Molar Mass and Percent Composition Calculator

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molar mass of common compounds

Molar mass of common compounds (H2O, CO2, NaCl, H2SO4 and more)

Every value below is the sum of CIAAW 2024 atomic weights and comes in two versions: the exact one, for the lab, and the rounded one that school exams use with H = 1, C = 12, O = 16. Type any other formula into the tool for the same calculation.

The values and why two numbers differ

With the CIAAW 2024 table and the conventional values for elements of variable weight, molar masses in g/mol are: water 18.015; carbon dioxide 44.009; ammonia 17.031; methane 16.043; hydrochloric acid 36.458; nitric acid 63.012; sodium hydroxide 39.997; calcium hydroxide 74.092; calcium carbonate 100.086; sodium chloride 58.440; sulfuric acid 98.072; ethanol 46.069; potassium permanganate 158.032 and glucose 180.156. Each one is a simple sum: in NaOH, 22.990 + 15.999 + 1.008 = 39.997.
Exams use whole-number weights, with two half-integer exceptions, Cl = 35.5 and Cu = 63.5. That gives 18 (water), 44 (CO2), 17 (NH3), 16 (CH4), 36.5 (HCl), 63 (HNO3), 40 (NaOH), 74 (Ca(OH)2), 100 (CaCO3), 58.5 (NaCl), 98 (H2SO4), 46 (ethanol), 158 (KMnO4) and 180 (glucose). The gap to the exact value runs from a few thousandths to a few tenths: 0.015 g/mol for water, 0.072 for H2SO4 and 0.156 for glucose. It only matters when the problem asks for decimal places, and it is best to follow the table the problem provides.
Reading the list by proportion helps remembering: the molar mass of CO2 (44) is almost 2.4 times that of water (18), which is why a mole of carbon dioxide weighs much more than a mole of water vapor while both fill the same volume at the same temperature and pressure. Glucose, with 24 atoms, weighs 180; methane, with 5, weighs 16. What decides the weight is the sum of the atomic weights, not the number of atoms: NaCl has only 2 atoms and weighs 58.5, more than ethanol with 9 atoms at 46.

Exam-style molar masses (rounded weights)

Water

Input: H2O
Expected output: 18 g/mol

Exact value 18.015 g/mol: 2 x 1.008 + 15.999.

Carbon dioxide

Input: CO2
Expected output: 44 g/mol

Exact value 44.009 g/mol.

Sulfuric acid

Input: H2SO4
Expected output: 98 g/mol

Exact value 98.072 g/mol; 2 H, 1 S and 4 O.

Limestone

Input: CaCO3
Expected output: 100 g/mol

Exact value 100.086 g/mol; the round 100 eases calculations.

Glucose

Input: C6H12O6
Expected output: 180 g/mol

Exact value 180.156 g/mol.

Potassium permanganate

Input: KMnO4
Expected output: 158 g/mol

Exact value 158.032 g/mol; K 39, Mn 55, O 4 x 16.

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

What is the molar mass of water?

18.015 g/mol with the CIAAW 2024 table, and 18 g/mol in exams. A mole of water, 6.022 x 10^23 molecules, weighs 18.015 g, which is why 1 liter of water (about 1000 g) holds close to 55.5 mol.

Is NaCl 58.44 or 58.5?

Both appear. 58.440 comes from Na = 22.990 and Cl = 35.45, the current table. 58.5 uses Na = 23 and Cl = 35.5, the exam convention. For 1 mol of table salt weigh 58.44 g.

How do I calculate one that is not on the list?

Type the formula into the tool: it accepts parentheses, hydrates and ions, shows atom by atom, and gives the rounded version in the Rounded mode.

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 values and why two numbers differ

With the CIAAW 2024 table and the conventional values for elements of variable weight, molar masses in g/mol are: water 18.015; carbon dioxide 44.009; ammonia 17.031; methane 16.043; hydrochloric acid 36.458; nitric acid 63.012; sodium hydroxide 39.997; calcium hydroxide 74.092; calcium carbonate 100.086; sodium chloride 58.440; sulfuric acid 98.072; ethanol 46.069; potassium permanganate 158.032 and glucose 180.156. Each one is a simple sum: in NaOH, 22.990 + 15.999 + 1.008 = 39.997.

Exams use whole-number weights, with two half-integer exceptions, Cl = 35.5 and Cu = 63.5. That gives 18 (water), 44 (CO2), 17 (NH3), 16 (CH4), 36.5 (HCl), 63 (HNO3), 40 (NaOH), 74 (Ca(OH)2), 100 (CaCO3), 58.5 (NaCl), 98 (H2SO4), 46 (ethanol), 158 (KMnO4) and 180 (glucose). The gap to the exact value runs from a few thousandths to a few tenths: 0.015 g/mol for water, 0.072 for H2SO4 and 0.156 for glucose. It only matters when the problem asks for decimal places, and it is best to follow the table the problem provides.

Reading the list by proportion helps remembering: the molar mass of CO2 (44) is almost 2.4 times that of water (18), which is why a mole of carbon dioxide weighs much more than a mole of water vapor while both fill the same volume at the same temperature and pressure. Glucose, with 24 atoms, weighs 180; methane, with 5, weighs 16. What decides the weight is the sum of the atomic weights, not the number of atoms: NaCl has only 2 atoms and weighs 58.5, more than ethanol with 9 atoms at 46.

Exam-style molar masses (rounded weights)

Water

Input: H2O
Expected output: 18 g/mol

Exact value 18.015 g/mol: 2 x 1.008 + 15.999.

Carbon dioxide

Input: CO2
Expected output: 44 g/mol

Exact value 44.009 g/mol.

Sulfuric acid

Input: H2SO4
Expected output: 98 g/mol

Exact value 98.072 g/mol; 2 H, 1 S and 4 O.

Limestone

Input: CaCO3
Expected output: 100 g/mol

Exact value 100.086 g/mol; the round 100 eases calculations.

Glucose

Input: C6H12O6
Expected output: 180 g/mol

Exact value 180.156 g/mol.

Potassium permanganate

Input: KMnO4
Expected output: 158 g/mol

Exact value 158.032 g/mol; K 39, Mn 55, O 4 x 16.

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

What is the molar mass of water?

18.015 g/mol with the CIAAW 2024 table, and 18 g/mol in exams. A mole of water, 6.022 x 10^23 molecules, weighs 18.015 g, which is why 1 liter of water (about 1000 g) holds close to 55.5 mol.

Is NaCl 58.44 or 58.5?

Both appear. 58.440 comes from Na = 22.990 and Cl = 35.45, the current table. 58.5 uses Na = 23 and Cl = 35.5, the exam convention. For 1 mol of table salt weigh 58.44 g.

How do I calculate one that is not on the list?

Type the formula into the tool: it accepts parentheses, hydrates and ions, shows atom by atom, and gives the rounded version in the Rounded mode.