The Mole

Molar Mass

Before you learn how to calculate using the mole and molarity, it is necessary to understand how to find gram formula mass, or molar mass. The gram formula mass is the mass in grams of the sum of the masses of all of the atoms in a compound. Basically, this means that you will need to find the masses of each atom present in a compound and then add all of those masses together to calculate the mass in grams of one mole of the compound. In introductory chemistry, the gram formula mass is also called the molar mass.

ChromiumYou may remember that the average atomic mass of an element is provided on the periodic table in atomic mass units (amu). The average atomic mass of any given element is equivalent to the number of grams per mole (g/mole) of that element. For example, in the snapshot here of the element chromium from the period table, you can see that chromium's symbol is Cr, its atomic number is 24, and it has an average atomic mass of 51.996 amu. This means that for each mole of chromium, there is 51.996 grams. If you were asked to calculate the molar mass of chromium trioxide (CrO3), you would know that there was one atom of chromium in the compound and that the atom of chromium had a mass of 51.996 g/mole. You will learn exactly what a mole is later, but now you will learn how to calculate molar masses.

baking sodaCalculating Molar Mass

Baking soda, or calcium phosphate, is a compound commonly used in cooking and baking. To calculate the molar mass of calcium phosphate, Ca3(PO4)2:

  1. First, find the mass of each element, based on the element atomic mass from the Periodic Table of Elements. Multiply the atomic mass by the number of atoms of each element. Remember, this is represented by the coefficient (or subscript) for each element in a compound. In Ca3(PO4)2, there are:
    • 3 atoms of Ca: 3 (40.08 g/mol) = 120.24 g/mol
    • 2 atoms of P: 2 (30.97 g/mol) = 61.94 g/mol
    • 8 atoms of O: 8 (16.00 g/mol) = 128.00 g/mol

  2. Next, add the products together: 120.24 g + 61.94 g + 128.00 g = 310.18 g/mol. The molar mass of Ca3(PO4)2 is 310.18 g/mol.

Molar Mass Review

Molar Mass Review PlayerSelf-Check iconNow that you have learned how to calculate the molar mass, review your knowledge in this non-graded activity. Examine each compound or molecule and calculate its gram formula mass. Once you have entered your answer, click SUBMIT. Click the interactivity thumbnail, and then click NEXT to get started.

 

The Mole

The Mole PlayerThe history of the molar concept can be traced several hundred years to the work of Joseph Louis Gay-Lussac and later by Amedeo Avogadro. View this presentation to learn about the mole. In addition, you will learn how to solve problems related to moles, particles, grams, and liters in compounds.

Download a printable version of the interactivity.

 

The Mole Review

The Mole Review PlayerSelf-Check iconReview your knowledge of mole calculations in this non-graded activity. Read each question, solve it on your own, and select the correct response. Then, click SUBMIT to check your response. Click the interactivity thumbnail, and then click NEXT to get started.