Molar mass of cysteine (C₃H₇NO₂S)

The molar mass of cysteine (C₃H₇NO₂S) is 121.154 g/mol, summed from the IUPAC standard atomic weights of C x 3, H x 7, N, O x 2, S.

Mass of sample

Amount of substance

mol

Molar mass

121.154g/mol

= Exact Molar mass of cysteine

Moles in the sample mass: 0.082 539 578 mol
Mass of that amount of substance: 121.154
Molecules in the sample mass: 4.97065e+22

Show the working

Mass of sample = 10
Amount of substance = 1
Molar mass = 12.011*3 + 1.008*7 + 14.007 + 15.999*2 + 32.06 = 121.154
Moles in the sample mass = 10 / 121.154 = 0.082 539 6
Mass of that amount of substance = 1 * 121.154 = 121.154
Molecules in the sample mass = 0.082 539 6 * NA = 4.97065e+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 C₃H₇NO₂S: 12.011 x 3 + 1.008 x 7 + 14.007 + 15.999 x 2 + 32.06 = 121.154 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
Carbon (C)	3	12.011	36.033	29.741 %
Sulfur (S)	1	32.06	32.06	26.462 %
Oxygen (O)	2	15.999	31.998	26.411 %
Nitrogen (N)	1	14.007	14.007	11.561 %
Hydrogen (H)	7	1.008	7.056	5.824 %

Contributions add to the molar mass of 121.154 g/mol. Atomic weights are the IUPAC 2021 standard values.

Frequently asked

What is the molar mass of cysteine?

121.154 g/mol, for the formula C₃H₇NO₂S.

How do you convert grams of cysteine to moles?

Divide the mass in grams by 121.154 g/mol. One gram of cysteine is 0.00825396 mol.

How many grams are in one mole of cysteine?

121.154 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.