Using Moles to Balance Equations Simplified Revision Notes for GCSE AQA Chemistry

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3.2.3 Using Moles to Balance Equations

Balancing Equations with Moles

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You can also use moles to help balance chemical equations. This involves converting the mass of each substance in the reaction to moles, finding the simplest ratio of moles, and then using that ratio to balance the equation.

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Worked Example:

Imagine you are making ammonia (NH₃) by reacting nitrogen (N₂) with hydrogen (H₂). You start with 5.6 g of nitrogen and 1.2 g of hydrogen, and you end up with 6.8 g of ammonia. Here's how you would write the balanced equation:

Calculate the moles:

Moles of nitrogen:

Moles of N_2 = \frac{5.6 g}{28 (Mr of N_2) = 0.2 moles}

Moles of hydrogen:

Moles of H_2 = \frac{1.2 g}{2 (Mr of H_2) = 0.6 moles}

Moles of ammonia:

Moles of NH_3 = \frac{6.8 g}{17 (Mr of NH_3) = 0.4 moles}

Simplify the ratio by dividing all the mole amounts by the smallest one (0.2):

Nitrogen:

\frac{0.2}{0.2}=1

Hydrogen:

\frac{0.6}{0.2}=3

Ammonia:

\frac{0.4}{0.2}=2

Write the balanced equation:

N_2+3H_2→2NH_3

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