Consider the balanced equation

We can balance the equation and we recognize it is an example of a formation reaction. How do we read this equation?

This equation reads 1 atom of magnesium combines with one molecule of chlorine to form one formula unit of the ionic compound magnesium chloride. If we wanted to do this reaction to prepare some magnesium chloride it would be difficult to find a single atom of magnesium and a single molecule of chlorine, even for the most experienced chemist. In the chemistry laboratory we must measure amounts of substances by mass. What is the relationship between the amount of substance and the number of atoms or molecules? How are we able to perform quantitative chemical reactions? There is an answer!

The answer to this question is found in the definition of a mol,

a mol and is defined as the number of 12C atoms in 12 g of 12C. This number has been experimentally determined as 6.022 x 1023 atoms. A mol of any substance is defined as the amount of the substance which contains the same number of units as are in 12 g of 12C or 6.023 x 1023 units of the substance.

Remember a unit is an atom, a molecule or a formula unit.

So if I have 1 mol of 12C, how many atoms of 12C do I have? Answer

If I have 1 mol of Mg atoms, how many Mg atoms do I have? Answer

If I have 1 mol of Cl2 molecules, how many molecules of Cl2 do I have? Answer

How many atoms of chlorine do I have in 1 mol of Cl2 molecules? Answer

How many formula units are in 1 mol of MgCl2? Answer

According to our definition of a mol 1 mol of 12C weighs 12 g. Interestingly the relative atomic mass of 12C is 12.000 u. 12.000u is the relative atomic mass of one 12C atom. 12.000 g is the mass of 1 mol of 12C atom. And 1 mol of 12C has 6.02 x 1023 12C atoms. The mass of one 12C atom in grams is;

12.000 g / 6.022 x 1023 12C atoms = 1.99 x 10-23 g per atom

How much does 1 mol of Mg atoms weigh? Answer

How much does 1 mol of Cl2 weigh? Answer

How much does 1 mol of MgCl2 weight? Answer

How much does 1 mol of C6Hl2O6 weigh? Answer

A molar mass is the mass in grams of 1 mol of a substance. So for the compounds listed above,

Formula

Molar Mass (g)

Mg

24.305

Cl2

70.9

MgCl2

95.2

C6Hl2O6

180

 

Here are few additional calculations.

Calculate how many mol are in 10.0 grams of glucose (C6Hl2O6).

To answer this question we need to convert grams to mol. Since 1 mol of has a mass of 180 g we can set up the conversion as;

How many molecules of glucose are in 10.0 g of C6Hl2O6?

We can now convert from mol of C6Hl2O6 to moelcules using Avogadro's number.

How many oxygen atoms are there in 10.0 grams of C6Hl2O6?

 

 

Sample Problem:

Determine the number of moles of N2O5 in 45.6 g of N2O5 gas and the number of nitrogen atoms.

Solution:

The molar mass of N2O5 is 108 g­mol-1. The sample is 45.6 g so we know there is less than a mol since a mol of N2O5 weighs 108 g. To calculate the number of moles we setup the problem in the following way;

To determine the number of nitrogen atoms in the sample we must first determine how many N2O5 molecules.

Now we can determine the number of nitrogen atoms;

 

Calculate the number of magnesium atoms in 10.0 g of magnesium.

Answer

 

Here is one of my favorite problems;