The molar mass of octane, C8H18, is 114.232 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 114.232 g, and a gram of it is 5.272e+21 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Octane is 8 × 12.011 (C) + 18 × 1.008 (H), which is 114.232 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Carbon makes up the largest share of the mass: 8 of the 26 atoms in a formula unit are carbon, and they account for 96.088 g of the 114.232 g, or 84.12% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of octane is 114.232 mg and one micromole is 114.232 µg, so a balance reading to a milligram places you within 8.754 µmol. A single formula unit weighs 1.8969e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
The reference hydrocarbon for petrol. Complete combustion of one mole needs 12.5 moles of oxygen, which is the calculation fuel-air ratios come from. At 114.232 g/mol it is heavier than 11 of the 21 organic compounds covered here, and that ranking matters more than it looks: creatinine has a molar mass of 113.12 g/mol, so a gram of it contains 0.983% more formula units than a gram of octane. Weigh by mass and you are not weighing equal amounts of substance.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 2.2846 g of every 114.232 g is not the compound — and any water picked up from the air does the same. Both are systematic, so they shift every solution made from that jar in the same direction.
The formula
m- The mass of octane in grams
114.232- The molar mass of octane in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of octane in grams.
- It is divided by the molar mass, 114.232 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of octane as a molecule count
0.5 g divided by 114.232 g/mol is 0.00437706 mol, and multiplying by the Avogadro constant gives 2.636e+21 formula units of octane. The same mass is 4.3771 mmol, which is the figure a reaction is actually planned in.
5 g of octane as a molecule count
5 g divided by 114.232 g/mol is 0.0437706 mol, and multiplying by the Avogadro constant gives 2.636e+22 formula units of octane. The same mass is 43.771 mmol, which is the figure a reaction is actually planned in.
50 g of octane as a molecule count
50 g divided by 114.232 g/mol is 0.437706 mol, and multiplying by the Avogadro constant gives 2.636e+23 formula units of octane. The same mass is 437.71 mmol, which is the figure a reaction is actually planned in.
How to read your score
Frequently asked questions
What is the molar mass of octane (C8H18)?
It is 114.232 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 8 × 12.011 (C) + 18 × 1.008 (H). One mole of octane therefore weighs 114.232 g, and a gram of it is 8.7541 mmol.
What percentage of octane is carbon?
84.12% by mass. Each formula unit contains 8 carbon atoms contributing 96.088 g of the 114.232 g total, so 100 g of octane contains 84.12 g of carbon and a kilogram contains 841.17 g of it.
How many molecules are in one gram of octane?
About 5.272e+21. One gram is 0.00875411 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 114.232.
How much does a single molecule of octane weigh?
1.8969e-22 g, which is 114.232 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for octane it is 114.232. Molar mass carries grams per mole: 114.232 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Carbon is quoted as 12.011 rather than a whole number for that reason, and the same applies to the other elements here, which is why octane comes out at 114.232 g/mol rather than 114.
Molar mass, composition and mole equivalents of octane
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Carbon (C) | 8 | 12.011 | 96.088 | 84.12% |
| Hydrogen (H) | 18 | 1.008 | 18.144 | 15.88% |
| Total — one mole of octane | 114.232 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 114.232 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000114232 | 0.114232 | 6.022e+17 |
| 10 µmol | 0.00114232 | 1.14232 | 6.022e+18 |
| 100 µmol | 0.0114232 | 11.4232 | 6.022e+19 |
| 1 mmol | 0.114232 | 114.232 | 6.022e+20 |
| 10 mmol | 1.14232 | 1142.32 | 6.022e+21 |
| 50 mmol | 5.7116 | 5711.6 | 3.011e+22 |
| 100 mmol | 11.4232 | 11,423.2 | 6.022e+22 |
| 250 mmol | 28.558 | 28,558 | 1.506e+23 |
| 500 mmol | 57.116 | 57,116 | 3.011e+23 |
| 750 mmol | 85.674 | 85,674 | 4.517e+23 |
| 1 mol | 114.232 | 114,232 | 6.022e+23 |
| 2 mol | 228.464 | 228,464 | 1.204e+24 |
| 5 mol | 571.16 | 571,160 | 3.011e+24 |
| 10 mol | 1142.32 | 1142320.000000 | 6.022e+24 |
Mass is the amount multiplied by 114.232 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Glycerol | C3H8O3 | 92.094 | 10.858 |
| Toluene | C7H8 | 92.141 | 10.853 |
| Creatinine | C4H7N3O | 113.12 | 8.8402 |
| Octane (this page) | C8H18 | 114.232 | 8.7541 |
| Tris base | C4H11NO3 | 121.136 | 8.2552 |
| Paracetamol | C8H9NO2 | 151.165 | 6.6153 |
| Uric acid | C5H4N4O3 | 168.112 | 5.9484 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.