Molar Mass and Percent Composition Calculator

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convert grams to moles and molecules

How to convert grams to moles and to a number of molecules

Three quantities, one bridge: molar mass links grams to moles, and the Avogadro constant links moles to particles. Enter any of the three in the conversion tab and the other two come out together.

The two bridges: molar mass and the Avogadro constant

From grams to moles you divide by the molar mass: n = m / M. From moles to particles you multiply by the Avogadro constant, N = n x N_A, with N_A = 6.02214076 x 10^23 mol^-1, an exact value since the SI redefinition in 2019. For 18 g of water (M = 18.015 g/mol): n = 18 / 18.015 = 0.9992 mol and N = 0.9992 x 6.02214076 x 10^23 = 6.017 x 10^23 molecules. In the reverse direction, 2.5 mol of CO2 weigh 2.5 x 44.009 = 110.02 g. With exam weights the arithmetic comes out round: 36 g of water is 2 mol, 44 g of CO2 is 1 mol and 360 g of glucose is 2 mol.
The number of particles depends on what is being counted. One mole of NaCl is 6.022 x 10^23 formula units, but 1.204 x 10^24 ions, because each unit brings Na+ and Cl-. One mole of H2O holds 6.022 x 10^23 molecules and 1.807 x 10^24 atoms. For a metal like gold the particle is the atom: 1.00 g of Au (196.967 g/mol) is 5.077 x 10^-3 mol, or 3.057 x 10^21 atoms. And the mass of a single water molecule is 18.015 / 6.022 x 10^23 = 2.99 x 10^-23 g, a number that explains why direct counting is impossible.
Type numbers in scientific notation without fear: the tool reads 6.02e23, 6.02 x 10^23 and 1e-3, and shows results with four significant figures, in scientific notation when the value goes above 100 thousand or below a thousandth. A quick check: 1 g of NaCl (58.44 g/mol) is 0.01711 mol, or 1.030 x 10^22 formula units, and a tablespoon of salt, about 18 g, holds close to 0.31 mol. The conversion needs no pressure or temperature, because molar mass and the Avogadro constant hold for any state of matter.

Conversions with exam weights (exact value in the notes)

Water

Input: 36 g H2O
Expected output: 2 mol

With the CIAAW table, 36 g of water is 1.998 mol.

Carbon dioxide

Input: 44 g CO2
Expected output: 1 mol

Exact: 44 / 44.009 = 0.9998 mol, or 6.021 x 10^23 molecules.

Sulfuric acid

Input: 2 mol H2SO4
Expected output: 196 g

Exact: 2 x 98.072 = 196.14 g.

Limestone

Input: 5 mol CaCO3
Expected output: 500 g

Exact: 5 x 100.086 = 500.43 g.

Glucose

Input: 360 g C6H12O6
Expected output: 2 mol

Exact: 360 / 180.156 = 1.998 mol, or 1.203 x 10^24 molecules.

Ammonia

Input: 3 mol NH3
Expected output: 51 g

Exact: 3 x 17.031 = 51.09 g.

Full tool FAQ

How do you calculate the molar mass of a compound?

Add the atomic weights of every atom in the formula. For aluminum sulfate, Al2(SO4)3: 2 Al + 3 S + 12 O = 2 x 26.9815 + 3 x 32.06 + 12 x 15.999 = 342.131 g/mol. A subscript outside parentheses multiplies everything inside them.

What is the difference between molar mass, molecular mass and molecular weight?

Why does the result differ from my textbook?

How do you calculate the molar mass of a hydrate?

How do you calculate percent composition?

How do you convert grams to moles and molecules?

How do you find the empirical formula?

Does the tool accept ions and isotopes?

Where do the atomic weights come from, and do they change?

Why do some elements show an asterisk or a dagger?

Frequently asked questions

How many molecules are in 1 mole?

6.02214076 x 10^23, the Avogadro constant. It is an exact value by definition since 2019, and applies to atoms, molecules, ions or formula units, depending on the substance.

Why does 18 g of water give 0.9992 mol and not 1 mol?

Because the real molar mass of water is 18.015 g/mol, not 18. With the Rounded mode, which uses H = 1 and O = 16, the result comes out at exactly 1 mol. The 0.08% deviation is irrelevant in practice, but shows up in exams that ask for decimals.

