Lesson 3: Chemical Reactions and Equations

The mole is a unit representing a certain number of things. Much the same as a dozen or a gross, but instead of being 12 or 144 things, a mole is 6.02 x 10²³ things. 

It is sometimes referred to as Avogadro's number and it is defined as the number of atoms in exactly 12 grams of Carbon-12. The mass number is the sum of the numbers of protons and neutrons present in the nucleus of an atom. The mass number is assigned a unit called atomic mass units (amu). In order to make more sensible, we use amu as gram/ mol. 

The mathematical relationship between mol, molar mass and mass can be expressed as 

 
n: mol
m: mass (gram)
M: molar mass (gram/mol) 

Using the periodic table, we can calculate the molar mass (mass of 1 mol) of the H₂O molecule. 

This brings us to the four steps that help us calculating a substance's molecular weight. 

1. Determine how many atoms of each different element are in the formula. 
2. Look up the atomic weight of each element in the periodic table
3. Add the results of step three together and round off as necessary. 

Example 2:
Calculate the number of mol found in 10.0 g of NaCl.

Solution:
Step 1. Atomic mass of Na= 22.9898, Cl= 35.4527  g/mol (again, found on the periodic table)

Step 2. The molar mass of NaCl= 22.9898+35.4527 g/mol= 58.4425 g/ mol 

Step 3.  If 58.4425 g of NaCl is 1 mol, then 10.0 g will be x mol. 
x= 10g/58.4425 g/mol= 0.1711 mol. 

Example 3:
In gram, calculate the mass of 0.50 mol of H₂O.

Solution:
Using the formula, 
  

m= n x M 
m= 0.50 mol x 18.0152 g/mol
m= 9.0076 g