It is natural, when thinking of the amount of a substance, to think of how much it weighs or how much space it takes up. However, because densities of different substances vary, weight and volume aren’t very good guidelines to amount. A small, dense object may weigh a lot, and have more molecules of substance than a large, hollow object. To meet the needs of scientific calculation, chemists and physicists have defined the “mole” as a measure of the amount of a substance. One mole is equal to approximately 6.022 times 10 to the 23rd power atoms or molecules. The molar mass of a substance is defined relative to the mole.
Molar mass is a ratio that is used to convert a mass measurement to an amount of substance. This amount is expressed as a number of particles, such as atoms, molecules or ions. It is the ratio between the mass of something and the number of particles that form it. It is usually expressed in grams per mole, often written g/mol.
Molar mass is of great importance when setting up an experiment. If you are testing principles involving specific amounts of a substance, the molar mass allows you to figure out how much you should weigh out on your scale. As an example, consider an experiment calling for 2 moles of pure carbon. Because you know that carbon has a molar mass of 12.01 g/mol, you just multiply this figure by 2 moles to discover that you need to weigh out 24.02 g of carbon.
Molar mass is also useful in analyzing the results of experiments. If two equal amounts of moles of different substances take up different volumes, then it indicates that the molecules of the substance with the larger volume are larger than the molecules of the substance with the smaller volume.
Mass Percentage Calculations
Molar mass is also used to determine what percentage any given element in a compound contributes to the total mass of the compound. For example, consider a sample of 28.00 g of carbon monoxide. Because you know that the molar mass of carbon is 12.01 g/mol and the molar mass of oxygen is 16.00 g/mol, the carbon is responsible for 12.01/28.00 times 100 equal 42.89 percent of the total mass.
Predicting Molar Masses
Molar masses of atoms can also be used to figure out the precise molar masses of complex molecules without experimenting on them directly. By adding the molar mass for each atom that makes up the molecule, you discover what the molar mass of the molecule as a whole is.