AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Using moles to balance equations (HT only): The balancing numbers in a symbol equation can be calculated from the masses of reactants and products by converting the masses in grams to amounts in moles and converting the numbers of moles to simple whole number ratios.
This Higher Tier skill finds the balancing numbers in a symbol equation from experimental masses.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This Higher Tier skill finds the balancing numbers in a symbol equation from experimental masses. Convert each mass in grams to moles by dividing by its relative formula mass (Mr). Then divide each number of moles by the smallest value to get a simple ratio. If the ratio is not whole numbers, multiply all values by the same factor to obtain the simplest whole number ratio. For example, if 2.4 g of Mg reacts with 1.6 g of O₂, moles Mg = 2.4 ÷ 24 = 0.10 mol and moles O₂ = 1.6 ÷ 32 = 0.050 mol. Dividing by 0.050 gives Mg:O₂ = 2:1, so the equation is 2Mg + O₂ → 2MgO. The ratio gives the balancing numbers.
What earns the marks
- Calculate the relative formula mass (Mr) for each reactant and product from its formula.
- Convert each given mass to moles using moles = mass ÷ Mr.
- Divide each number of moles by the smallest number of moles to obtain a simple ratio.
- If necessary, multiply all values by the same whole number to convert the ratio to the simplest whole numbers.
- Write the balanced symbol equation using the whole number ratio as the balancing numbers.
Common mistakes and how to put them right
Dividing mass by mass instead of converting each mass to moles first.
Put it right: Convert grams to moles using Mr before finding the ratio.
Rounding mole values too early, which changes the final ratio.
Put it right: Keep values unrounded until the ratio is found, then round only the final whole numbers.
Forgetting to multiply all parts of the ratio by the same factor when a fractional ratio appears.
Put it right: Multiply every value by the same number to reach whole numbers.
Exam tips
- Show the moles of each substance clearly, for example 'moles of Mg = 2.4 ÷ 24 = 0.10 mol'.
- State the ratio before writing the equation, for example 'Mg:O₂ = 2:1', so your reasoning is visible.
- Check that the final equation is balanced by counting atoms of each element on both sides.
Try the lesson
The start of the Using moles to balance equations (HT only): The balancing numbers in a symbol equation can be calculated from the masses of reactants and products by converting the masses in grams to amounts in moles and converting the numbers of moles to simple whole number ratios. lesson, made from checked teaching notes for this specification point.
Have a go first
In a reaction, 5.6 g of metal Y (Aᵣ = 56) reacts completely with 10.65 g of chlorine gas (Cl₂, Mᵣ = 71) to form 16.25 g of a chloride. Which equation correctly represents this reaction?
Listen
This Higher Tier skill finds the balancing numbers in a symbol equation from experimental masses. Convert each mass in grams to moles by dividing by its relative formula mass (Mr). Then divide each number of moles by the smallest value to get a simple ratio. If the ratio is not whole numbers, multiply all values by the same factor to obtain the simplest whole number ratio. For example, if 2.4 g of Mg reacts with 1.6 g of O₂, moles Mg = 2.4 ÷ 24 = 0.10 mol and moles O₂ = 1.6 ÷ 32 = 0.050 mol. Dividing by 0.050 gives Mg:O₂ = 2:1, so the equation is 2Mg + O₂ → 2MgO. The ratio gives the balancing numbers. A strong answer earns marks for these points. Calculate the relative formula mass (Mr) for each reactant and product from its formula. Convert each given mass to moles using moles = mass ÷ Mr. Divide each number of moles by the smallest number of moles to obtain a simple ratio. If necessary, multiply all values by the same whole number to convert the ratio to the simplest whole numbers. A common slip: Dividing mass by mass instead of converting each mass to moles first. Instead, convert grams to moles using Mr before finding the ratio. Exam tip: Show the moles of each substance clearly, for example 'moles of Mg = 2.4 ÷ 24 = 0.10 mol'.
See it
Key words
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Mole
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Relative formula mass
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Simple whole number ratio
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Balancing numbers
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See every definition
- Mole
- An amount obtained by converting a mass in grams, used with relative formula mass to find the balancing numbers in a symbol equation.
- Relative formula mass
- The value each mass in grams is divided by to convert it to an amount in moles.
- Simple whole number ratio
- What the numbers of moles are converted to, after dividing each by the smallest number of moles, to give the balancing numbers.
- Balancing numbers
- The values in a symbol equation that can be calculated from the masses of reactants and products using moles and a simple whole number ratio.
Label the diagram
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1Box 1
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3Box 3
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