The Mole Concept Explained By Analogy
Let's say you know that each bowling ball in the box weighs 1 kg and the total weight inside the box is 2 kg. Can you figure out how many bowling balls are in the box without opening it?
Of course you can! The answer is 2 bowling balls.
Easy right? If each bowling ball weighs 1 kg and we have 2 kg of bowling balls, we have 2 bowling balls.
In this situation we used the total mass of the bowling balls in the box along with the mass of each individual bowling ball to determine the number of bowling balls. This is what we call a mass to number conversion.
A general equation for this conversion is:
In chemistry, atoms are way too small to count. However, in many situations we need to know the number of atoms we have in a sample. Just like what we did with the bowling ball problem, we use the total mass of a chemical sample along with the mass of each individual atom (known as the atomic mass) to determine the total number of atoms in the sample. We can do the same thing with molecules.
So, in other words, if you had a box with 12g of carbon in it, there would be 6.02 x 1023 atoms in that box.
In chemistry the word mole is another word for Avagadro's number, 6.02x1023. A mole of eggs is equal to 6.02x1023 eggs (wow, that's a lot of eggs!). A mole of carbon atoms is equal to 6.02x1023 carbon atoms.
We use the term mole to quantify things in chemistry such as atoms or molecules. The mole makes dealing with large numbers much easier. You will encounter this term a lot in your chemistry class.
For instance, let's say we had 12.04 x 1027 atoms in a sample. Instead of writing a large number like 12.04 x 1027 we can simply write 2 moles (12.04 x 1027 = 2 x 6.02 x 1023). This is similar to writing 2 dozen instead of the number 24 (24 = 2 x 12).
Consider the following analogy. Let's say you formed a "bowling ball molecule" by attaching two 1 kg bowling balls to one 3 kg bowling ball:
Calculating the molecular mass of a molecule is no different or any more complicated. To calculate molecular mass you simply add up the atomic masses of the atoms in the molecule.
Calculate the molecular mass of a molecule of H2O. Assume that the molecule is made up of the 1H isotope of hydrogen and the 16O isotope of oxygen.
The atomic mass of 1H (hydrogen-1) is 1.01 amu. The atomic mass of 16O (oxygen-16) is 16.00 amu.
The molecular mass = 16.00 amu + 2 x 1.01 amu = 18.02 amu.
In the case of atoms, the molar mass of an element is equal to the atomic weight of the element expressed in the units of g/mole. See the examples below:
The atomic weight of hydrogen (atomic symbol = H) is 1.01. The molar mass of hydrogen is 1.01 g/mole.
The atomic weight of silver (atomic symbol = Ag) is 107.87. The molar mass of silver is 107.87 g/mole.
The atomic weight of iron (atomic symbol = Fe) = 55.85. The molar mass of iron is 55.85 g/mole.
Calculate the molar mass of CaCl2.
The first thing we want to do is find the atomic weights of the elements in the molecule on the periodic table.
We see that Ca (calcium) has an atomic weight of 40.08
We see that Cl (chlorine) has an atomic weight of 35.45
We now add the atomic weights together. We multiply the atomic weight of Cl x 2 because there are 2 Cl atoms in the molecule:
40.08 + 2 x 35.45 = 110.98 g/mole
When doing this you are converting from mass (usually in grams) to numbers (in moles). This is a very important skill that you will need to know for your chemistry class.
When we convert from grams to moles we use the general equation we talked about above:
When converting between from grams to moles we use the molar mass as the "mass of the thing". This is very important to remember.
How many moles are present in 2.0 g of CaCl2?
The first thing we need is the molar mass. We figured out above that the molar mass of CaCl2 is 110.98 g/mole.
2g/110.98 g/mole = 0.018 moles of CaCl2