Theoretical Masses of Products (AQA GCSE Chemistry): Revision Notes

Theoretical mass is the maximum amount of product that could be formed from a given amount of reactants, assuming the reaction goes perfectly with no losses or side reactions.

How to Calculate Theoretical Mass:

• To find the theoretical mass of a product:

1. Use the balanced chemical equation to see the ratio of reactants to products.
2. Calculate the moles of reactants you're starting with.
3. Use the mole ratio from the equation to find the moles of product you should get.
4. Convert moles to grams using the relative formula mass (Mr) of the product.

Step 1: Use the balanced chemical equation: The balanced equation for the reaction between hydrogen and oxygen to form water is:

This equation tells us that 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water.

Step 2: Calculate the moles of reactants:

• First, find the relative formula mass (Mr) of hydrogen gas (H₂):

• The atomic mass of hydrogen (H) is 1, so:

• Now, calculate the moles of hydrogen gas you have:

Step 3: Use the mole ratio:

• The balanced equation tells us that 2 moles of H₂ produce 2 moles of H₂O.
• Since we have 4 moles of H₂, and the ratio is 1:1, we will produce 4 moles of H₂O.

Step 4: Convert moles to grams:

• Find the relative formula mass (Mr) of water (H₂O):
• The atomic mass of hydrogen (H) is 1, and oxygen (O) is 16, so:

• Now, calculate the theoretical mass of water: