Molar MassMolar mass of aspirin (C9H8O4)
The molar mass of aspirin, C9H8O4, is 180.16 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 180.16 grams.
180.16 g/mol
9 × 12.011 + 8 × 1.0080 + 4 × 15.999 = 180.159 · 21 atoms in the formula · also called acetylsalicylic acid
About aspirin
Aspirin is acetylsalicylic acid, a pain reliever made from salicylic acid. Its preparation and titration are common practical exercises.
Other names: acetylsalicylic acid.
The working
The formula C9H8O4 has 9 carbon atoms, 8 hydrogen atoms and 4 oxygen atoms, 21 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) | 9 | 12.011 | 108.099 | 60.00% |
| Hydrogen (H) | 8 | 1.0080 | 8.064 | 4.48% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 35.52% |
9 × 12.011 + 8 × 1.0080 + 4 × 15.999 = 180.159 g/mol
With the rounded atomic masses used in many school books (C = 12, H = 1, O = 16) the answer is 180 g/mol. The difference from 180.16 is too small to matter in most school sums.
Percentage composition
By mass, C9H8O4 is 60.00% carbon, 4.48% hydrogen and 35.52% oxygen. The largest share comes from carbon. In a 100 g sample there are 60.0 g of carbon, 4.5 g of hydrogen and 35.5 g of oxygen.
Worked examples
Grams to moles. How many moles are in 50 g of C9H8O4? Divide the mass by the molar mass: 50 ÷ 180.16 = 0.2775 mol.
Moles to grams. What is the mass of 0.4 mol of C9H8O4? Multiply: 0.4 × 180.16 = 72.06 g.
Counting particles. 50 g contains 0.2775 mol × 6.022 × 1023 = 1.671 × 1023 molecules, and 3.510 × 1024 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 18.02 g |
| 0.5 mol | 90.08 g |
| 1 mol | 180.2 g |
| 2 mol | 360.3 g |
| 5 mol | 900.8 g |
| Mass | Moles |
|---|---|
| 1 g | 0.005551 mol |
| 10 g | 0.05551 mol |
| 100 g | 0.5551 mol |
| 1 kg | 5.551 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of C9H8O4?
The molar mass of aspirin (C9H8O4) is 180.16 g/mol, or 180.159 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of C9H8O4?
100 ÷ 180.16 = 0.5551 mol.
What is the mass of one molecule of C9H8O4?
Divide the molar mass by the Avogadro constant: 180.16 ÷ 6.022 × 1023 = 2.992 × 10-22 g.
What is the percentage of carbon in C9H8O4?
Carbon makes up 60.00% of the mass of C9H8O4: 9 × 12.011 ÷ 180.159 × 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.