What we will cover

• Revision of:

  • basic stoich concepts

  • question types

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​Important Calculation

​How to approach stoich problems

1. What do I need to get?

2. What do I need to get there?

3. Label the question!

4. Plan out an answer

5. If in doubt, convert something to moles!

   1. This will give you a molar value, which you can these use to inform other species within the reaction ​

​What does an equation tell us?

An equation gives us qualitative and quantitative information

• nitrogen gas reacts with hydrogen gas to give ammonia

• ​1 mole of nitrogen reacts with 3 moles of hydrogen, to give 2 moles of ammonia

​This can give us information about HOW MUCH of something is needed within a chemical reaction

​How much is needed?

Common question type! ​

​How many moles of oxygen are needed to react with aluminium to form aluminium oxide (Al₂O₃)?

1. Write a balanced equation

2. ​Determine the ratio between the reactants and products

3. Multiply ratio and the given amount of moles

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​How much mass is needed?

1. Need: mass of HC​l (need the molar amount to get this)

2. Have already: mass of magnesium carbonate, written reaction

3. Plan​:

   1. write full balanced equation

   2. determine mole ratio

   3. determine actual mole amount of magnesium carbonate

   4. calculate actual mole amount of HCl by mole ratio

   5. use n = m/mm to determine mass of HCl

​Limiting Reactant Calculations

• Presents you with a reaction, where one reactant will run out before the other. ​Will often ask you to determine what will be left over, and also how much.

• Other molar amounts (especially of products) are based on the limiting reactant molar amount

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• ​There is a method to working out limiting reactant questions

1. Determine molar ratio between ​the reactants

2. Determine the actual molar amount needed of each

3. ​Compare values to values given in the question, determine which will be limiting and which will have some left over

4. ​Use the molar amount of the limiting reactant to calculate the amount of moles of product

​Try it yourself

​Try it yourself