To prepare a solution of disodium phosphate you need the grams, and grams is molarity × volume in litres × 141.9573 g/mol. A litre of 1 M takes 141.9573 g; 500 mL of 0.1 M takes 7.0979 g; 250 mL of 0.01 M takes 354.89 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. Disodium phosphate is 2 × 22.9898 (Na) + 1.008 (H) + 30.9738 (P) + 4 × 15.999 (O), which is 141.9573 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 4 of the 8 atoms in a formula unit are oxygen, and they account for 63.996 g of the 141.9573 g, or 45.08% 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.
The basic half of a phosphate buffer, and an emulsifying salt in processed cheese. The dihydrate and dodecahydrate are both sold and weigh more. At 141.9573 g/mol it is heavier than 16 of the 34 salts covered here, and that ranking matters more than it looks: monopotassium phosphate has a molar mass of 136.0841 g/mol, so a gram of it contains 4.32% more formula units than a gram of disodium phosphate. Weigh by mass and you are not weighing equal amounts of substance.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 2.8391 g of every 141.9573 g is not the compound — and any water picked up from the air does the same. Both are systematic, so they shift every solution made from that jar in the same direction.
The formula
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
141.9573- The molar mass of disodium phosphate in g/mol, derived from its formula
m- The mass of disodium 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 141.9573 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 disodium phosphate
Weigh 3.54893 g. That is 0.1 M × 0.25 L × 141.9573 g/mol, and it comes to 3548.93 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 disodium phosphate
Weigh 35.4893 g. That is 0.5 M × 0.5 L × 141.9573 g/mol, and it comes to 35,489.3 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 disodium phosphate
Weigh 141.957 g. That is 1 M × 1 L × 141.9573 g/mol, and it comes to 141,957 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 disodium phosphate (Na2HPO4)?
It is 141.9573 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 22.9898 (Na) + 1.008 (H) + 30.9738 (P) + 4 × 15.999 (O). One mole of disodium phosphate therefore weighs 141.9573 g, and a gram of it is 7.0444 mmol.
What percentage of disodium phosphate is oxygen?
45.08% by mass. Each formula unit contains 4 oxygen atoms contributing 63.996 g of the 141.9573 g total, so 100 g of disodium phosphate contains 45.08 g of oxygen and a kilogram contains 450.81 g of it.
How much disodium phosphate do I need for 1 litre of 1 M solution?
141.9573 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 14.1957 g, and for 1 L of 0.01 M it is 1.41957 g.
How much for 100 mL of 0.1 M?
1.41957 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 141.9573 g. In milligrams that is 1419.57 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 709.79 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes for disodium phosphate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 4 | 15.999 | 63.996 | 45.08% |
| Sodium (Na) | 2 | 22.98977 | 45.97954 | 32.39% |
| Phosphorus (P) | 1 | 30.97376 | 30.97376 | 21.82% |
| Hydrogen (H) | 1 | 1.008 | 1.008 | 0.7101% |
| Total — one mole of disodium phosphate | 141.9573 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 141.9573 g/mol.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 14.1957 mg | 35.4893 mg | 70.9787 mg | 141.957 mg |
| 0.005 M | 70.9787 mg | 177.447 mg | 354.893 mg | 709.787 mg |
| 0.01 M | 141.957 mg | 354.893 mg | 709.787 mg | 1.41957 g |
| 0.05 M | 709.787 mg | 1.77447 g | 3.54893 g | 7.09787 g |
| 0.1 M | 1.41957 g | 3.54893 g | 7.09787 g | 14.1957 g |
| 0.15 M | 2.12936 g | 5.3234 g | 10.6468 g | 21.2936 g |
| 0.2 M | 2.83915 g | 7.09787 g | 14.1957 g | 28.3915 g |
| 0.25 M | 3.54893 g | 8.87233 g | 17.7447 g | 35.4893 g |
| 0.5 M | 7.09787 g | 17.7447 g | 35.4893 g | 70.9787 g |
| 1 M | 14.1957 g | 35.4893 g | 70.9787 g | 141.957 g |
| 2 M | 28.3915 g | 70.9787 g | 141.957 g | 283.915 g |
Each cell is molarity × volume in litres × 141.9573 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 |
|---|---|---|---|
| Magnesium sulfate | MgSO4 | 120.361 | 8.3083 |
| Ammonium sulfate | (NH4)2SO4 | 132.134 | 7.5681 |
| Monopotassium phosphate | KH2PO4 | 136.0841 | 7.3484 |
| Disodium phosphate (this page) | Na2HPO4 | 141.9573 | 7.0444 |
| Sodium sulfate | Na2SO4 | 142.0355 | 7.0405 |
| Potassium permanganate | KMnO4 | 158.0323 | 6.3278 |
| Copper(II) sulfate | CuSO4 | 159.602 | 6.2656 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.