Going from grams to moles for trisodium phosphate means dividing by its molar mass, 163.93907 g/mol. One gram is 0.00609983 mol, or 6.09983 mmol; 100 mg is 0.60998 mmol; and a 163.9391 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, Na3PO4, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.061 mmol of trisodium phosphate; on a three-decimal balance it is 1 mg, or 6.1 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
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 you weighed, in grams
163.93907- The molar mass of trisodium phosphate in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 163.93907 g/mol, the molar mass of trisodium phosphate.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of trisodium phosphate in moles
0.25 ÷ 163.9391 = 0.00152496 mol, or 1.525 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.1 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of trisodium phosphate in moles
2 ÷ 163.9391 = 0.0121997 mol, or 12.2 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.1 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of trisodium phosphate in moles
25 ÷ 163.9391 = 0.152496 mol, or 152.5 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.1 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
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 do I convert grams of trisodium phosphate to moles?
Divide the mass in grams by 163.93907 g/mol. So 1 g is 0.00609983 mol, 10 g is 0.0609983 mol and 100 g is 0.609983 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of trisodium phosphate?
0.609983 mol, which is 609.983 mmol. Read the other way, one mole of trisodium phosphate is 163.9391 g, so 100 g is 0.61 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of trisodium phosphate is 6.0998 µmol, 10 mg is 60.998 µmol and 100 mg is 0.60998 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of trisodium phosphate, which is 0.00597783 mol rather than 0.00609983 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference for 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000006 | 0.00609983 | 6.09983 |
| 5 mg | 0.000030 | 0.0304991 | 30.4991 |
| 10 mg | 0.000061 | 0.0609983 | 60.9983 |
| 25 mg | 0.000152496 | 0.152496 | 152.496 |
| 50 mg | 0.000304991 | 0.304991 | 304.991 |
| 100 mg | 0.000609983 | 0.609983 | 609.983 |
| 250 mg | 0.00152496 | 1.52496 | 1524.96 |
| 500 mg | 0.00304991 | 3.04991 | 3049.91 |
| 1 g | 0.00609983 | 6.09983 | 6099.83 |
| 2 g | 0.0121997 | 12.1997 | 12,199.7 |
| 5 g | 0.0304991 | 30.4991 | 30,499.1 |
| 10 g | 0.0609983 | 60.9983 | 60,998.3 |
| 25 g | 0.152496 | 152.496 | 152,496 |
| 50 g | 0.304991 | 304.991 | 304,991 |
| 100 g | 0.609983 | 609.983 | 609,983 |
| 250 g | 1.52496 | 1524.96 | 1524956.804056 |
| 500 g | 3.04991 | 3049.91 | 3049913.608111 |
Every row is the mass divided by 163.93907 g/mol. A balance reading to 1 mg resolves 6.1 µmol of this compound, which is the smallest step the middle columns can really move in.
| 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.