1.1.4 The Mole

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The Mole
A mole is just a very large number of particles. Chemists often talk about `amount of substance'.
Basically all they mean is number of particles. Amount of substance is measured using a unit
called the mole (mol for short) and is given the symbol n. One mole is roughly 6.02 x 1023
particles, the Avogadro Constant.
You can calculate the number of moles from the number of atoms or molecules:
Number of moles = Number of particles you have
Number of particles in a mole
Molar mass is the mass of one mole of something. But the main thing to remember is molar
mass is just the same as the relative molecular mass.
E.g. Find the molar mass of CaCO3:
Relative formula mass of CaCO3 = 40 + 12 + (3 x 16) = 100
So the molar mass is 100 g mol-1 i.e. 1 mole of CaCO3 weighs 100g
A really important formula is:
Example 1: How many moles of aluminum oxide are present in 5.1g of Al2O3?
Molar mass of Al2O3 = (2 x 27) + (3 x 16)
= 102 g mol-1
Number of moles of Al2O3 = 5.1 = 0.05 moles
Example 2: How many moles of chlorine molecules are present in 71g of chlorine gas?
Molar mass of Cl2 = (2 x 35.3) = 71 g mol-1
Number of moles of Cl2 = 71 = 1 mole
In a solution the concentration is measured in mol dm-3. It's how many moles are dissolved in 1
dm3 of solution. The formula is:

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Example 1: What mass of sodium hydroxide needs to be dissolved to make 50cm3 of a 2M
Number of moles = 2 x 50 = 0.1 moles of NaOH
Molar mass of NaOH = 23 + 16 + 1 = 40 g mol-1
Mass = number of moles x molar mass = 0.1 x 40 = 4g
A solution that has more moles per dm3 than another is more concentrated. A solution that has
fewer mole per dm3 than another is more dilute.…read more


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