Formulae, Equations and Amounts of Substance

Based on topic 1.1 of CCEA syllabus

  • Created by: Mary
  • Created on: 03-03-13 13:18

Balanced Equations for Unfamiliar Reactions

Quite often the question will tell you the reactants and some of the products. Use this information to write out the word equation.

1. Write out the word equation:

Calcium carbonate = Calcium oxide + carbon dioxide

2. Next work out the formulae of each of the individual reactants / products

Calcium carbonate = CaCO3

Calcium oxide = CaO

Carbon dioxide = CO2

3. Put it all together in the formula

CaCO3 = CaO + CO2

4. Make sure it balances!

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Balanced Equations for Familiar Reactions

For every reaction you come across at AS level, you should be able to write out the balanced equation.

If you know the products and reactants this is simple. 

Even if your teacher doesn't give you the formula you should try to work it out because theoretically you can be asked for the balanced formula for any reaction. 

If you still can't work it out, ask me!

Ionic equations

1. Write down the equation in terms of ions

2. Cancel out any ions that appear on both sides of the equation (spectator ions)

3. Make sure the charges balance, if necessary add electrons to either side of the equation

4. Balance the equation in terms of atoms too!

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Avogadro's Number and Molar Mass

Avogadro's number relates the mass of substance to the number of particles in the atom.

To find out how many particles there are in a given mass of substance we simply divide it by Avogadro's number.

You DON'T need to learn this, it is given to you in the exam!

Molar mass = this is the mass that is equal to one mole of substance. 

For carbon this is 12g, for carbon dioxide it is 44g and for calcium carbonate it is 100g.

To work out the molar mass all you need to do is add up the RAMs of the individual atoms - whatever answer this is, this tells you how many grams equal one mole of substance

Therefore if we know the molar mass of a substance and we know Avogadro's number, we can work out the number of moles in the substance. You simply divide the molar mass by Avogadro's number (NA)

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Reacting Masses of Substance

Working out the number of moles of substance is easy when all the reactants are in the proper amounts, but when a substance is in excess, we can't use it to calculate the moles of other substances. 

A reagent that is in excess CANNOT be used to work out the moles of other products in the balanced equation!

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Water of Crystallisation

Anhydrous = without water of crystallisation. Eg: CuSO4

Hydrated = with water of crystallisation. Eg: CuSO4.5H2O

Water of crystallisation = water that is bound to a substance as part of a crystal. 

Writted as .xH2O after the compound.

Sometimes you are asked to calculate the water of crystallisation in a compound.

1. To do this, you must first work out the mass of the original compound (eg: CuSO4)

2. Then you must work out the mass of the hydrated compound (eg: CuSO4.xH2O)

3. To work out the mass of water you subtract them:

Mass of water = Hydrated - Anhydrous

4. To work out the number of moles of water you divide the mass of water by its RMM (18)

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