How to Calculate Mass Ratio

By Kevin Lee
Carbon dioxide is made of one carbon atom and two oxygen atoms.

Elements consist of atoms that combine in predictable ways to form compounds. When observing chemical reactions or studying chemical properties, it's sometimes important to know how to compute mass ratio -- the ratio of atoms in each of a compound's elements. You can do this once you know a couple of important properties a compound's elements possess.

Identify the elements that make up your compound. For instance, when you work with water, that compound's elements are hydrogen and oxygen. If the compound is carbon dioxide, the elements are carbon and oxygen.

Write down each element's atomic mass. That value is the sum of the masses of an atom's electrons, protons and neutrons. You can find an element's atomic mass on a table that displays those values.

Write your compound's chemical formula. Water's formula, for instance, is H2O, and the 2 next to H tells you that one water molecule consists of two hydrogen atoms. Because no number appears after the oxygen symbol, O, its coefficient defaults to 1. This means that a water molecule contains one oxygen molecule.

Compute the compound's total mass by multiplying each element's atomic mass by the element's subscript and adding the two products. If your compound is carbon dioxide, whose formula is CO2, write the following equation:

compound_total_mass = (carbon atomic mass * number of carbon atoms in a CO2 molecule) + (oxygen atomic mass * number of oxygen atoms in a CO2 molecule)

After you plug in the real values, the equation appears as shown below:

compound_total_mass = (12.01 * 1) + (16.00 * 2)

Solve the equation to get 44.01 as the compound's total mass.

Divide the mass that one of the elements contributes by the compound's total mass and multiple the result by 100 to get the percentage that the element contributes to the compound's total mass. The following equation demonstrates finding carbon's contribution to CO2:

carbon_contribution = 12.01/44.01 * 100 = 27.29% carbon

Solve the equation to get 27.29 percent as carbon's percent contribution to CO2's mass. If you'd rather compute oxygen's mass ratio, solve this equation where 32, or 2 x 16.00, is the mass that oxygen contributes to CO2:

oxygen_contribution = 32/44.01 * 100 = 72.71% oxygen


These steps describe working with a carbon dioxide sample that consists of two elements. However, zinc carbonate (ZnCO3) and other compounds can contain several elements. Use the same steps to compute the mass ratios of any element within a compound.

When you search a periodic table for atomic masses, you may see other properties, such as atomic number and atomic weight. Atomic number is the number of protons in an atom and periodic tables order elements by this value. Isotopes make it possible for an element to have different masses. Atomic weight is the weighted average of the masses an element may possess.

There's a reason elements combine to form compounds in a way that seems proportionate. In 1794, Joseph Proust published the Law of Definite Proportions, which sates that that the proportion of elements by mass in a chemical compound are always the same regardless of how you prepare the compound.

About the Author

After majoring in physics, Kevin Lee began writing professionally in 1989 when, as a software developer, he also created technical articles for the Johnson Space Center. Today this urban Texas cowboy continues to crank out high-quality software as well as non-technical articles covering a multitude of diverse topics ranging from gaming to current affairs.