Remember that a mole of any particle will not have the same mass as a mole of another particle. For example, a mole of iron will not have the same mass as a mole of copper or a mole of tin or a mole of iodine. The picture below shows one mole of atoms of iron, copper, tin, and iodine.
Source: Visualizing Moles, Newfoundland and Labrador Center for Distance Learning and Innovation
It is important to remember the following:
The mass of one mole is based on the average atomic mass on the periodic table.
The average atomic mass (amu) is an average of all elements and their isotopes that occur in nature.
Ex: One water molecule has an atomic mass of 18 amu, so its molar mass is 18 g/mol.
1 amu = molar mass of a compound in the units g/mol.
Some elements occur as diatomic molecules. An easy way to remember these molecules is in the sentence: Horses Need Oats For Clear Brown I’s (H, N, O, F, Cl, Br, and I).
Molar mass may be called atomic mass or formula mass. Use the term molar mass for the mass of one mole of any substances regardless of its composition.
In order to convert between mass and moles, you need to know the molar mass of the substance.
This equality can be written as a set of two conversion factors. They are as follows:
1 mole over molar mass (g)1 molemolar mass (g) or
molar mass (g) over 1 molemolar mass (g) 1 mole
Let’s practice a few mole-molar mass conversions. You will use the steps for dimensional analysis to help solve these problems.
Write the given information as a fraction by placing it over 1. (Placing it over 1 makes it a fraction but does not change its value.)
Write a conversion factor that has the unit you want to remove in the denominator and the unit you want to end up with in the numerator. After you fill in your units, add the numbers. (Usually one of the numbers is a 1, but it can be in either the denominator or the numerator.) Note: In some cases you may need to repeat this step a number of times in order to get the unit you want to end up with in the numerator.
Mark through the units to double check that they all cancel and that you are left with the units you want.
Multiply the numbers in the numerators, and then multiply the numbers in the denominators.
Divide the numerator by the denominator. Be sure to add your units to your final answer.
Practice Problem 1
How many moles are in 28.0 grams of carbon dioxide, CO2?
