The molar mass of disodium phosphate, Na2HPO4, is 141.9573 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 141.9573 g, and a gram of it is 4.242e+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. 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.
The numbers a bench actually uses sit at the millimole scale. One millimole of disodium phosphate is 141.957 mg and one micromole is 141.957 µg, so a balance reading to a milligram places you within 7.044 µmol. A single formula unit weighs 2.3573e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
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
m- The mass of disodium phosphate in grams
141.9573- The molar mass of disodium 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 disodium phosphate in grams.
- It is divided by the molar mass, 141.9573 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 disodium phosphate as a molecule count
0.5 g divided by 141.9573 g/mol is 0.00352219 mol, and multiplying by the Avogadro constant gives 2.121e+21 formula units of disodium phosphate. The same mass is 3.5222 mmol, which is the figure a reaction is actually planned in.
5 g of disodium phosphate as a molecule count
5 g divided by 141.9573 g/mol is 0.0352219 mol, and multiplying by the Avogadro constant gives 2.121e+22 formula units of disodium phosphate. The same mass is 35.222 mmol, which is the figure a reaction is actually planned in.
50 g of disodium phosphate as a molecule count
50 g divided by 141.9573 g/mol is 0.352219 mol, and multiplying by the Avogadro constant gives 2.121e+23 formula units of disodium phosphate. The same mass is 352.22 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 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 many molecules are in one gram of disodium phosphate?
About 4.242e+21. One gram is 0.00704437 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 141.9573.
How much does a single molecule of disodium phosphate weigh?
2.3573e-22 g, which is 141.9573 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 disodium phosphate it is 141.9573. Molar mass carries grams per mole: 141.9573 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. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why disodium phosphate comes out at 141.9573 g/mol rather than 142.
Molar mass, composition and mole equivalents of 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.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000141957 | 0.141957 | 6.022e+17 |
| 10 µmol | 0.00141957 | 1.41957 | 6.022e+18 |
| 100 µmol | 0.0141957 | 14.1957 | 6.022e+19 |
| 1 mmol | 0.141957 | 141.957 | 6.022e+20 |
| 10 mmol | 1.41957 | 1419.57 | 6.022e+21 |
| 50 mmol | 7.09787 | 7097.87 | 3.011e+22 |
| 100 mmol | 14.1957 | 14,195.7 | 6.022e+22 |
| 250 mmol | 35.4893 | 35,489.3 | 1.506e+23 |
| 500 mmol | 70.9787 | 70,978.7 | 3.011e+23 |
| 750 mmol | 106.468 | 106,468 | 4.517e+23 |
| 1 mol | 141.957 | 141,957 | 6.022e+23 |
| 2 mol | 283.915 | 283,915 | 1.204e+24 |
| 5 mol | 709.787 | 709,787 | 3.011e+24 |
| 10 mol | 1419.57 | 1419573.005580 | 6.022e+24 |
Mass is the amount multiplied by 141.9573 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 |
|---|---|---|---|
| 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.