204e+24 |
| 5 mol | 1240.86 | 1240857.692800 | 3.011e+24 |
| 10 mol | 2481.72 | 2481715.385600 | 6.022e+24 |
Mass is the amount multiplied by 248.17154 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 |
|---|---|---|---|
| Gypsum | CaSO4·2H2O | 172.164 | 5.8084 |
| Potassium sulfate | K2SO4 | 174.2526 | 5.7388 |
| Epsom salt | MgSO4·7H2O | 246.466 | 4.0574 |
| Sodium thiosulfate pentahydrate (this page) | Na2S2O3·5H2O | 248.1715 | 4.0295 |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.677 | 4.0052 |
| Sodium citrate | Na3C6H5O7 | 258.0683 | 3.8749 |
| Iron(II) sulfate heptahydrate | FeSO4·7H2O | 278.006 | 3.597 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.