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What are Moles?

A mole is the mass of a substance that contains 6.02 x 10²³ particles of that substance. 
Symbol: n
SI unit: mol

1 mol of carbon-12 has a mass of 12 grams and contains 6.02 x 10²² of carbon atoms. 

To find the number of moles of a substance, we need to use the following formula:
Number of moles = Mass of substance ÷ Mr of molecule
n = M ÷ Mr

In the above equation,


Find the number of moles in 10 grams of H₂O. 
(Ar of H = 1, O = 16)

We first need to find the Mr of H₂O.
There are two atoms of Hydrogen.
Ar of each atom of H = 1. Ar of two atoms of H = 1 x 2 = 2
There is one atom of Oxygen. Ar of each atom of O = 16. 
Mr of H₂O = 2 + 16 = 18

We are given the mass = 10 g
n = M ÷ Mr
n = 10 ÷ 18
n = 0.5555 ≈ 0.56 mol

You can rearrange the formula to make (M) or (Mr) the subject of the formula. 

Question can also involve calculating moles from concentration and volume. If we are given the concentration and volume of a substance, then we can calculate the mole with the following equation:

Moles = Concentration x Volume
n = c x v

Majority of the times, the volume will be given in cm³ and the concentration will be given in mol dm⁻³
You will have to convert the volume given into dm³.

To convert cm³ to dm³, we need to divide the volume by 1000. 
100 cm³ = 0.1 dm³
25 cm³ = 0.025 dm³
50 cm³ = 0.05 dm³

You are given a 25 cm³ sample of HCl of concentration 0.02 mol dm⁻³. Find the moles of HCl present.

We are given the concentration and the volume of HCl. 
We first need to convert the 25 cm³ to dm³.
25 cm³ = 0.025 dm³

n = c x v
n = 0.02 x 0.025 = 0.0005 mol

You can rearrange the formula to make concentration or volume the subject of the formula. 

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