Molar MassMolar mass of formaldehyde (HCHO)
The molar mass of formaldehyde, HCHO, is 30.03 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 30.03 grams.
30.03 g/mol
2 × 1.0080 + 12.011 + 15.999 = 30.026 · 4 atoms in the formula · also called methanal
About formaldehyde
Formaldehyde is the simplest aldehyde. Its solution in water, formalin, preserves biological specimens. It shares the empirical formula CH2O with glucose.
Other names: methanal.
The working
The formula HCHO has 2 hydrogen atoms, 1 carbon atom and 1 oxygen atom, 4 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 | 6.71% |
| Carbon (C) | 1 | 12.011 | 12.011 | 40.00% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 53.28% |
2 × 1.0080 + 12.011 + 15.999 = 30.026 g/mol
With the rounded atomic masses used in many school books (H = 1, C = 12, O = 16) the answer is 30 g/mol. The difference from 30.03 is too small to matter in most school sums.
Percentage composition
By mass, HCHO is 6.71% hydrogen, 40.00% carbon and 53.28% oxygen. The largest share comes from oxygen. In a 100 g sample there are 6.7 g of hydrogen, 40.0 g of carbon and 53.3 g of oxygen.
Notice that hydrogen has more atoms in the formula (2) than oxygen (1), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 5 g of HCHO? Divide the mass by the molar mass: 5 ÷ 30.03 = 0.1665 mol.
Moles to grams. What is the mass of 0.25 mol of HCHO? Multiply: 0.25 × 30.03 = 7.506 g.
Counting particles. 5 g contains 0.1665 mol × 6.022 × 1023 = 1.003 × 1023 molecules, and 4.011 × 1023 atoms in all.
As a gas. At 0 °C and 1 atmosphere one mole of an ideal gas occupies 22.4 litres, so the density of HCHO is about 30.03 ÷ 22.4 = 1.34 g per litre. Air averages about 29 g/mol, so this gas is heavier than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 3.003 g |
| 0.5 mol | 15.01 g |
| 1 mol | 30.03 g |
| 2 mol | 60.05 g |
| 5 mol | 150.1 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0333 mol |
| 10 g | 0.333 mol |
| 100 g | 3.33 mol |
| 1 kg | 33.3 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of HCHO?
The molar mass of formaldehyde (HCHO) is 30.03 g/mol, or 30.026 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of HCHO?
100 ÷ 30.03 = 3.33 mol.
What is the mass of one molecule of HCHO?
Divide the molar mass by the Avogadro constant: 30.03 ÷ 6.022 × 1023 = 4.986 × 10-23 g.
What is the percentage of oxygen in HCHO?
Oxygen makes up 53.28% of the mass of HCHO: 1 × 15.999 ÷ 30.026 × 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.