The mass percent composition of a molecule is the percent contribution of each element to its molecular mass. The mass percent of an element in a compound is expressed as the ratio of the mass contribution of the element to the total molecular mass of the compound multiplied by 100%. This sounds complicated, but finding the mass percent is actually a simple process!
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Part 1 of 2: Understanding the Basics
Step 1. Understand about atoms
Atom is the basic unit of an object, which consists of protons, neutrons, and electrons. These are the basic ingredients of everything around you.
Step 2. Understand about molecules
A molecule is an electrically neutral group of chemical substances, consisting of two or more atoms, which are held together by chemical bonds.
For example, water is made up of H2O molecules. Each water molecule consists of two hydrogen atoms, which are chemically bonded to one oxygen atom
Step 3. Understand about moles
The mole is a unit of measurement used to express the amount of a chemical in a sample. A mole is defined as the amount of any substance that contains 6.02 x10^23 basic entities. Use moles as a way of expressing the exact amount of a chemical.
6.02 x 10^23, which is approximately the number of carbon atoms in 12 grams of pure carbon, is also known as "Avogadro's Number"
Step 4. Understand about chemical compounds
A chemical compound is a pure chemical substance consisting of two or more chemical elements.
Water, which consists of H2O molecules, is a chemical compound
Step 5. Understand about atomic mass
Atomic mass is the mass of atomic particles, sub-atomic particles, or molecules. The atomic mass 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 a particle, sub-atomic particle, or molecule at rest
Step 6. Understand about molecular mass
Molecular mass is the mass of a single molecule. In other words, the molecular mass is the total mass of all the individual atoms that make up a given molecule.
Like atomic mass, molecular mass is different from molecular weight
Part 2 of 2: Calculating Mass Percent
Step 1. Write down the elements in the compound
For example, you must calculate the mass percent of citric acid, C6H8O7, as shown in the figure. The elements of this compound are carbon (C), hydrogen (H), and oxygen (O)
Step 2. Write down the atomic mass of each individual element
Use the periodic table to locate these numbers. Usually, the atomic mass is located below the atomic symbol, in units of g/mol.
In the example above, you will 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
Step 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 times the number of atoms in the element, in a single molecule of that compound. Small numbers written at the bottom after each atomic symbol, indicate the number of atoms of that element in the compound.
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In the example above, recall that each element of your compound consists of 6 carbon atoms, 8 hydrogen atoms, and 7 oxygen atoms. Thus, you will calculate the mass contribution of the element as shown in the figure.
Carbon mass contribution: 6 x 12.01 g/mol
Mass contribution of hydrogen: 8 x 1.00 g/mol = 8.00 g/mol
Mass contribution of oxygen: 7 x 15.99 g/mol = 111.93 g/mol
Step 4. Calculate the total molecular mass of your compound
The total molecular mass of a compound, denoted as the sum of the mass contributions of the individual elements in the compound. This total mass represents the mass of a single molecule.
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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
Step 5. Find the mass percent composition
The mass percent composition of an element is denoted as the ratio between the mass contribution of the element to the total molecular mass of the compound, multiplied by 100%.
