Moles to grams for sodium thiosulfate pentahydrate is a multiplication by 248.17154 g/mol. A mole weighs 248.1715 g, 100 mmol weighs 24.8172 g, 10 mmol weighs 2.48172 g and a millimole is 248.172 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of sodium thiosulfate pentahydrate you need, in moles
248.17154- The molar mass of sodium thiosulfate pentahydrate in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 248.17154 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of sodium thiosulfate pentahydrate in grams
5 mmol × 248.1715 g/mol = 1.24086 g, which is 1240.86 mg. Doubling the amount doubles the mass to 2.48172 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of sodium thiosulfate pentahydrate in grams
250 mmol × 248.1715 g/mol = 62.0429 g, which is 62,042.9 mg. Doubling the amount doubles the mass to 124.086 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of sodium thiosulfate pentahydrate in grams
2 mol × 248.1715 g/mol = 496.343 g, which is 496,343 mg. Doubling the amount doubles the mass to 992.686 g, because the relationship is a plain proportion once the molar mass is fixed.
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 grams is one mole of sodium thiosulfate pentahydrate?
248.17154 g, by definition of the molar mass. Two moles is 496.3431 g and half a mole is 124.0858 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of sodium thiosulfate pentahydrate is 24.8172 g. One millimole is 248.172 mg and one micromole is 248.172 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 2.482 mg of sodium thiosulfate pentahydrate, weigh 24.82 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the hydrated form, Na2S2O3·5H2O, at 248.17154 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. A hydrate carries its water into the weighing, which is why the two figures are not interchangeable.
Moles to grams reference for 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000248172 | 0.248172 | 0.000253135 |
| 10 µmol | 0.00248172 | 2.48172 | 0.00253135 |
| 100 µmol | 0.0248172 | 24.8172 | 0.0253135 |
| 1 mmol | 0.248172 | 248.172 | 0.253135 |
| 10 mmol | 2.48172 | 2481.72 | 2.53135 |
| 50 mmol | 12.4086 | 12,408.6 | 12.6567 |
| 100 mmol | 24.8172 | 24,817.2 | 25.3135 |
| 250 mmol | 62.0429 | 62,042.9 | 63.2837 |
| 500 mmol | 124.086 | 124,086 | 126.567 |
| 750 mmol | 186.129 | 186,129 | 189.851 |
| 1 mol | 248.172 | 248,172 | 253.135 |
| 2 mol | 496.343 | 496,343 | 506.27 |
| 5 mol | 1240.86 | 1240857.692800 | 1265.67 |
| 10 mol | 2481.72 | 2481715.385600 | 2531.35 |
Weigh is the amount multiplied by 248.17154 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.