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If you’re currently preparing for your Sec 3 Chemistry test/exam, or your O Level Chemistry test/exam, and you’re not too sure about how to start solving mole calculation questions, here’s a simple 3 Step Method we use at Chem Made Easy to get students started.

This 3 step method will help you answer almost all Mole Calculation questions (if youre taking Combined Chemistry) and roughly half of the mole calculation questions (if you’re taking pure chem!) It does not include concepts on excess limiting, percentage yield and purity.

However, as the steps are pretty universal, once you get a firm grasp of the basics, you should be able to apply the skills to answer the tougher questions on your own!

Curious about just how useful this 3 Step Method is? Let’s check it out!

The 3 Step Mole Calculation Method
Step 1. Find the number of moles
Step 2. Mole ratio
Step 3. Find the unknown

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Step 1. Finding the number of moles

The first thing you want to do when you get to a mole calculation question is to calculate the number of moles of whatever information you’re given.

If you’re given the volume of gas –> use formula #2
If you’re given the mass of a reactant –> use formula #1
If you’re given concentration and volume –> use formula #3

Once you have this, you can then use the Balanced Chemical equation (if you’re not given the balanced chemical equation, you need to come up with it first) to…

Step 2. Find the mole ratio.

Given substance : Unknown substance

Based on the balanced chemical equation, you should be able to tell how many moles of product is formed, based on the given number of moles of reactants. Or you should be able to determine how many moles of the other reactant is needed, starting from a certain number of moles. (please download the pdf file for a better idea on this!)

Once you determine the number of moles of unknown, you can then move on to Step 3, which is to…

Step 3. Find the unknown
To find the unknown (volume of gas, mass, or concentration) we will need to re-use one of the formulas (1, 2 or 3).

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Hope this short explanation has helped make the chapter on mole calculation a little clearer for you to understand!