How do I convert a number of atoms of an element into grams?

Divide by Avogadro's number to get moles and multiply by the atomic mass. For 3.01 x 10^23 carbon atoms that is 0.5 mol and 0.5 x 12.011 = 6.01 g. In the conversion tab, choose Particles and use the element's molar mass.

Next steps

Contribution of each element to the molar mass
Element	Atoms	Atomic mass	Subtotal (g/mol)	Mass %
C	6	12.011*	72.066	40.00%
H	12	1.008*	12.096	6.71%
O	6	15.999*	95.994	53.28%
Molar mass	180.156	100%

Contribution of each element to the molar mass

Element

Atoms

Atomic mass

Subtotal (g/mol)

Mass %

12.011*

72.066

40.00%

1.008*

12.096

6.71%

15.999*

95.994

53.28%

Molar mass

180.156

100%

The two bridges: molar mass and the Avogadro constant

From grams to moles you divide by the molar mass: n = m / M. From moles to particles you multiply by the Avogadro constant, N = n x N_A, with N_A = 6.02214076 x 10^23 mol^-1, an exact value since the SI redefinition in 2019. For 18 g of water (M = 18.015 g/mol): n = 18 / 18.015 = 0.9992 mol and N = 0.9992 x 6.02214076 x 10^23 = 6.017 x 10^23 molecules. In the reverse direction, 2.5 mol of CO2 weigh 2.5 x 44.009 = 110.02 g. With exam weights the arithmetic comes out round: 36 g of water is 2 mol, 44 g of CO2 is 1 mol and 360 g of glucose is 2 mol.

The number of particles depends on what is being counted. One mole of NaCl is 6.022 x 10^23 formula units, but 1.204 x 10^24 ions, because each unit brings Na+ and Cl-. One mole of H2O holds 6.022 x 10^23 molecules and 1.807 x 10^24 atoms. For a metal like gold the particle is the atom: 1.00 g of Au (196.967 g/mol) is 5.077 x 10^-3 mol, or 3.057 x 10^21 atoms. And the mass of a single water molecule is 18.015 / 6.022 x 10^23 = 2.99 x 10^-23 g, a number that explains why direct counting is impossible.

Type numbers in scientific notation without fear: the tool reads 6.02e23, 6.02 x 10^23 and 1e-3, and shows results with four significant figures, in scientific notation when the value goes above 100 thousand or below a thousandth. A quick check: 1 g of NaCl (58.44 g/mol) is 0.01711 mol, or 1.030 x 10^22 formula units, and a tablespoon of salt, about 18 g, holds close to 0.31 mol. The conversion needs no pressure or temperature, because molar mass and the Avogadro constant hold for any state of matter.

Conversions with exam weights (exact value in the notes)

Water

Input: 36 g H2O
Expected output: 2 mol

With the CIAAW table, 36 g of water is 1.998 mol.

Carbon dioxide

Input: 44 g CO2
Expected output: 1 mol

Exact: 44 / 44.009 = 0.9998 mol, or 6.021 x 10^23 molecules.

Sulfuric acid

Input: 2 mol H2SO4
Expected output: 196 g

Exact: 2 x 98.072 = 196.14 g.

Limestone

Input: 5 mol CaCO3
Expected output: 500 g

Exact: 5 x 100.086 = 500.43 g.

Glucose

Input: 360 g C6H12O6
Expected output: 2 mol

Exact: 360 / 180.156 = 1.998 mol, or 1.203 x 10^24 molecules.

Ammonia

Input: 3 mol NH3
Expected output: 51 g

Exact: 3 x 17.031 = 51.09 g.

Full tool FAQ

How do you calculate the molar mass of a compound?

What is the difference between molar mass, molecular mass and molecular weight?

Why does the result differ from my textbook?

How do you calculate the molar mass of a hydrate?

How do you calculate percent composition?

How do you convert grams to moles and molecules?

How do you find the empirical formula?

Does the tool accept ions and isotopes?

Where do the atomic weights come from, and do they change?

Why do some elements show an asterisk or a dagger?

Frequently asked questions

How many molecules are in 1 mole?

6.02214076 x 10^23, the Avogadro constant. It is an exact value by definition since 2019, and applies to atoms, molecules, ions or formula units, depending on the substance.