To prepare a solution of sodium citrate you need the grams, and grams is molarity × volume in litres × 258.06831 g/mol. A litre of 1 M takes 258.0683 g; 500 mL of 0.1 M takes 12.903 g; 250 mL of 0.01 M takes 645.17 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. 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.
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 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
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
258.06831- The molar mass of sodium citrate in g/mol, derived from its formula
m- The mass of sodium citrate 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 258.06831 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 sodium citrate
Weigh 6.45171 g. That is 0.1 M × 0.25 L × 258.0683 g/mol, and it comes to 6451.71 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 sodium citrate
Weigh 64.5171 g. That is 0.5 M × 0.5 L × 258.0683 g/mol, and it comes to 64,517.1 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 sodium citrate
Weigh 258.068 g. That is 1 M × 1 L × 258.0683 g/mol, and it comes to 258,068 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 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 much sodium citrate do I need for 1 litre of 1 M solution?
258.06831 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 25.8068 g, and for 1 L of 0.01 M it is 2.58068 g.
How much for 100 mL of 0.1 M?
2.58068 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 258.0683 g. In milligrams that is 2580.68 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 1290.3 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes 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.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 25.8068 mg | 64.5171 mg | 129.034 mg | 258.068 mg |
| 0.005 M | 129.034 mg | 322.585 mg | 645.171 mg | 1.29034 g |
| 0.01 M | 258.068 mg | 645.171 mg | 1.29034 g | 2.58068 g |
| 0.05 M | 1.29034 g | 3.22585 g | 6.45171 g | 12.9034 g |
| 0.1 M | 2.58068 g | 6.45171 g | 12.9034 g | 25.8068 g |
| 0.15 M | 3.87102 g | 9.67756 g | 19.3551 g | 38.7102 g |
| 0.2 M | 5.16137 g | 12.9034 g | 25.8068 g | 51.6137 g |
| 0.25 M | 6.45171 g | 16.1293 g | 32.2585 g | 64.5171 g |
| 0.5 M | 12.9034 g | 32.2585 g | 64.5171 g | 129.034 g |
| 1 M | 25.8068 g | 64.5171 g | 129.034 g | 258.068 g |
| 2 M | 51.6137 g | 129.034 g | 258.068 g | 516.137 g |
Each cell is molarity × volume in litres × 258.06831 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 |
|---|---|---|---|
| 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.