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Molar mass of sodium thiosulphate pentahydrate (Na2S2O3·5H2O)

The molar mass of sodium thiosulphate pentahydrate, Na2S2O3·5H2O, is 248.17 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 248.17 grams.

248.17 g/mol

2 × 22.990 + 2 × 32.06 + 8 × 15.999 + 10 × 1.0080 = 248.172 · 22 atoms in the formula · also called hypo

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About sodium thiosulphate pentahydrate

This is the crystalline form of hypo that is weighed out for iodine titrations, so its molar mass, not that of the anhydrous salt, belongs in those sums.

Other names: hypo.

The working

The formula Na2S2O3·5H2O has 2 sodium atoms, 2 sulphur atoms, 8 oxygen atoms and 10 hydrogen atoms, 22 atoms in all. Multiply each count by the atomic weight of the element and add:

ElementAtomsAtomic weightTotalShare by mass
Sodium (Na)222.99045.98018.53%
Sulphur (S)232.0664.12025.84%
Oxygen (O)815.999127.99251.57%
Hydrogen (H)101.008010.0804.06%

2 × 22.990 + 2 × 32.06 + 8 × 15.999 + 10 × 1.0080 = 248.172 g/mol

With the rounded atomic masses used in many school books (Na = 23, S = 32, O = 16, H = 1) the answer is 248 g/mol. The difference from 248.17 is too small to matter in most school sums.

The water in the crystal

The part after the dot is water of crystallisation: 5 molecules of water for each Na2S2O3. That water has a mass of 5 × 18.015 = 90.08 g/mol, which is 36.3% of the crystal. Without it, the anhydrous part Na2S2O3 has a molar mass of 158.10 g/mol. Heating drives the water off, so a sample loses up to 36.3% of its mass. How this is used to find the number of waters is shown on the hydrates page.

Percentage composition

By mass, Na2S2O3·5H2O is 18.53% sodium, 25.84% sulphur, 51.57% oxygen and 4.06% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 18.5 g of sodium, 25.8 g of sulphur, 51.6 g of oxygen and 4.1 g of hydrogen.

Notice that hydrogen has more atoms in the formula (10) than oxygen (8), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.

Worked examples

Grams to moles. How many moles are in 5 g of Na2S2O3·5H2O? Divide the mass by the molar mass: 5 ÷ 248.17 = 0.02015 mol.

Moles to grams. What is the mass of 0.25 mol of Na2S2O3·5H2O? Multiply: 0.25 × 248.17 = 62.04 g.

Counting particles. 5 g contains 0.02015 mol × 6.022 × 1023 = 1.213 × 1022 formula units.

Making a solution. A 0.1 mol/L solution contains 0.1 × 248.17 = 24.82 g of Na2S2O3·5H2O in each litre. For 250 mL, weigh 6.204 g and make it up to the mark with water.

Quick conversion tables

AmountMass
0.1 mol24.82 g
0.5 mol124.1 g
1 mol248.2 g
2 mol496.3 g
5 mol1241 g
MassMoles
1 g0.004029 mol
10 g0.04029 mol
100 g0.4029 mol
1 kg4.029 mol

For other amounts use the grams and moles converter.

Questions people ask

What is the molar mass of Na2S2O3.5H2O?

The molar mass of sodium thiosulphate pentahydrate (Na2S2O3.5H2O) is 248.17 g/mol, or 248.172 g/mol before rounding, using IUPAC abridged atomic weights.

How many moles are in 100 g of Na2S2O3.5H2O?

100 ÷ 248.17 = 0.4029 mol.

What is the mass of one formula unit of Na2S2O3.5H2O?

Divide the molar mass by the Avogadro constant: 248.17 ÷ 6.022 × 1023 = 4.121 × 10-22 g.

What is the percentage of oxygen in Na2S2O3.5H2O?

Oxygen makes up 51.57% of the mass of Na2S2O3.5H2O: 8 × 15.999 ÷ 248.172 × 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.

More hydrates

All hydrates · All compounds