Going from grams to moles for oxalic acid means dividing by its molar mass, 90.034 g/mol. One gram is 0.0111069 mol, or 11.1069 mmol; 100 mg is 1.1107 mmol; and a 90.034 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, C2H2O4, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.1111 mmol of oxalic acid; on a three-decimal balance it is 1 mg, or 11.11 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.
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 you weighed, in grams
90.034- The molar mass of oxalic acid 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 90.034 g/mol, the molar mass of oxalic acid.
- 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 oxalic acid in moles
0.25 ÷ 90.034 = 0.00277673 mol, or 2.7767 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 11.11 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of oxalic acid in moles
2 ÷ 90.034 = 0.0222138 mol, or 22.214 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 11.11 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of oxalic acid in moles
25 ÷ 90.034 = 0.277673 mol, or 277.67 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 11.11 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 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 convert grams of oxalic acid to moles?
Divide the mass in grams by 90.034 g/mol. So 1 g is 0.0111069 mol, 10 g is 0.111069 mol and 100 g is 1.11069 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of oxalic acid?
1.11069 mol, which is 1110.69 mmol. Read the other way, one mole of oxalic acid is 90.034 g, so 100 g is 1.111 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of oxalic acid is 11.107 µmol, 10 mg is 111.07 µmol and 100 mg is 1.1107 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 oxalic acid, which is 0.0108848 mol rather than 0.0111069 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 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000011 | 0.0111069 | 11.1069 |
| 5 mg | 0.000056 | 0.0555346 | 55.5346 |
| 10 mg | 0.000111069 | 0.111069 | 111.069 |
| 25 mg | 0.000277673 | 0.277673 | 277.673 |
| 50 mg | 0.000555346 | 0.555346 | 555.346 |
| 100 mg | 0.00111069 | 1.11069 | 1110.69 |
| 250 mg | 0.00277673 | 2.77673 | 2776.73 |
| 500 mg | 0.00555346 | 5.55346 | 5553.46 |
| 1 g | 0.0111069 | 11.1069 | 11,106.9 |
| 2 g | 0.0222138 | 22.2138 | 22,213.8 |
| 5 g | 0.0555346 | 55.5346 | 55,534.6 |
| 10 g | 0.111069 | 111.069 | 111,069 |
| 25 g | 0.277673 | 277.673 | 277,673 |
| 50 g | 0.555346 | 555.346 | 555,346 |
| 100 g | 1.11069 | 1110.69 | 1110691.516538 |
| 250 g | 2.77673 | 2776.73 | 2776728.791345 |
| 500 g | 5.55346 | 5553.46 | 5553457.582691 |
Every row is the mass divided by 90.034 g/mol. A balance reading to 1 mg resolves 11.11 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 |
|---|---|---|---|
| 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.