To prepare a solution of sodium thiosulfate pentahydrate you need the grams, and grams is molarity × volume in litres × 248.17154 g/mol. A litre of 1 M takes 248.1715 g; 500 mL of 0.1 M takes 12.409 g; 250 mL of 0.01 M takes 620.43 mg. Enter the strength you want and the volume you want it in, and the calculator returns the mass to weigh together with the same figure in milligrams and as a percentage strength.
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.
Order of operations matters more than precision here. Dissolve the solid in rather less than the final volume, then make up to the mark once it has all gone into solution and come back to room temperature — warm solutions read low when they cool. If the calculated mass is under about 20 mg, prepare ten times the strength and dilute it tenfold by pipette instead, because C₁V₁ = C₂V₂ carries less error than a marginal 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
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
248.17154- The molar mass of sodium thiosulfate pentahydrate in g/mol, derived from its formula
m- The mass of sodium thiosulfate pentahydrate to weigh out, in grams
How it works, step by step
- Enter the molarity you want, in mol/L.
- Enter the volume you want to make, in millilitres.
- The two are multiplied together and by 248.17154 g/mol.
- The result is the mass to weigh; dissolve it in part of the volume and then make up to the mark.
Worked examples
250 mL of 0.1 M sodium thiosulfate pentahydrate
Weigh 6.20429 g. That is 0.1 M × 0.25 L × 248.1715 g/mol, and it comes to 6204.29 mg if your balance is set to milligrams. Dissolve in about 175 mL first, then make up to 250 mL.
500 mL of 0.5 M sodium thiosulfate pentahydrate
Weigh 62.0429 g. That is 0.5 M × 0.5 L × 248.1715 g/mol, and it comes to 62,042.9 mg if your balance is set to milligrams. Dissolve in about 350 mL first, then make up to 500 mL.
1000 mL of 1 M sodium thiosulfate pentahydrate
Weigh 248.172 g. That is 1 M × 1 L × 248.1715 g/mol, and it comes to 248,172 mg if your balance is set to milligrams. Dissolve in about 700 mL first, then make up to 1000 mL.
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 much sodium thiosulfate pentahydrate do I need for 1 litre of 1 M solution?
248.17154 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 24.8172 g, and for 1 L of 0.01 M it is 2.48172 g.
How much for 100 mL of 0.1 M?
2.48172 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 248.1715 g. In milligrams that is 2481.72 mg.
Do I dissolve the solid first or make up the volume first?
Dissolve first, in perhaps 70% of the final volume, then make up to the mark. Adding solid to a full flask overshoots the volume and leaves the solution weak, and undissolved solid at the mark means the strength keeps changing as it goes in.
Can I dilute a stock solution instead of weighing?
Yes, and it is more accurate for small amounts. C₁V₁ = C₂V₂: to get 500 mL of 0.01 M from a 0.5 M stock, take 10 mL of stock and make up to 500 mL. Weighing the 1240.9 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes 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.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 24.8172 mg | 62.0429 mg | 124.086 mg | 248.172 mg |
| 0.005 M | 124.086 mg | 310.214 mg | 620.429 mg | 1.24086 g |
| 0.01 M | 248.172 mg | 620.429 mg | 1.24086 g | 2.48172 g |
| 0.05 M | 1.24086 g | 3.10214 g | 6.20429 g | 12.4086 g |
| 0.1 M | 2.48172 g | 6.20429 g | 12.4086 g | 24.8172 g |
| 0.15 M | 3.72257 g | 9.30643 g | 18.6129 g | 37.2257 g |
| 0.2 M | 4.96343 g | 12.4086 g | 24.8172 g | 49.6343 g |
| 0.25 M | 6.20429 g | 15.5107 g | 31.0214 g | 62.0429 g |
| 0.5 M | 12.4086 g | 31.0214 g | 62.0429 g | 124.086 g |
| 1 M | 24.8172 g | 62.0429 g | 124.086 g | 248.172 g |
| 2 M | 49.6343 g | 124.086 g | 248.172 g | 496.343 g |
Each cell is molarity × volume in litres × 248.17154 g/mol. Dissolve the solid first and then make up to the marked volume — adding solid to a full volume overshoots it.
| 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.