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Terms of UseHow to Find Mass Percent Explore this Article parts 1 Understanding the Fundamentals 2 Calculating Mass Percent Other Sections Tips and Warnings Related Articles References Article Summary Author Info

Last Updated: January 31, 2021 References

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wikiHow is a “wiki,” similar to Wikipedia, which means that many of our articles are co-written by multiple authors. To create this article, 11 people, some anonymous, worked to edit and improve it over time. This article has been viewed 19,758 times.

The mass percent composition of a molecule is the percent contribution of each individual element to a molecule's mass. Percent of mass of one element in a compound is expressed as a ratio of mass contribution to the element and the total molecular mass of the compound multiplied by 100%. It sounds complicated, but finding the mass percent is actually a simple process!

Steps

Part 1 Part 1 of 2:

Understanding the Fundamentals

1. 1 Understand atoms. Atoms are basic units of matter, composed of protons, neutrons, and electrons. These are the fundamental building blocks of everything around you.
2. 2 Understand molecules. Molecules are electrically neutral chemical groups of two or more atoms, which are held together by chemical bonds.
   • Water, for example, is made of the molecule H2O. Each water molecule is composed of two hydrogen atoms, which are chemically bonded to one oxygen atom.
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3. 3 Understand moles. A mole is a unit of measurement used to express the amount of a chemical substance in a sample. It is defined as the amount of any substance that contains 6.02 x10^ 23 elementary entities. Use moles as a way to concisely express amounts of chemical substances.
   • 6.02 x 10^23, which is approximately the number of carbon atoms in 12 grams (0.42 oz) of pure carbon, is also called “Avogadro's Number.”
4. 4 Understand chemical compounds. Chemical compounds are pure chemical substances that consist of two or more chemical elements.
   • Water, made up of the molecule H2O, is a chemical compound.
5. 5 Understand atomic mass. Atomic mass is the mass of an atomic particle, sub-atomic particle, or molecule. It is expressed in grams per mole (g/mol).
   • Note that atomic mass is different from atomic weight and specifically refers to the total mass of the particle, subatomic particle, or molecule at rest.
6. 6 Understand molecular mass. Molecular mass is the mass of a single molecule. In other words, it is the total mass of all of the individual atoms that make up a given molecule.
   • As with atomic mass, molecular mass differs from molecular weight.
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Part 2 Part 2 of 2:

Calculating Mass Percent

1. 1 List the elements in the compound.
   • For example, say you need to calculate the mass percent of citric acid, C6H8O7, as shown in the picture. The elements of this compound are carbon (C), hydrogen (H), and oxygen (O).
2. 2 Write down the atomic mass of each individual element. Use the periodic table to locate these numbers. It is usually located just below the atomic symbol, in the units of g/mol.
   • In the example above, you would note that the atomic mass of carbon is 12.01 g/mol, the atomic mass of hydrogen is 1.00 g/mol, and the atomic mass of oxygen is 15.99 g/mol.
3. 3 Find the mass contribution of each element in your compound. The mass contribution of an element in a compound is the atomic mass of the element multiplied by the number of atoms in the element in a single molecule of the compound. The numeric subscript after each atomic symbol signifies the number of atoms of that element in the compound.
   • In the example above, recall that each molecule of your compound is made up of 6 atoms of carbon, 8 atoms of hydrogen, and 7 atoms of oxygen. Therefore, you would calculate the mass contribution of the elements as shown in the picture. Mass contribution of carbon: 6 x 12.01 g/mol = 72.06 g/molMass contribution of hydrogen: 8 x 1.00 g/mol = 8.00 g/molMass contribution of oxygen: 7 x 15.99 g/mol = 111.93 g/mol
4. 4 Calculate the total molecular mass of your compound. Total molecular mass of a compound is expressed as a sum of the mass contributions of the individual elements in the compound. It represents the mass of a single molecule.
   • In the example above, calculate your total molecular mass as follows:Molecular mass of C6H8O7 = 72.06 g/mol + 8.00 g/mol + 111.93 g/mol = 191.99 g/mol
5. 5 Find the mass percent composition. The mass percent composition of an element is expressed as a ratio of mass contribution of the element and total molecular mass of the compound multiplied by 100%.
   • In the example above, calculate your mass percent of each element as shown in the photo. You can therefore conclude that C6H8O7 consists of 37.53% carbon, 4.16% hydrogen, and 58.29% oxygen.
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Tips

• You can double check your work by adding your percentages together and verifying that they add up to 100%. Be aware, though, that you may not get exactly 100%. This is because mass contributions of each element are rounded to two places after the decimal point. Thanks Helpful 0 Not Helpful 0

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References

1. http://chemistry.about.com/od/workedchemistryproblems/a/How-To-Calculate-Mass-Percent.htm
2. https://en.wikipedia.org/wiki/Atom
3. http://chemistry.about.com/od/moleculescompounds/f/What-Is-A-Molecule.htm
4. http://chemistry.about.com/cs/generalchemistry/f/blmole.htm
5. http://www.universetoday.com/73476/what-is-atomic-mass/

About this article

wikiHow is a “wiki,” similar to Wikipedia, which means that many of our articles are co-written by multiple authors. To create this article, 11 people, some anonymous, worked to edit and improve it over time. This article has been viewed 19,758 times. 4 votes - 30% Co-authors: 11 Updated: January 31, 2021 Views: 19,758 Article SummaryX

To find mass percent, start by listing the elements in the compound, and using the periodic table to find the atomic mass of each element. Next, find the mass contribution of the elements by multiplying the atomic mass by the number of atoms in the element. Then, calculate the total molecular mass of your compound by adding the mass contributions of the individual elements in the compound. Finally, find the mass percent composition by creating a ratio of mass contribution of the element and the total molecular mass of the compound multiplied by 100%. To learn how to calculate the mass percent of citric acid, keep reading! Did this summary help you?YesNo

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About this article

4 votes - 30% Click a star to vote 30% of people told us that this article helped them. Co-authors: 11 Updated: January 31, 2021 Views: 19,758

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