Molar MassMolar mass of sodium thiosulphate (Na2S2O3)
The molar mass of sodium thiosulphate, Na2S2O3, is 158.10 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 158.10 grams.
158.10 g/mol
2 × 22.990 + 2 × 32.06 + 3 × 15.999 = 158.097 · 7 atoms in the formula · also called sodium thiosulfate, hypo
About sodium thiosulphate
Sodium thiosulphate is the hypo of photography and the standard reagent for titrating iodine. The crystals in the bottle are the pentahydrate, so a titration sum normally needs that larger figure.
Other names: sodium thiosulfate, hypo.
The working
The formula Na2S2O3 has 2 sodium atoms, 2 sulphur atoms and 3 oxygen atoms, 7 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Sodium (Na) | 2 | 22.990 | 45.980 | 29.08% |
| Sulphur (S) | 2 | 32.06 | 64.120 | 40.56% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 30.36% |
2 × 22.990 + 2 × 32.06 + 3 × 15.999 = 158.097 g/mol
With the rounded atomic masses used in many school books (Na = 23, S = 32, O = 16) the answer is 158 g/mol. The difference from 158.10 is too small to matter in most school sums.
Percentage composition
By mass, Na2S2O3 is 29.08% sodium, 40.56% sulphur and 30.36% oxygen. The largest share comes from sulphur. In a 100 g sample there are 29.1 g of sodium, 40.6 g of sulphur and 30.4 g of oxygen.
Worked examples
Grams to moles. How many moles are in 20 g of Na2S2O3? Divide the mass by the molar mass: 20 ÷ 158.10 = 0.1265 mol.
Moles to grams. What is the mass of 0.5 mol of Na2S2O3? Multiply: 0.5 × 158.10 = 79.05 g.
Counting particles. 20 g contains 0.1265 mol × 6.022 × 1023 = 7.618 × 1022 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 158.10 = 15.81 g of Na2S2O3 in each litre. For 250 mL, weigh 3.952 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 15.81 g |
| 0.5 mol | 79.05 g |
| 1 mol | 158.1 g |
| 2 mol | 316.2 g |
| 5 mol | 790.5 g |
| Mass | Moles |
|---|---|
| 1 g | 0.006325 mol |
| 10 g | 0.06325 mol |
| 100 g | 0.6325 mol |
| 1 kg | 6.325 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Na2S2O3?
The molar mass of sodium thiosulphate (Na2S2O3) is 158.10 g/mol, or 158.097 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Na2S2O3?
100 ÷ 158.10 = 0.6325 mol.
What is the mass of one formula unit of Na2S2O3?
Divide the molar mass by the Avogadro constant: 158.10 ÷ 6.022 × 1023 = 2.625 × 10-22 g.
What is the percentage of sulphur in Na2S2O3?
Sulphur makes up 40.56% of the mass of Na2S2O3: 2 × 32.06 ÷ 158.097 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.