Avogadro’s number and the mole

Often students arrive at A level knowing how to calculate the number of moles of a substance but with a shaky understanding of what exactly a mole is or why we need it at all.

Atoms are really, really small. Too small to count. But we still need to know the actual numbers of particles (atoms, molecules or ions) taking part in a reaction, rather than just the mass or volume of the substances.

A mole is an amount of substance. If I asked you to get me a pair of rabbits, you would fetch me two. We just know that a pair = 2. Similarly, if I asked you to bring me a dozen kiwis, you would bring me 12.

A mole of substance contains 602200000000000000000000 particles and this is known as Avogadro’s number or the Avogadro constant. It is the number of atoms in 12g of carbon. 12, I hope you will realise, is also the relative atomic mass of carbon. This is the clever part. One mole of any substance weighs its atomic mass or molar mass in grams.

So if we know how many moles of a substance we have (divide the mass in grams by the molar mass), we can easily figure out how many particles of substance we have.

In the video, I show you how to calculate the number of particles of different substances and work through a few A level questions.

Practice questions and answers

Avogadro’s number = 6.02 x 10²³

Use the accurate relative atomic mass from your periodic table / data sheet, and remember not to round up until you get to the end of your calculation. 

1.  Calculate the number of atoms in:

(a) 18g of carbon C

(b) 14g of copper Cu

2.  Calculate the number of molecules in:

(a) 1.0g of ammonia  NH₃

(b) 7.2g of sulphur  S₈

3.  Calculate the number of ions present in:

(a) 0.5 mol of sodium chloride NaCl

(b) 14.6 g of lithium oxide. Li₂O

4.   What is the number of hydrogen atoms in 0.125mol C₂H₅OH?

Answers

1 (a) 9.03 x 10²³ 1(b) 1.33 x 10²³

2 (a) 3.54 x 10²² 2(b) 1.69 x 10²²

3 (a) 0.5 mol NaCl gives 1 mole of ions … 6.02 x 10²³

   (b) 0.4899mol of Li₂O gives 1.4698mol ions … 8.85 x 10²³

4  0.125mol of ethanol gives 0.75mol of hydrogen atoms … 4.515 x 10²³