Molar mass of oxalic acid dihydrate (H₂C₂O₄·2H₂O)

The molar mass of oxalic acid dihydrate, H₂C₂O₄·2H₂O, is 126.06 g/mol. One mole of it, which is 6.022 × 10²³ molecules, has a mass of 126.06 grams.

About oxalic acid dihydrate

Crystalline oxalic acid holds two molecules of water. It is a primary standard, so this is the figure to use when a question says 6.3 g of oxalic acid crystals in a litre.

The working

The formula H₂C₂O₄·2H₂O has 6 hydrogen atoms, 2 carbon atoms and 6 oxygen atoms, 14 atoms in all. Multiply each count by the atomic weight of the element and add:

6 × 1.0080 + 2 × 12.011 + 6 × 15.999 = 126.064 g/mol

With the rounded atomic masses used in many school books (H = 1, C = 12, O = 16) the answer is 126 g/mol. The difference from 126.06 is too small to matter in most school sums.

The water in the crystal

The part after the dot is water of crystallisation: 2 molecules of water for each H₂C₂O₄. That water has a mass of 2 × 18.015 = 36.03 g/mol, which is 28.6% of the crystal. Without it, the anhydrous part H₂C₂O₄ has a molar mass of 90.03 g/mol. Heating drives the water off, so a sample loses up to 28.6% of its mass. How this is used to find the number of waters is shown on the hydrates page.

Percentage composition

By mass, H₂C₂O₄·2H₂O is 4.80% hydrogen, 19.06% carbon and 76.15% oxygen. The largest share comes from oxygen. In a 100 g sample there are 4.8 g of hydrogen, 19.1 g of carbon and 76.1 g of oxygen.

Worked examples

Grams to moles. How many moles are in 10 g of H₂C₂O₄·2H₂O? Divide the mass by the molar mass: 10 ÷ 126.06 = 0.07932 mol.

Moles to grams. What is the mass of 1.5 mol of H₂C₂O₄·2H₂O? Multiply: 1.5 × 126.06 = 189.1 g.

Counting particles. 10 g contains 0.07932 mol × 6.022 × 10²³ = 4.777 × 10²² molecules, and 6.688 × 10²³ atoms in all.

Making a solution. A 0.1 mol/L solution contains 0.1 × 126.06 = 12.61 g of H₂C₂O₄·2H₂O in each litre. For 250 mL, weigh 3.152 g and make it up to the mark with water.

Quick conversion tables

For other amounts use the grams and moles converter.

Questions people ask

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.

More hydrates

• Zinc sulphate heptahydrate ZnSO₄·7H₂O · 287.54
• Ammonium iron(II) sulphate hexahydrate FeSO₄·(NH₄)₂SO₄·6H₂O · 392.12
• Potassium aluminium sulphate dodecahydrate KAl(SO₄)₂·12H₂O · 474.37
• Barium chloride dihydrate BaCl₂·2H₂O · 244.26
• Calcium chloride dihydrate CaCl₂·2H₂O · 147.01
• Copper(II) sulphate pentahydrate CuSO₄·5H₂O · 249.68

All hydrates · All compounds