Mass fraction is the ratio of the mass of a component to the total mass of all parts in a molecule, mixture, or reaction. It can be calculated using the periodic table and basic math skills. Fractions are used to calculate the mass fraction of atoms or molecules, and all fractions should add up to 1. Molecules can be combined into a mixture, and the mass fraction can be calculated for each molecule. The same can be done for atomic fraction by adding all of the same atoms together and creating a graph.
Mass fraction is the ratio of the mass that one part of a molecule, mixture, or reaction contributes to the total mass of all its parts. Molecules are groups of atoms, mixtures are groups of molecules, and reactions rearrange atoms into new molecules. All atoms and molecules have a specific mass, and when they are combined, it is often useful to know the mass fraction of an atom or molecule in a mixture or reaction. There are several ways to find mass fraction, but most require a periodic table of elements and basic math skills.
All of the mass fraction examples use a fraction calculation. With fractions, it’s important to think of parts of the whole, and the mass fraction of any component is the part of the whole mass that makes up a particular component. This leads to dividing the mass of a single component by the total mass in the molecule or mixture to calculate the fraction of that component, denoted Xf. There are three atoms in water: two hydrogen atoms and one oxygen atom. Using the Periodic Table of the Elements to find the atomic numbers of hydrogen and oxygen, each mass fraction can be calculated, and all fractions should always add up to 1.
Molecules can be combined into a mixture such as acid and water. This leads to a similar calculation that gives the mass fraction of each molecule instead of each atom. Water is a common solvent for acid, the mass of which is 18 atomic mass units (amu). Hydrochloric acid is approximately 36 amu and is often added to water to produce hydrochloric acid. If one of each molecule is added to a mixture, or a one-to-one mixture is made, the fraction for each is not ½ because hydrochloric acid weighs twice as much water per molecule.
The same example can be used to show the atomic fraction instead of the molecular fraction. To calculate how much mass certain atoms contribute to the total mass, you first need to add all of the same atoms together and create a graph. In hydrochloric acid, there are three hydrogen atoms for every oxygen atom in chlorine. When the mass fraction of hydrogen is calculated, it becomes three times the atomic number divided by the total mass, which is approximately 54 amu. In this example, hydrogen contributes 0.056 of the total mass because three times 1 amu divided by 54 amu equals 0.056.
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