The molar mass of sodium citrate, Na3C6H5O7, is 258.06831 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 258.0683 g, and a gram of it is 2.334e+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. Sodium citrate is 3 × 22.9898 (Na) + 6 × 12.011 (C) + 5 × 1.008 (H) + 7 × 15.999 (O), which is 258.06831 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: 7 of the 21 atoms in a formula unit are oxygen, and they account for 111.993 g of the 258.0683 g, or 43.4% 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 sodium citrate is 258.068 mg and one micromole is 258.068 µg, so a balance reading to a milligram places you within 3.875 µmol. A single formula unit weighs 4.2853e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
The anticoagulant in blood bags, the emulsifying salt in smooth cheese sauce, and the buffer in effervescent tablets. The dihydrate is 294.10 g per mole. At 258.0683 g/mol it is heavier than 29 of the 34 salts covered here, and that ranking matters more than it looks: copper(II) sulfate pentahydrate has a molar mass of 249.677 g/mol, so a gram of it contains 3.36% more formula units than a gram of sodium citrate. 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 5.1614 g of a nominal 258.0683 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 sodium citrate in grams
258.06831- The molar mass of sodium citrate 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 sodium citrate in grams.
- It is divided by the molar mass, 258.06831 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 sodium citrate as a molecule count
0.5 g divided by 258.0683 g/mol is 0.00193747 mol, and multiplying by the Avogadro constant gives 1.167e+21 formula units of sodium citrate. The same mass is 1.9375 mmol, which is the figure a reaction is actually planned in.
5 g of sodium citrate as a molecule count
5 g divided by 258.0683 g/mol is 0.0193747 mol, and multiplying by the Avogadro constant gives 1.167e+22 formula units of sodium citrate. The same mass is 19.375 mmol, which is the figure a reaction is actually planned in.
50 g of sodium citrate as a molecule count
50 g divided by 258.0683 g/mol is 0.193747 mol, and multiplying by the Avogadro constant gives 1.167e+23 formula units of sodium citrate. The same mass is 193.75 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 sodium citrate (Na3C6H5O7)?
It is 258.06831 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 3 × 22.9898 (Na) + 6 × 12.011 (C) + 5 × 1.008 (H) + 7 × 15.999 (O). One mole of sodium citrate therefore weighs 258.0683 g, and a gram of it is 3.8749 mmol.
What percentage of sodium citrate is oxygen?
43.4% by mass. Each formula unit contains 7 oxygen atoms contributing 111.993 g of the 258.0683 g total, so 100 g of sodium citrate contains 43.4 g of oxygen and a kilogram contains 433.97 g of it.
How many molecules are in one gram of sodium citrate?
About 2.334e+21. One gram is 0.00387494 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 258.0683.
How much does a single molecule of sodium citrate weigh?
4.2853e-22 g, which is 258.06831 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 sodium citrate it is 258.06831. Molar mass carries grams per mole: 258.06831 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 sodium citrate comes out at 258.06831 g/mol rather than 258.
Molar mass, composition and mole equivalents of sodium citrate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 7 | 15.999 | 111.993 | 43.4% |
| Carbon (C) | 6 | 12.011 | 72.066 | 27.93% |
| Sodium (Na) | 3 | 22.98977 | 68.96931 | 26.73% |
| Hydrogen (H) | 5 | 1.008 | 5.04 | 1.953% |
| Total — one mole of sodium citrate | 258.06831 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 258.06831 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000258068 | 0.258068 | 6.022e+17 |
| 10 µmol | 0.00258068 | 2.58068 | 6.022e+18 |
| 100 µmol | 0.0258068 | 25.8068 | 6.022e+19 |
| 1 mmol | 0.258068 | 258.068 | 6.022e+20 |
| 10 mmol | 2.58068 | 2580.68 | 6.022e+21 |
| 50 mmol | 12.9034 | 12,903.4 | 3.011e+22 |
| 100 mmol | 25.8068 | 25,806.8 | 6.022e+22 |
| 250 mmol | 64.5171 | 64,517.1 | 1.506e+23 |
| 500 mmol | 129.034 | 129,034 | 3.011e+23 |
| 750 mmol | 193.551 | 193,551 | 4.517e+23 |
| 1 mol | 258.068 | 258,068 | 6.022e+23 |
| 2 mol | 516.137 | 516,137 | 1.204e+24 |
| 5 mol | 1290.34 | 1290341.539200 | 3.011e+24 |
| 10 mol | 2580.68 | 2580683.078400 | 6.022e+24 |
Mass is the amount multiplied by 258.06831 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 |
|---|---|---|---|
| Epsom salt | MgSO4·7H2O | 246.466 | 4.0574 |
| Sodium thiosulfate pentahydrate | Na2S2O3·5H2O | 248.1715 | 4.0295 |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.677 | 4.0052 |
| Sodium citrate (this page) | Na3C6H5O7 | 258.0683 | 3.8749 |
| Iron(II) sulfate heptahydrate | FeSO4·7H2O | 278.006 | 3.597 |
| Zinc sulfate heptahydrate | ZnSO4·7H2O | 287.541 | 3.4778 |
| Potassium dichromate | K2Cr2O7 | 294.1818 | 3.3993 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.