Molar MassMolar mass of octane (C8H18)
The molar mass of octane, C8H18, is 114.23 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 114.23 grams.
About octane
Octane stands for the hydrocarbons in petrol in many combustion questions. Several isomers share this formula.
The working
The formula C8H18 has 8 carbon atoms and 18 hydrogen atoms, 26 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) | 8 | 12.011 | 96.088 | 84.12% |
| Hydrogen (H) | 18 | 1.0080 | 18.144 | 15.88% |
8 × 12.011 + 18 × 1.0080 = 114.232 g/mol
With the rounded atomic masses used in many school books (C = 12, H = 1) the answer is 114 g/mol. The difference from 114.23 is too small to matter in most school sums.
Percentage composition
By mass, C8H18 is 84.12% carbon and 15.88% hydrogen. The largest share comes from carbon. In a 100 g sample there are 84.1 g of carbon and 15.9 g of hydrogen.
Notice that hydrogen has more atoms in the formula (18) than carbon (8), 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 100 g of C8H18? Divide the mass by the molar mass: 100 ÷ 114.23 = 0.8754 mol.
Moles to grams. What is the mass of 2 mol of C8H18? Multiply: 2 × 114.23 = 228.5 g.
Counting particles. 100 g contains 0.8754 mol × 6.022 × 1023 = 5.272 × 1023 molecules, and 1.371 × 1025 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 11.42 g |
| 0.5 mol | 57.12 g |
| 1 mol | 114.2 g |
| 2 mol | 228.5 g |
| 5 mol | 571.2 g |
| Mass | Moles |
|---|---|
| 1 g | 0.008754 mol |
| 10 g | 0.08754 mol |
| 100 g | 0.8754 mol |
| 1 kg | 8.754 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of C8H18?
The molar mass of octane (C8H18) is 114.23 g/mol, or 114.232 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of C8H18?
100 ÷ 114.23 = 0.8754 mol.
What is the mass of one molecule of C8H18?
Divide the molar mass by the Avogadro constant: 114.23 ÷ 6.022 × 1023 = 1.897 × 10-22 g.
What is the percentage of carbon in C8H18?
Carbon makes up 84.12% of the mass of C8H18: 8 × 12.011 ÷ 114.232 × 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.