To prepare a solution of oxalic acid you need the grams, and grams is molarity × volume in litres × 90.034 g/mol. A litre of 1 M takes 90.034 g; 500 mL of 0.1 M takes 4.5017 g; 250 mL of 0.01 M takes 225.08 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. Oxalic acid is 2 × 12.011 (C) + 2 × 1.008 (H) + 4 × 15.999 (O), which is 90.034 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 90.034 g, or 71.08% 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.
Rust remover, wood bleach and the acid in rhubarb leaves. The dihydrate is the form usually sold, at 126.07 g per mole, and it is genuinely toxic. At 90.034 g/mol it is heavier than 5 of the 12 acids covered here, and that ranking matters more than it looks: nitric acid has a molar mass of 63.012 g/mol, so a gram of it contains 42.9% more formula units than a gram of oxalic acid. Weigh by mass and you are not weighing equal amounts of substance.
One more step applies if you are starting from a concentrated bottle. Concentrated acids are labelled as a percentage by weight together with a density, not as a molarity, so getting to mol/L means multiplying density by that percentage and dividing by 90.034 g/mol. A page like this one gives you the last of those three numbers; the first two are on the bottle, and they vary between grades.
The formula
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
90.034- The molar mass of oxalic acid in g/mol, derived from its formula
m- The mass of oxalic acid 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 90.034 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 oxalic acid
Weigh 2.25085 g. That is 0.1 M × 0.25 L × 90.034 g/mol, and it comes to 2250.85 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 oxalic acid
Weigh 22.5085 g. That is 0.5 M × 0.5 L × 90.034 g/mol, and it comes to 22,508.5 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 oxalic acid
Weigh 90.034 g. That is 1 M × 1 L × 90.034 g/mol, and it comes to 90,034 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 oxalic acid (C2H2O4)?
It is 90.034 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 12.011 (C) + 2 × 1.008 (H) + 4 × 15.999 (O). One mole of oxalic acid therefore weighs 90.034 g, and a gram of it is 11.107 mmol.
What percentage of oxalic acid is oxygen?
71.08% by mass. Each formula unit contains 4 oxygen atoms contributing 63.996 g of the 90.034 g total, so 100 g of oxalic acid contains 71.08 g of oxygen and a kilogram contains 710.8 g of it.
How much oxalic acid do I need for 1 litre of 1 M solution?
90.034 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 9.0034 g, and for 1 L of 0.01 M it is 0.90034 g.
How much for 100 mL of 0.1 M?
0.90034 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 90.034 g. In milligrams that is 900.34 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 450.17 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes for oxalic acid
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 4 | 15.999 | 63.996 | 71.08% |
| Carbon (C) | 2 | 12.011 | 24.022 | 26.68% |
| Hydrogen (H) | 2 | 1.008 | 2.016 | 2.239% |
| Total — one mole of oxalic acid | 90.034 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 90.034 g/mol.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 9.0034 mg | 22.5085 mg | 45.017 mg | 90.034 mg |
| 0.005 M | 45.017 mg | 112.542 mg | 225.085 mg | 450.17 mg |
| 0.01 M | 90.034 mg | 225.085 mg | 450.17 mg | 900.34 mg |
| 0.05 M | 450.17 mg | 1.12542 g | 2.25085 g | 4.5017 g |
| 0.1 M | 900.34 mg | 2.25085 g | 4.5017 g | 9.0034 g |
| 0.15 M | 1.35051 g | 3.37627 g | 6.75255 g | 13.5051 g |
| 0.2 M | 1.80068 g | 4.5017 g | 9.0034 g | 18.0068 g |
| 0.25 M | 2.25085 g | 5.62712 g | 11.2542 g | 22.5085 g |
| 0.5 M | 4.5017 g | 11.2542 g | 22.5085 g | 45.017 g |
| 1 M | 9.0034 g | 22.5085 g | 45.017 g | 90.034 g |
| 2 M | 18.0068 g | 45.017 g | 90.034 g | 180.068 g |
Each cell is molarity × volume in litres × 90.034 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 |
|---|---|---|---|
| Acetic acid | CH3COOH | 60.052 | 16.652 |
| Boric acid | H3BO3 | 61.831 | 16.173 |
| Nitric acid | HNO3 | 63.012 | 15.87 |
| Oxalic acid (this page) | C2H2O4 | 90.034 | 11.107 |
| Lactic acid | C3H6O3 | 90.078 | 11.101 |
| Phosphoric acid | H3PO4 | 97.99376 | 10.205 |
| Sulfuric acid | H2SO4 | 98.072 | 10.197 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.