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Year 10
AQA
Higher

Determining an equation from given values

I can deduce the balanced symbol equation from the masses of reactants and products of a given reaction.

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New
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Year 10
AQA
Higher

Determining an equation from given values

I can deduce the balanced symbol equation from the masses of reactants and products of a given reaction.

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Share activities with pupils
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Lesson details

Key learning points

  1. The stoichiometry of a chemical reaction is the molar ratio of each substance in a balanced symbol equation.
  2. Stoichiometric values can be calculated using masses found by experiment.
  3. Subscript values in chemical formula indicate the ratio of atoms/ions in a molecule/formula unit.

Keywords

  • Balanced symbol equation - Describes a reaction using a symbol equation with coefficients, which ensure there are equal numbers of atoms of each element on both sides of the symbol equation.

  • Stoichiometry - Refers to the molar ratio of the reactants and products in a chemical reaction.

  • Mole - A mole of something is 6.02 × 10²³ of it. The mass of a mole of a substance is its relative mass expressed in grams.

  • Ratio - Ratio can be used to determine the size of one quantity in relation to another.

Common misconception

Some pupils incorrectly interpret subscript and stoichiometric values in a balanced symbol equation.

Both subscript and stoichiometric values represent ratios. Subscripts indicate ratios *within* substances; stoichiometry. Molar ratios *between* substances in a reaction. Much practice distinguishing between/ interpreting these is time well spent.

The process of ratios is a frequent feature of calculations in chemistry. If not part of the course already, consider extending this idea to incorporate empirical formula calculations.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
A beaker contains six pens. Three pens are black, two are red and one is green. Which of the follow ratios are correct?
The ratio of black pens to green pens is 1 : 3
Correct answer: The ratio of red pens to green pens is 2 : 1
Correct answer: The ratio of green pens to black pens is 1 : 3
The ratio of red pens to black pens is 3 : 2
Q2.
Which empirical formula shows a metal to non-metal ratio of 1 : 2?
Correct answer: CuCl₂
H₂O
Na₂O
NaCl
Q3.
True or false? The unit for relative atomic mass is grams.
True
Correct answer: False
Q4.
State the unit for amount of substance.
Correct answer: Mole
Avogadro's constant
Relative atomic mass
Kilogram
Weight
Q5.
When a chemical is __________ there will always be some left over at the end of the reaction.
Correct answer: in excess
limiting
a reactant
Q6.
Calculate the relative formula mass of ammonium nitrate, NH₄NO₃. Relative atomic masses: N = 14; H = 1; O = 16.
Correct Answer: 80, Eighty

Exit quiz

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6 Questions

Q1.
The formula for sulfuric acid is H₂SO₄. What is the ratio of oxygen to sulfur in the formula?
Correct Answer: 4 : 1, 4:1, 4 :1, 4: 1
Q2.
Which of the following chemical equations shows a similar molar ratio to the reaction?: Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O
Correct answer: 2Ca(OH)₂ + 4HNO₃ → 2Ca(NO₃)₂ + 4H₂O
2Ca(OH)₂ + 2HNO₃ → 2Ca(NO₃)₂ + 2H₂O
Ca(OH)₂ + 4HNO₃ → Ca(NO₃)₂ + 4H₂O
4Ca(OH)₂ + 2HNO₃ → 4Ca(NO₃)₂ + 2H₂O
Q3.
6 moles of calcium hydroxide, Ca(OH)₂, react with excess nitric acid, HNO₃. How many moles of water are formed in the reaction? Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O
Correct Answer: 12, 12 mol, 12 moles, 12mol, 12moles
Q4.
0.5 g of magnesium reacts with oxygen to form 0.8 g of magnesium oxide. Calculate the number of moles of magnesium oxide formed during the reaction. Relative atomic masses: Mg = 24; O = 16.
0.8
Correct answer: 0.02
0.01
0.5
Q5.
What is the reacting ratio when 12.0 g of magnesium, Mg, reacts with 8.0 g of oxygen, O₂, to produce magnesium oxide?
Correct answer: Mg : O₂ = 2 : 1
Mg : O₂ = 1 : 2
Mg : O₂ = 3 : 2
Mg : O₂ = 1.5 : 1
Q6.
It is possible to determine the balanced symbol equation of a reaction from the molar ratios using the following step. Place the steps into the correct order.
1 - Write down the chemical formula for each reactant and product.
2 - Calculate the relative formula mass for each substance.
3 - Calculate the number of moles for each substance.
4 - Find the unitary ratio (one ratio value should be 1).
5 - Find the molar ratio (these should be whole numbers).
6 - Convert the molar ratio to a balanced symbol equation.