Moles to grams for oxalic acid is a multiplication by 90.034 g/mol. A mole weighs 90.034 g, 100 mmol weighs 9.0034 g, 10 mmol weighs 0.90034 g and a millimole is 90.034 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of oxalic acid you need, in moles
90.034- The molar mass of oxalic acid in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 90.034 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of oxalic acid in grams
5 mmol × 90.034 g/mol = 0.45017 g, which is 450.17 mg. Doubling the amount doubles the mass to 0.90034 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of oxalic acid in grams
250 mmol × 90.034 g/mol = 22.5085 g, which is 22,508.5 mg. Doubling the amount doubles the mass to 45.017 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of oxalic acid in grams
2 mol × 90.034 g/mol = 180.068 g, which is 180,068 mg. Doubling the amount doubles the mass to 360.136 g, because the relationship is a plain proportion once the molar mass is fixed.
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 many grams is one mole of oxalic acid?
90.034 g, by definition of the molar mass. Two moles is 180.068 g and half a mole is 45.017 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of oxalic acid is 9.0034 g. One millimole is 90.034 mg and one micromole is 90.034 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 0.9003 mg of oxalic acid, weigh 9.003 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, C2H2O4, at 90.034 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000090 | 0.090034 | 0.000092 |
| 10 µmol | 0.00090034 | 0.90034 | 0.000918347 |
| 100 µmol | 0.0090034 | 9.0034 | 0.00918347 |
| 1 mmol | 0.090034 | 90.034 | 0.0918347 |
| 10 mmol | 0.90034 | 900.34 | 0.918347 |
| 50 mmol | 4.5017 | 4501.7 | 4.59173 |
| 100 mmol | 9.0034 | 9003.4 | 9.18347 |
| 250 mmol | 22.5085 | 22,508.5 | 22.9587 |
| 500 mmol | 45.017 | 45,017 | 45.9173 |
| 750 mmol | 67.5255 | 67,525.5 | 68.876 |
| 1 mol | 90.034 | 90,034 | 91.8347 |
| 2 mol | 180.068 | 180,068 | 183.669 |
| 5 mol | 450.17 | 450,170 | 459.173 |
| 10 mol | 900.34 | 900,340 | 918.347 |
Weigh is the amount multiplied by 90.034 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.