Molar Mass and Percent Composition Calculator

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how to calculate mass percent composition

How to calculate the mass percent composition of a compound

Percent composition is the fraction of the molar mass that each element accounts for, times 100. It is used to compare the nitrogen content of fertilizers, the iron content of ores and to check whether a sample is pure.

Element mass divided by molar mass

The formula is % of element = (subscript x atomic mass) / molar mass x 100, and all the percentages add up to 100. In water, H2O = 18.015 g/mol: hydrogen contributes 2.016 and gives 11.19%, and oxygen, with 15.999, gives 88.81%. In table salt, NaCl = 58.440: sodium 22.990 / 58.440 = 39.34% and chlorine 60.66%. Iron(III) oxide Fe2O3 (159.687 g/mol) has 111.69 g of Fe and 47.997 g of O per mole, or 69.94% iron and 30.06% oxygen, which is why 1 tonne of pure hematite yields about 699 kg of iron.
Nitrogen content is the number that matters in a fertilizer. Urea, CO(NH2)2 (60.056 g/mol), has 28.014 g of N per mole, or 46.65%, the highest among common solid nitrogen fertilizers. Ammonium sulfate, (NH4)2SO4 (132.134 g/mol), has 21.20% N and 24.26% sulfur. Potassium nitrate, KNO3 (101.102 g/mol), has 13.85% N and 38.67% K. To apply 100 kg of N per hectare you need 214 kg of urea (100 / 0.4665) or 472 kg of ammonium sulfate (100 / 0.2120), a ratio that changes the transport cost.
Two different formulas can have the same percent composition. Formaldehyde (CH2O), acetic acid (C2H4O2) and glucose (C6H12O6) all have 40.00% C, 6.71% H and 53.28% O, and the percentage alone cannot tell them apart: you need the molar mass (30, 60 or 180 g/mol) to know which one it is. This is the starting point of the empirical formula. Another trap is rounding too early: 11.19% + 88.81% adds up exactly, but with exam-rounded values (H = 1, O = 16) the results are 11.11% and 88.89%.

Mass percentages, rounded to whole numbers

Water

Input: H2O
Expected output: H 11% | O 89%

Exact: H 11.19% and O 88.81%.

Table salt

Input: NaCl
Expected output: Na 39% | Cl 61%

Exact: Na 39.34% and Cl 60.66%.

Hematite

Input: Fe2O3
Expected output: Fe 70% | O 30%

Exact: Fe 69.94% and O 30.06%.

Urea

Input: CO(NH2)2
Expected output: C 20% | O 27% | N 47% | H 7%

Exact: N 46.65%; the fertilizer's nitrogen content.

Ammonium sulfate

Input: (NH4)2SO4
Expected output: N 21% | H 6% | S 24% | O 48%

Exact: N 21.20%, H 6.10%, S 24.26% and O 48.43%.

Limestone

Input: CaCO3
Expected output: Ca 40% | C 12% | O 48%

Exact: Ca 40.04%, C 12.00% and O 47.96%.

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 do I add the percentages if there is rounding?

The sum can come to 99.99% or 100.01% because each row is rounded, and that is normal. The tool keeps the exact values as fractions and rounds only for display, so the display error does not propagate into the result.

What is the difference between percent by mass and by moles?

Percent by mass compares grams of each element; by moles it compares the number of atoms. In water, hydrogen is 11.19% of the mass but 66.7% of the atoms, 2 out of 3. Textbook percent composition is always by mass.

Can I go the other way, from percentages to the formula?

Yes, in the Empirical formula tab: enter the percentage or mass of each element and, if you have it, the molar mass, and the tool returns the empirical and molecular formula.

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%

Element mass divided by molar mass

The formula is % of element = (subscript x atomic mass) / molar mass x 100, and all the percentages add up to 100. In water, H2O = 18.015 g/mol: hydrogen contributes 2.016 and gives 11.19%, and oxygen, with 15.999, gives 88.81%. In table salt, NaCl = 58.440: sodium 22.990 / 58.440 = 39.34% and chlorine 60.66%. Iron(III) oxide Fe2O3 (159.687 g/mol) has 111.69 g of Fe and 47.997 g of O per mole, or 69.94% iron and 30.06% oxygen, which is why 1 tonne of pure hematite yields about 699 kg of iron.

Nitrogen content is the number that matters in a fertilizer. Urea, CO(NH2)2 (60.056 g/mol), has 28.014 g of N per mole, or 46.65%, the highest among common solid nitrogen fertilizers. Ammonium sulfate, (NH4)2SO4 (132.134 g/mol), has 21.20% N and 24.26% sulfur. Potassium nitrate, KNO3 (101.102 g/mol), has 13.85% N and 38.67% K. To apply 100 kg of N per hectare you need 214 kg of urea (100 / 0.4665) or 472 kg of ammonium sulfate (100 / 0.2120), a ratio that changes the transport cost.

Two different formulas can have the same percent composition. Formaldehyde (CH2O), acetic acid (C2H4O2) and glucose (C6H12O6) all have 40.00% C, 6.71% H and 53.28% O, and the percentage alone cannot tell them apart: you need the molar mass (30, 60 or 180 g/mol) to know which one it is. This is the starting point of the empirical formula. Another trap is rounding too early: 11.19% + 88.81% adds up exactly, but with exam-rounded values (H = 1, O = 16) the results are 11.11% and 88.89%.

Mass percentages, rounded to whole numbers

Water

Input: H2O
Expected output: H 11% | O 89%

Exact: H 11.19% and O 88.81%.

Table salt

Input: NaCl
Expected output: Na 39% | Cl 61%

Exact: Na 39.34% and Cl 60.66%.

Hematite

Input: Fe2O3
Expected output: Fe 70% | O 30%

Exact: Fe 69.94% and O 30.06%.

Urea

Input: CO(NH2)2
Expected output: C 20% | O 27% | N 47% | H 7%

Exact: N 46.65%; the fertilizer's nitrogen content.

Ammonium sulfate

Input: (NH4)2SO4
Expected output: N 21% | H 6% | S 24% | O 48%

Exact: N 21.20%, H 6.10%, S 24.26% and O 48.43%.

Limestone

Input: CaCO3
Expected output: Ca 40% | C 12% | O 48%

Exact: Ca 40.04%, C 12.00% and O 47.96%.

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 do I add the percentages if there is rounding?

What is the difference between percent by mass and by moles?

Can I go the other way, from percentages to the formula?

Yes, in the Empirical formula tab: enter the percentage or mass of each element and, if you have it, the molar mass, and the tool returns the empirical and molecular formula.