Molar MassMolar mass of benzoic acid (C6H5COOH)
The molar mass of benzoic acid, C6H5COOH, is 122.12 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 122.12 grams.
122.12 g/mol
7 × 12.011 + 6 × 1.0080 + 2 × 15.999 = 122.123 · 15 atoms in the formula
About benzoic acid
Benzoic acid is a white solid whose sodium salt is a common food preservative. It is often used in school to practise recrystallisation.
The working
The formula C6H5COOH has 7 carbon atoms, 6 hydrogen atoms and 2 oxygen atoms, 15 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) | 7 | 12.011 | 84.077 | 68.85% |
| Hydrogen (H) | 6 | 1.0080 | 6.048 | 4.95% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 26.20% |
7 × 12.011 + 6 × 1.0080 + 2 × 15.999 = 122.123 g/mol
With the rounded atomic masses used in many school books (C = 12, H = 1, O = 16) the answer is 122 g/mol. The difference from 122.12 is too small to matter in most school sums.
Percentage composition
By mass, C6H5COOH is 68.85% carbon, 4.95% hydrogen and 26.20% oxygen. The largest share comes from carbon. In a 100 g sample there are 68.8 g of carbon, 5.0 g of hydrogen and 26.2 g of oxygen.
Worked examples
Grams to moles. How many moles are in 25 g of C6H5COOH? Divide the mass by the molar mass: 25 ÷ 122.12 = 0.2047 mol.
Moles to grams. What is the mass of 0.1 mol of C6H5COOH? Multiply: 0.1 × 122.12 = 12.21 g.
Counting particles. 25 g contains 0.2047 mol × 6.022 × 1023 = 1.233 × 1023 molecules, and 1.849 × 1024 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 12.21 g |
| 0.5 mol | 61.06 g |
| 1 mol | 122.1 g |
| 2 mol | 244.2 g |
| 5 mol | 610.6 g |
| Mass | Moles |
|---|---|
| 1 g | 0.008188 mol |
| 10 g | 0.08188 mol |
| 100 g | 0.8188 mol |
| 1 kg | 8.188 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of C6H5COOH?
The molar mass of benzoic acid (C6H5COOH) is 122.12 g/mol, or 122.123 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of C6H5COOH?
100 ÷ 122.12 = 0.8188 mol.
What is the mass of one molecule of C6H5COOH?
Divide the molar mass by the Avogadro constant: 122.12 ÷ 6.022 × 1023 = 2.028 × 10-22 g.
What is the percentage of carbon in C6H5COOH?
Carbon makes up 68.85% of the mass of C6H5COOH: 7 × 12.011 ÷ 122.123 × 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.