Molar mass of orthoclase feldspar (KAlSi₃O₈)

The molar mass of orthoclase feldspar (KAlSi₃O₈) is 278.326838 g/mol, summed from the IUPAC standard atomic weights of K, Al, Si x 3, O x 8.

Mass of sample

Amount of substance

mol

Molar mass

278.326 838g/mol

= Exact Molar mass of orthoclase feldspar

Moles in the sample mass: 0.035 928 982 mol
Mass of that amount of substance: 278.326 84
Molecules in the sample mass: 2.16369e+22

Show the working

Mass of sample = 10
Amount of substance = 1
Molar mass = 39.0983 + 26.9815384 + 28.085*3 + 15.999*8 = 278.327
Moles in the sample mass = 10 / 278.327 = 0.035 929
Mass of that amount of substance = 1 * 278.327 = 278.327
Molecules in the sample mass = 0.035 929 * NA = 2.16369e+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 KAlSi₃O₈: 39.0983 + 26.9815384 + 28.085 x 3 + 15.999 x 8 = 278.326838 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.

Composition by mass

Element	Atoms	Atomic weight	Contribution (g/mol)	Share of mass
Oxygen (O)	8	15.999	127.992	45.986 %
Silicon (Si)	3	28.085	84.255	30.272 %
Potassium (K)	1	39.0983	39.0983	14.048 %
Aluminium (Al)	1	26.9815384	26.981538	9.6942 %

Contributions add to the molar mass of 278.326838 g/mol. Atomic weights are the IUPAC 2021 standard values.

Frequently asked

What is the molar mass of orthoclase feldspar?

278.326838 g/mol, for the formula KAlSi₃O₈.

How do you convert grams of orthoclase feldspar to moles?

Divide the mass in grams by 278.326838 g/mol. One gram of orthoclase feldspar is 0.0035929 mol.

How many grams are in one mole of orthoclase feldspar?

278.326838 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.