Going from grams to moles for octane means dividing by its molar mass, 114.232 g/mol. One gram is 0.00875411 mol, or 8.75411 mmol; 100 mg is 0.87541 mmol; and a 114.232 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, C8H18, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.08754 mmol of octane; on a three-decimal balance it is 1 mg, or 8.754 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
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 you weighed, in grams
114.232- The molar mass of octane in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 114.232 g/mol, the molar mass of octane.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of octane in moles
0.25 ÷ 114.232 = 0.00218853 mol, or 2.1885 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 8.754 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of octane in moles
2 ÷ 114.232 = 0.0175082 mol, or 17.508 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 8.754 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of octane in moles
25 ÷ 114.232 = 0.218853 mol, or 218.85 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 8.754 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
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 do I convert grams of octane to moles?
Divide the mass in grams by 114.232 g/mol. So 1 g is 0.00875411 mol, 10 g is 0.0875411 mol and 100 g is 0.875411 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of octane?
0.875411 mol, which is 875.411 mmol. Read the other way, one mole of octane is 114.232 g, so 100 g is 0.8754 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of octane is 8.7541 µmol, 10 mg is 87.541 µmol and 100 mg is 0.87541 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of octane, which is 0.00857903 mol rather than 0.00875411 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference for 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000009 | 0.00875411 | 8.75411 |
| 5 mg | 0.000044 | 0.0437706 | 43.7706 |
| 10 mg | 0.000088 | 0.0875411 | 87.5411 |
| 25 mg | 0.000218853 | 0.218853 | 218.853 |
| 50 mg | 0.000437706 | 0.437706 | 437.706 |
| 100 mg | 0.000875411 | 0.875411 | 875.411 |
| 250 mg | 0.00218853 | 2.18853 | 2188.53 |
| 500 mg | 0.00437706 | 4.37706 | 4377.06 |
| 1 g | 0.00875411 | 8.75411 | 8754.11 |
| 2 g | 0.0175082 | 17.5082 | 17,508.2 |
| 5 g | 0.0437706 | 43.7706 | 43,770.6 |
| 10 g | 0.0875411 | 87.5411 | 87,541.1 |
| 25 g | 0.218853 | 218.853 | 218,853 |
| 50 g | 0.437706 | 437.706 | 437,706 |
| 100 g | 0.875411 | 875.411 | 875,411 |
| 250 g | 2.18853 | 2188.53 | 2188528.608446 |
| 500 g | 4.37706 | 4377.06 | 4377057.216892 |
Every row is the mass divided by 114.232 g/mol. A balance reading to 1 mg resolves 8.754 µmol of this compound, which is the smallest step the middle columns can really move in.
| 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.