Going from grams to moles for sodium citrate means dividing by its molar mass, 258.06831 g/mol. One gram is 0.00387494 mol, or 3.87494 mmol; 100 mg is 0.38749 mmol; and a 258.0683 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, Na3C6H5O7, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.03875 mmol of sodium citrate; on a three-decimal balance it is 1 mg, or 3.875 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.
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.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 5.1614 g of every 258.0683 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 you weighed, in grams
258.06831- The molar mass of sodium citrate 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 258.06831 g/mol, the molar mass of sodium citrate.
- 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 sodium citrate in moles
0.25 ÷ 258.0683 = 0.000968736 mol, or 0.96874 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 3.875 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of sodium citrate in moles
2 ÷ 258.0683 = 0.00774989 mol, or 7.7499 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 3.875 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of sodium citrate in moles
25 ÷ 258.0683 = 0.0968736 mol, or 96.874 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 3.875 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 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 do I convert grams of sodium citrate to moles?
Divide the mass in grams by 258.06831 g/mol. So 1 g is 0.00387494 mol, 10 g is 0.0387494 mol and 100 g is 0.387494 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of sodium citrate?
0.387494 mol, which is 387.494 mmol. Read the other way, one mole of sodium citrate is 258.0683 g, so 100 g is 0.3875 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of sodium citrate is 3.8749 µmol, 10 mg is 38.749 µmol and 100 mg is 0.38749 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 sodium citrate, which is 0.00379744 mol rather than 0.00387494 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 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000004 | 0.00387494 | 3.87494 |
| 5 mg | 0.000019 | 0.0193747 | 19.3747 |
| 10 mg | 0.000039 | 0.0387494 | 38.7494 |
| 25 mg | 0.000097 | 0.0968736 | 96.8736 |
| 50 mg | 0.000193747 | 0.193747 | 193.747 |
| 100 mg | 0.000387494 | 0.387494 | 387.494 |
| 250 mg | 0.000968736 | 0.968736 | 968.736 |
| 500 mg | 0.00193747 | 1.93747 | 1937.47 |
| 1 g | 0.00387494 | 3.87494 | 3874.94 |
| 2 g | 0.00774989 | 7.74989 | 7749.89 |
| 5 g | 0.0193747 | 19.3747 | 19,374.7 |
| 10 g | 0.0387494 | 38.7494 | 38,749.4 |
| 25 g | 0.0968736 | 96.8736 | 96,873.6 |
| 50 g | 0.193747 | 193.747 | 193,747 |
| 100 g | 0.387494 | 387.494 | 387,494 |
| 250 g | 0.968736 | 968.736 | 968,736 |
| 500 g | 1.93747 | 1937.47 | 1937471.532963 |
Every row is the mass divided by 258.06831 g/mol. A balance reading to 1 mg resolves 3.875 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 |
|---|---|---|---|
| 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.