To prepare a solution of trisodium phosphate you need the grams, and grams is molarity × volume in litres × 163.93907 g/mol. A litre of 1 M takes 163.9391 g; 500 mL of 0.1 M takes 8.197 g; 250 mL of 0.01 M takes 409.85 mg. Enter the strength you want and the volume you want it in, and the calculator returns the mass to weigh together with the same figure in milligrams and as a percentage strength.
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.
Order of operations matters more than precision here. Dissolve the solid in rather less than the final volume, then make up to the mark once it has all gone into solution and come back to room temperature — warm solutions read low when they cool. If the calculated mass is under about 20 mg, prepare ten times the strength and dilute it tenfold by pipette instead, because C₁V₁ = C₂V₂ carries less error than a marginal 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
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
163.93907- The molar mass of trisodium phosphate in g/mol, derived from its formula
m- The mass of trisodium phosphate to weigh out, in grams
How it works, step by step
- Enter the molarity you want, in mol/L.
- Enter the volume you want to make, in millilitres.
- The two are multiplied together and by 163.93907 g/mol.
- The result is the mass to weigh; dissolve it in part of the volume and then make up to the mark.
Worked examples
250 mL of 0.1 M trisodium phosphate
Weigh 4.09848 g. That is 0.1 M × 0.25 L × 163.9391 g/mol, and it comes to 4098.48 mg if your balance is set to milligrams. Dissolve in about 175 mL first, then make up to 250 mL.
500 mL of 0.5 M trisodium phosphate
Weigh 40.9848 g. That is 0.5 M × 0.5 L × 163.9391 g/mol, and it comes to 40,984.8 mg if your balance is set to milligrams. Dissolve in about 350 mL first, then make up to 500 mL.
1000 mL of 1 M trisodium phosphate
Weigh 163.939 g. That is 1 M × 1 L × 163.9391 g/mol, and it comes to 163,939 mg if your balance is set to milligrams. Dissolve in about 700 mL first, then make up to 1000 mL.
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 much trisodium phosphate do I need for 1 litre of 1 M solution?
163.93907 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 16.3939 g, and for 1 L of 0.01 M it is 1.63939 g.
How much for 100 mL of 0.1 M?
1.63939 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 163.9391 g. In milligrams that is 1639.39 mg.
Do I dissolve the solid first or make up the volume first?
Dissolve first, in perhaps 70% of the final volume, then make up to the mark. Adding solid to a full flask overshoots the volume and leaves the solution weak, and undissolved solid at the mark means the strength keeps changing as it goes in.
Can I dilute a stock solution instead of weighing?
Yes, and it is more accurate for small amounts. C₁V₁ = C₂V₂: to get 500 mL of 0.01 M from a 0.5 M stock, take 10 mL of stock and make up to 500 mL. Weighing the 819.7 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes 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.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 16.3939 mg | 40.9848 mg | 81.9695 mg | 163.939 mg |
| 0.005 M | 81.9695 mg | 204.924 mg | 409.848 mg | 819.695 mg |
| 0.01 M | 163.939 mg | 409.848 mg | 819.695 mg | 1.63939 g |
| 0.05 M | 819.695 mg | 2.04924 g | 4.09848 g | 8.19695 g |
| 0.1 M | 1.63939 g | 4.09848 g | 8.19695 g | 16.3939 g |
| 0.15 M | 2.45909 g | 6.14772 g | 12.2954 g | 24.5909 g |
| 0.2 M | 3.27878 g | 8.19695 g | 16.3939 g | 32.7878 g |
| 0.25 M | 4.09848 g | 10.2462 g | 20.4924 g | 40.9848 g |
| 0.5 M | 8.19695 g | 20.4924 g | 40.9848 g | 81.9695 g |
| 1 M | 16.3939 g | 40.9848 g | 81.9695 g | 163.939 g |
| 2 M | 32.7878 g | 81.9695 g | 163.939 g | 327.878 g |
Each cell is molarity × volume in litres × 163.93907 g/mol. Dissolve the solid first and then make up to the marked volume — adding solid to a full volume overshoots it.
| 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.