Molar mass of octane (C₈H₁₈)

The molar mass of octane (C₈H₁₈) is 114.232 g/mol, summed from the IUPAC standard atomic weights of C x 8, H x 18. The reference hydrocarbon for petrol combustion arithmetic.

Mass of sample

Amount of substance

mol

Molar mass

114.232g/mol

= Exact Molar mass of octane

Moles in the sample mass: 0.087 541 144 mol
Mass of that amount of substance: 114.232
Molecules in the sample mass: 5.27185e+22

Show the working

Mass of sample = 10
Amount of substance = 1
Molar mass = 12.011*8 + 1.008*18 = 114.232
Moles in the sample mass = 10 / 114.232 = 0.087 541 1
Mass of that amount of substance = 1 * 114.232 = 114.232
Molecules in the sample mass = 0.087 541 1 * NA = 5.27185e+22

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

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The formula

Molar mass = sum over elements of (atomic weight x number of atoms)
For C₈H₁₈: 12.011 x 8 + 1.008 x 18 = 114.232 g/mol
Moles = mass / molar mass; mass = moles x molar mass
Molecules = moles x 6.02214076 x 10^23 (the Avogadro constant, exact by definition)

Molar mass in grams per mole is numerically equal to the molecular mass in daltons, so this figure doubles as the molecular weight.

Atomic weights are the IUPAC 2021 standard values. Several elements have a range rather than a single value because isotope ratios vary between sources; the conventional single value is used here.

The reference hydrocarbon for petrol combustion arithmetic.

Composition by mass

Element	Atoms	Atomic weight	Contribution (g/mol)	Share of mass
Carbon (C)	8	12.011	96.088	84.117 %
Hydrogen (H)	18	1.008	18.144	15.883 %

Contributions add to the molar mass of 114.232 g/mol. Atomic weights are the IUPAC 2021 standard values.

Frequently asked

What is the molar mass of octane?

114.232 g/mol, for the formula C₈H₁₈.

How do you convert grams of octane to moles?

Divide the mass in grams by 114.232 g/mol. One gram of octane is 0.00875411 mol.

How many grams are in one mole of octane?

114.232 g, by definition of the molar mass.

Is molar mass the same as molecular weight?

Numerically yes, for the purposes of this calculation. Molecular weight is a dimensionless ratio and molar mass carries units of g/mol, but the digits are identical.

Where the numbers come from

IUPAC 2021 standard atomic weights; Avogadro constant fixed by the 2019 SI.