The molar mass of trisodium phosphate, Na3PO4, is 163.93907 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 163.9391 g, and a gram of it is 3.673e+21 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Trisodium phosphate is 3 × 22.9898 (Na) + 30.9738 (P) + 4 × 15.999 (O), which is 163.93907 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Sodium makes up the largest share of the mass: 3 of the 8 atoms in a formula unit are sodium, and they account for 68.9693 g of the 163.9391 g, or 42.07% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of trisodium phosphate is 163.939 mg and one micromole is 163.939 µg, so a balance reading to a milligram places you within 6.1 µmol. A single formula unit weighs 2.7223e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
A heavy-duty cleaner and degreaser, and a food additive. Its solutions are strongly alkaline — around pH 12 at 1% — which is what does the cleaning. At 163.9391 g/mol it is heavier than 21 of the 34 salts covered here, and that ranking matters more than it looks: iron(III) chloride has a molar mass of 162.195 g/mol, so a gram of it contains 1.08% more formula units than a gram of trisodium phosphate. Weigh by mass and you are not weighing equal amounts of substance.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 3.2788 g of a nominal 163.9391 g is something else, so for exact work you divide the weighed mass by the assay figure on the certificate. For most purposes the difference is smaller than the error in reading the meniscus, but it is systematic rather than random, so it does not average out.
The formula
m- The mass of trisodium phosphate in grams
163.93907- The molar mass of trisodium phosphate in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of trisodium phosphate in grams.
- It is divided by the molar mass, 163.93907 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of trisodium phosphate as a molecule count
0.5 g divided by 163.9391 g/mol is 0.00304991 mol, and multiplying by the Avogadro constant gives 1.837e+21 formula units of trisodium phosphate. The same mass is 3.0499 mmol, which is the figure a reaction is actually planned in.
5 g of trisodium phosphate as a molecule count
5 g divided by 163.9391 g/mol is 0.0304991 mol, and multiplying by the Avogadro constant gives 1.837e+22 formula units of trisodium phosphate. The same mass is 30.499 mmol, which is the figure a reaction is actually planned in.
50 g of trisodium phosphate as a molecule count
50 g divided by 163.9391 g/mol is 0.304991 mol, and multiplying by the Avogadro constant gives 1.837e+23 formula units of trisodium phosphate. The same mass is 304.99 mmol, which is the figure a reaction is actually planned in.
How to read your score
Frequently asked questions
What is the molar mass of trisodium phosphate (Na3PO4)?
It is 163.93907 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 3 × 22.9898 (Na) + 30.9738 (P) + 4 × 15.999 (O). One mole of trisodium phosphate therefore weighs 163.9391 g, and a gram of it is 6.0998 mmol.
What percentage of trisodium phosphate is sodium?
42.07% by mass. Each formula unit contains 3 sodium atoms contributing 68.9693 g of the 163.9391 g total, so 100 g of trisodium phosphate contains 42.07 g of sodium and a kilogram contains 420.7 g of it.
How many molecules are in one gram of trisodium phosphate?
About 3.673e+21. One gram is 0.00609983 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 163.9391.
How much does a single molecule of trisodium phosphate weigh?
2.7223e-22 g, which is 163.93907 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for trisodium phosphate it is 163.93907. Molar mass carries grams per mole: 163.93907 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Sodium is quoted as 22.98977 rather than a whole number for that reason, and the same applies to the other elements here, which is why trisodium phosphate comes out at 163.93907 g/mol rather than 164.
Molar mass, composition and mole equivalents of trisodium phosphate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Sodium (Na) | 3 | 22.98977 | 68.96931 | 42.07% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 39.04% |
| Phosphorus (P) | 1 | 30.97376 | 30.97376 | 18.89% |
| Total — one mole of trisodium phosphate | 163.93907 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 163.93907 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000163939 | 0.163939 | 6.022e+17 |
| 10 µmol | 0.00163939 | 1.63939 | 6.022e+18 |
| 100 µmol | 0.0163939 | 16.3939 | 6.022e+19 |
| 1 mmol | 0.163939 | 163.939 | 6.022e+20 |
| 10 mmol | 1.63939 | 1639.39 | 6.022e+21 |
| 50 mmol | 8.19695 | 8196.95 | 3.011e+22 |
| 100 mmol | 16.3939 | 16,393.9 | 6.022e+22 |
| 250 mmol | 40.9848 | 40,984.8 | 1.506e+23 |
| 500 mmol | 81.9695 | 81,969.5 | 3.011e+23 |
| 750 mmol | 122.954 | 122,954 | 4.517e+23 |
| 1 mol | 163.939 | 163,939 | 6.022e+23 |
| 2 mol | 327.878 | 327,878 | 1.204e+24 |
| 5 mol | 819.695 | 819,695 | 3.011e+24 |
| 10 mol | 1639.39 | 1639390.698380 | 6.022e+24 |
Mass is the amount multiplied by 163.93907 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Potassium permanganate | KMnO4 | 158.0323 | 6.3278 |
| Copper(II) sulfate | CuSO4 | 159.602 | 6.2656 |
| Iron(III) chloride | FeCl3 | 162.195 | 6.1654 |
| Trisodium phosphate (this page) | Na3PO4 | 163.9391 | 6.0998 |
| Potassium iodide | KI | 166.0028 | 6.024 |
| Silver nitrate | AgNO3 | 169.8722 | 5.8868 |
| Gypsum | CaSO4·2H2O | 172.164 | 5.8084 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.