Molar MassMolar mass of oxalic acid (H2C2O4)
The molar mass of oxalic acid, H2C2O4, is 90.03 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 90.03 grams.
90.03 g/mol
2 × 1.0080 + 2 × 12.011 + 4 × 15.999 = 90.034 · 8 atoms in the formula · also called ethanedioic acid
About oxalic acid
Oxalic acid is a dibasic organic acid found in spinach and rhubarb leaves. In school laboratories its dihydrate is a favourite primary standard for titrating sodium hydroxide and potassium permanganate, so be sure which form a question means.
Other names: ethanedioic acid.
The working
The formula H2C2O4 has 2 hydrogen atoms, 2 carbon atoms and 4 oxygen atoms, 8 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 2.24% |
| Carbon (C) | 2 | 12.011 | 24.022 | 26.68% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 71.08% |
2 × 1.0080 + 2 × 12.011 + 4 × 15.999 = 90.034 g/mol
With the rounded atomic masses used in many school books (H = 1, C = 12, O = 16) the answer is 90 g/mol. The difference from 90.03 is too small to matter in most school sums.
Percentage composition
By mass, H2C2O4 is 2.24% hydrogen, 26.68% carbon and 71.08% oxygen. The largest share comes from oxygen. In a 100 g sample there are 2.2 g of hydrogen, 26.7 g of carbon and 71.1 g of oxygen.
Worked examples
Grams to moles. How many moles are in 10 g of H2C2O4? Divide the mass by the molar mass: 10 ÷ 90.03 = 0.1111 mol.
Moles to grams. What is the mass of 1.5 mol of H2C2O4? Multiply: 1.5 × 90.03 = 135.1 g.
Counting particles. 10 g contains 0.1111 mol × 6.022 × 1023 = 6.689 × 1022 molecules, and 5.351 × 1023 atoms in all.
Making a solution. A 0.1 mol/L solution contains 0.1 × 90.03 = 9.003 g of H2C2O4 in each litre. For 250 mL, weigh 2.251 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 9.003 g |
| 0.5 mol | 45.02 g |
| 1 mol | 90.03 g |
| 2 mol | 180.1 g |
| 5 mol | 450.2 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01111 mol |
| 10 g | 0.1111 mol |
| 100 g | 1.111 mol |
| 1 kg | 11.11 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of H2C2O4?
The molar mass of oxalic acid (H2C2O4) is 90.03 g/mol, or 90.034 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of H2C2O4?
100 ÷ 90.03 = 1.111 mol.
What is the mass of one molecule of H2C2O4?
Divide the molar mass by the Avogadro constant: 90.03 ÷ 6.022 × 1023 = 1.495 × 10-22 g.
What is the percentage of oxygen in H2C2O4?
Oxygen makes up 71.08% of the mass of H2C2O4: 4 × 15.999 ÷ 90.034 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.