Going from grams to moles for sodium thiosulfate pentahydrate means dividing by its molar mass, 248.17154 g/mol. One gram is 0.00402947 mol, or 4.02947 mmol; 100 mg is 0.40295 mmol; and a 248.1715 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, Na2S2O3·5H2O, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.04029 mmol of sodium thiosulfate pentahydrate; on a three-decimal balance it is 1 mg, or 4.029 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
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 you weighed, in grams
248.17154- The molar mass of sodium thiosulfate pentahydrate in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 248.17154 g/mol, the molar mass of sodium thiosulfate pentahydrate.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of sodium thiosulfate pentahydrate in moles
0.25 ÷ 248.1715 = 0.00100737 mol, or 1.0074 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 4.029 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of sodium thiosulfate pentahydrate in moles
2 ÷ 248.1715 = 0.00805894 mol, or 8.0589 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 4.029 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of sodium thiosulfate pentahydrate in moles
25 ÷ 248.1715 = 0.100737 mol, or 100.74 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 4.029 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
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 do I convert grams of sodium thiosulfate pentahydrate to moles?
Divide the mass in grams by 248.17154 g/mol. So 1 g is 0.00402947 mol, 10 g is 0.0402947 mol and 100 g is 0.402947 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of sodium thiosulfate pentahydrate?
0.402947 mol, which is 402.947 mmol. Read the other way, one mole of sodium thiosulfate pentahydrate is 248.1715 g, so 100 g is 0.4029 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of sodium thiosulfate pentahydrate is 4.0295 µmol, 10 mg is 40.295 µmol and 100 mg is 0.40295 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of sodium thiosulfate pentahydrate, which is 0.00394888 mol rather than 0.00402947 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000004 | 0.00402947 | 4.02947 |
| 5 mg | 0.000020 | 0.0201474 | 20.1474 |
| 10 mg | 0.000040 | 0.0402947 | 40.2947 |
| 25 mg | 0.000100737 | 0.100737 | 100.737 |
| 50 mg | 0.000201474 | 0.201474 | 201.474 |
| 100 mg | 0.000402947 | 0.402947 | 402.947 |
| 250 mg | 0.00100737 | 1.00737 | 1007.37 |
| 500 mg | 0.00201474 | 2.01474 | 2014.74 |
| 1 g | 0.00402947 | 4.02947 | 4029.47 |
| 2 g | 0.00805894 | 8.05894 | 8058.94 |
| 5 g | 0.0201474 | 20.1474 | 20,147.4 |
| 10 g | 0.0402947 | 40.2947 | 40,294.7 |
| 25 g | 0.100737 | 100.737 | 100,737 |
| 50 g | 0.201474 | 201.474 | 201,474 |
| 100 g | 0.402947 | 402.947 | 402,947 |
| 250 g | 1.00737 | 1007.37 | 1007367.732217 |
| 500 g | 2.01474 | 2014.74 | 2014735.464434 |
Every row is the mass divided by 248.17154 g/mol. A balance reading to 1 mg resolves 4.029 µmol of this compound, which is the smallest step the middle columns can really move in.
| 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.