Molar MassMolar mass of propanoic acid (C2H5COOH)
The molar mass of propanoic acid, C2H5COOH, is 74.08 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 74.08 grams.
74.08 g/mol
3 × 12.011 + 6 × 1.0080 + 2 × 15.999 = 74.079 · 11 atoms in the formula · also called propionic acid
About propanoic acid
Propanoic acid is the third member of the carboxylic acid family. Its salts are used to stop mould in bread.
Other names: propionic acid.
The working
The formula C2H5COOH has 3 carbon atoms, 6 hydrogen atoms and 2 oxygen atoms, 11 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Carbon (C) | 3 | 12.011 | 36.033 | 48.64% |
| Hydrogen (H) | 6 | 1.0080 | 6.048 | 8.16% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 43.19% |
3 × 12.011 + 6 × 1.0080 + 2 × 15.999 = 74.079 g/mol
With the rounded atomic masses used in many school books (C = 12, H = 1, O = 16) the answer is 74 g/mol. The difference from 74.08 is too small to matter in most school sums.
Percentage composition
By mass, C2H5COOH is 48.64% carbon, 8.16% hydrogen and 43.19% oxygen. The largest share comes from carbon. In a 100 g sample there are 48.6 g of carbon, 8.2 g of hydrogen and 43.2 g of oxygen.
Notice that hydrogen has more atoms in the formula (6) than carbon (3), 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 20 g of C2H5COOH? Divide the mass by the molar mass: 20 ÷ 74.08 = 0.27 mol.
Moles to grams. What is the mass of 0.5 mol of C2H5COOH? Multiply: 0.5 × 74.08 = 37.04 g.
Counting particles. 20 g contains 0.27 mol × 6.022 × 1023 = 1.626 × 1023 molecules, and 1.788 × 1024 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 7.408 g |
| 0.5 mol | 37.04 g |
| 1 mol | 74.08 g |
| 2 mol | 148.2 g |
| 5 mol | 370.4 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0135 mol |
| 10 g | 0.135 mol |
| 100 g | 1.35 mol |
| 1 kg | 13.5 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of C2H5COOH?
The molar mass of propanoic acid (C2H5COOH) is 74.08 g/mol, or 74.079 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of C2H5COOH?
100 ÷ 74.08 = 1.35 mol.
What is the mass of one molecule of C2H5COOH?
Divide the molar mass by the Avogadro constant: 74.08 ÷ 6.022 × 1023 = 1.230 × 10-22 g.
What is the percentage of carbon in C2H5COOH?
Carbon makes up 48.64% of the mass of C2H5COOH: 3 × 12.011 ÷ 74.079 × 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.