For sodium thiosulfate pentahydrate (Na2S2O3·5H2O), the calculated molar mass is approximately 248.172 g/mol using CIAAW abridged standard atomic weights.
The atom-count table below shows the derivation. Standard atomic weights describe typical natural isotopic composition; their uncertainty means these are approximate molar masses, not exact constants.
The formula shown defines the compound and hydration state used. Check the reagent label: a hydrate, different chemical form, isotopic enrichment or impure sample needs a different calculation.
The formula
M- 248.172 g/mol for Na2S2O3·5H2O
n- Amount of substance in moles
m- Mass in grams
How to use this calculator
- Confirm the chemical formula and hydration state.
- Enter the requested amount and, where relevant, final solution volume.
- Apply the stated formula. Retain only as many significant digits as your measurements and atomic weights justify.
Worked examples
One mole
One mole of Na2S2O3·5H2O has a calculated mass of about 248.172 g. A mass of 248.172 g divided by 248.172 g/mol corresponds to 1 mol.
Frequently asked questions
Why might another table give a slightly different molar mass?
Atomic-weight tables use different editions or rounding. Natural isotopic composition also varies. Check the chemical formula, hydration state and precision before comparing.
Does this correct for purity or water absorbed during storage?
No. It assumes the stated pure chemical formula. Use the reagent specification and a suitable purity model when the actual composition differs.
Na2S2O3·5H2O: atom-by-atom molar mass
| Element | Atom count | Atomic weight used | Contribution (g/mol) |
|---|---|---|---|
| Na | 2 | 22.99 | 45.98 |
| S | 2 | 32.06 | 64.12 |
| O | 8 | 15.999 | 127.992 |
| H | 10 | 1.008 | 10.08 |