Mole concept – What are its applications?

Having understood the mole concept, let’s see how it can be applied to solve problems.

Consider the following chemical equation, which is the combustion of methane to give carbon dioxide and water:

CH_{4}\left ( g \right )+2O_{2}\left ( g \right )\rightarrow CO_{2}\left ( g \right )+2H_{2}O\left ( l \right )\; \; \; \; \;\; \; \; (3)

It clearly states that 1 molecule of methane reacts with 2 molecules of oxygen to give 1 molecule of carbon dioxide and 2 molecules of water. Since the reactant ratio of 1:2 gives the product ratio of 1:2, we can say that 1 mole of methane reacts with 2 moles of oxygen to give 1 mole of carbon dioxide and 2 moles of water if we want the equation to depict the reaction from a macroscopic point of view. For convenience, you may regard all chemical equations as if they are written from a macroscopic perspective, i.e. the numbers in front of chemical formulas in all chemical equations are in moles (molar format). Note that the number ‘1’ is not added in front of CHand COas it is redundant. The letters in brackets signify the physical states of the molecules: (g) for gaseous, (l) for liquid, (s) for solid and (aq) for aqueous.


Question 1

With reference to eq3, how many moles of COare produced from 1 mole of CHand 1.5 moles of O2?

Answer 1

From eq3, 2 moles of O2 react with 1 mole of CHto give 1 mole of COand 2 moles of water. So, 1.5 moles of Omust react with only 1.5/2 moles of CHto give 1.5/2 moles of COand 1.5 moles of water. There will be an excess of 1 – (1.5/2) = 0.5/2  moles of CHthat is unreacted. Note that Ois the limiting reactant in this example, i.e. the chemical that is totally consumed when the reaction is complete.


Let’s try another one.

Question 2

How many oxygen atoms are there in two moles of sulphate ions, SO42-and how many moles of SO42- are there in 5g of CaSO4?

Answer 2

In one ion of SO42-, there are 1 atom of sulphur and 4 atoms of oxygen (note that the superscript “2-” refers to the charge of the ion). In one mole of SO42-, there are 1 mole of sulphur and 4 moles of oxygen. Using eq1,

number (of atoms) = number of moles x 6.02 x 1023

number (of oxygen atoms) = 2 x 4 x 6.02 x 1023

Therefore, there are 4.82 x 1024 atoms of oxygen in two moles of SO42-. 1 mole of CaSOcontains 1 mole of SO42-. Using eq2 where n=\frac{m}{M}, the number of moles of SO42- in 5g of CaSOis:

n=\frac{5}{40.1+32.1+\left ( 16.0\times 4 \right)}=3.67\times 10^{-2}\: moles


next article: milestones related to the avogadro constant
Previous article: How was the avogadro constant determined?
Content page of Mole concept
Content page of basic chemistry
Main content page