Molar MassMolar mass of sodium phosphate (Na3PO4)
The molar mass of sodium phosphate, Na3PO4, is 163.94 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 163.94 grams.
163.94 g/mol
3 × 22.990 + 30.974 + 4 × 15.999 = 163.940 · 8 atoms in the formula · also called trisodium phosphate
About sodium phosphate
Trisodium phosphate is a strongly alkaline salt used as a cleaning agent and water softener. It is the salt formed when all three acidic hydrogens of phosphoric acid are replaced by sodium.
Other names: trisodium phosphate.
The working
The formula Na3PO4 has 3 sodium atoms, 1 phosphorus atom and 4 oxygen atoms, 8 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) | 3 | 22.990 | 68.970 | 42.07% |
| Phosphorus (P) | 1 | 30.974 | 30.974 | 18.89% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 39.04% |
3 × 22.990 + 30.974 + 4 × 15.999 = 163.940 g/mol
With the rounded atomic masses used in many school books (Na = 23, P = 31, O = 16) the answer is 164 g/mol. The difference from 163.94 is too small to matter in most school sums.
Percentage composition
By mass, Na3PO4 is 42.07% sodium, 18.89% phosphorus and 39.04% oxygen. The largest share comes from sodium. In a 100 g sample there are 42.1 g of sodium, 18.9 g of phosphorus and 39.0 g of oxygen.
Worked examples
Grams to moles. How many moles are in 5 g of Na3PO4? Divide the mass by the molar mass: 5 ÷ 163.94 = 0.0305 mol.
Moles to grams. What is the mass of 0.25 mol of Na3PO4? Multiply: 0.25 × 163.94 = 40.98 g.
Counting particles. 5 g contains 0.0305 mol × 6.022 × 1023 = 1.837 × 1022 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 163.94 = 16.39 g of Na3PO4 in each litre. For 250 mL, weigh 4.099 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 16.39 g |
| 0.5 mol | 81.97 g |
| 1 mol | 163.9 g |
| 2 mol | 327.9 g |
| 5 mol | 819.7 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0061 mol |
| 10 g | 0.061 mol |
| 100 g | 0.61 mol |
| 1 kg | 6.1 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Na3PO4?
The molar mass of sodium phosphate (Na3PO4) is 163.94 g/mol, or 163.940 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Na3PO4?
100 ÷ 163.94 = 0.61 mol.
What is the mass of one formula unit of Na3PO4?
Divide the molar mass by the Avogadro constant: 163.94 ÷ 6.022 × 1023 = 2.722 × 10-22 g.
What is the percentage of sodium in Na3PO4?
Sodium makes up 42.07% of the mass of Na3PO4: 3 × 22.990 ÷ 163.940 × 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.