Molarity is moles of solute per litre of solution, so for oxalic acid it is the mass divided by 90.034 g/mol and then by the volume in litres. One gram in 100 mL is 0.11107 mol/L; the same gram in a litre is 0.011107 mol/L; and 90.034 g in a litre is a 1 M solution. Enter a mass and a volume and the calculator does both divisions, and also reports the strength as g/L and % w/v.
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.
The volume in the denominator is the volume of the finished solution, not the volume of water you started with. Dissolving 90.034 g of a solid adds a little volume of its own, so a solution made up to the mark in a volumetric flask is at the molarity you calculated while the same solid stirred into a litre of water is slightly weaker. At dilute strengths the difference is negligible; at 1 M and above it is not.
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
m- The mass of oxalic acid dissolved, in grams
V- The final volume of the solution, in millilitres
90.034- The molar mass of oxalic acid in g/mol, derived from its formula
c- The concentration in moles per litre
How it works, step by step
- Enter the mass of oxalic acid you dissolved, in grams.
- Enter the final volume of the solution in millilitres.
- The mass is divided by 90.034 g/mol and then by the volume in litres.
- The result is molarity in mol/L, with mmol/L, g/L and % w/v given alongside.
Worked examples
1 g of oxalic acid made up to 100 mL
1 g is 0.0111069 mol. Divided by 0.1 L that is 0.111069 mol/L, or 111.07 mmol/L. The same solution is 10 g/L and 1% w/v — four ways of writing one strength.
5 g of oxalic acid made up to 500 mL
5 g is 0.0555346 mol. Divided by 0.5 L that is 0.111069 mol/L, or 111.07 mmol/L. The same solution is 10 g/L and 1% w/v — four ways of writing one strength.
25 g of oxalic acid made up to 1000 mL
25 g is 0.277673 mol. Divided by 1 L that is 0.277673 mol/L, or 277.67 mmol/L. The same solution is 25 g/L and 2.5% w/v — four ways of writing one strength.
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 do I calculate the molarity of a oxalic acid solution?
Divide the mass in grams by 90.034 g/mol to get moles, then divide by the volume of solution in litres. Both divisions are in the calculator: enter grams and millilitres and it reports mol/L.
What is the molarity of 1 g of oxalic acid in 100 mL?
0.111069 mol/L, or 111.07 mmol/L. The same gram in 250 mL gives 0.0444277 mol/L and in a litre 0.0111069 mol/L, since molarity falls in inverse proportion to the volume.
Is molarity per litre of water or per litre of solution?
Per litre of finished solution. That is why volumetric glassware is filled to a mark after the solid has dissolved rather than measured out first. Molality, which is per kilogram of solvent, is the quantity defined the other way and is not what this page computes.
How do mol/L, g/L and % w/v relate for oxalic acid?
One mol/L of oxalic acid is 90.034 g/L, which is 9.0034% w/v. Going the other way, 1% w/v is 10 g/L and therefore 0.11107 mol/L. All three describe the same solution.
Molarity of oxalic acid by mass and volume
| 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.
| Solute | in 100 mL | in 250 mL | in 500 mL | in 1 L |
|---|---|---|---|---|
| 10 mg | 0.0011107 | 0.00044428 | 0.00022214 | 0.00011107 |
| 50 mg | 0.0055535 | 0.0022214 | 0.0011107 | 0.00055535 |
| 100 mg | 0.011107 | 0.0044428 | 0.0022214 | 0.0011107 |
| 250 mg | 0.027767 | 0.011107 | 0.0055535 | 0.0027767 |
| 500 mg | 0.055535 | 0.022214 | 0.011107 | 0.0055535 |
| 1 g | 0.11107 | 0.044428 | 0.022214 | 0.011107 |
| 2 g | 0.22214 | 0.088855 | 0.044428 | 0.022214 |
| 5 g | 0.55535 | 0.22214 | 0.11107 | 0.055535 |
| 10 g | 1.1107 | 0.44428 | 0.22214 | 0.11107 |
| 25 g | 2.7767 | 1.1107 | 0.55535 | 0.27767 |
| 50 g | 5.5535 | 2.2214 | 1.1107 | 0.55535 |
| 100 g | 11.107 | 4.4428 | 2.2214 | 1.1107 |
All figures in mol/L. Each is the mass divided by 90.034 g/mol and then by the volume in litres, so the columns differ only by that volume.
| 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.