Practising calculations in chemistry | 14-16 years

This activity enables students to check and practise simple calculations in chemistry. As groups of students become ‘expert’, demonstrating their ability to their peers and their teacher, they progressively validate the work of other groups.

This resource is best used to consolidate and to check on learning. Since a number of different types of calculation are included, it might well be revisited over a period of time.

Learning objectives

Students will be able to:

• Calculate the relative formula mass of a compound given its formula and appropriate atomic masses.
• Calculate the percentage of an element from a given formula.
• Calculate masses of reactants and products from balanced equations.

Sequence of activities

Introduction

1. Ask students to use ’traffic light’ cards to indicate whether they think that they can calculate the relative formula mass of a compound, if they are given its formula and appropriate relative atomic masses:
   • Green – yes
   • Red – no
   • Yellow – not sure
2. Share the first of the three learning objectives with students and explain that they will be working in groups to achieve this.
3. Repeat this process at an appropriate stage to share the second and third learning objectives with students.

Activity, stage 1: practising calculations

Divide students into groups of four, making sure that those students who showed a red or yellow card are in the same group as some who showed a green card. Ask the groups to:

1. Do the four practice calculations on their ‘Calculations in chemistry student sheet’.
2. Help each other within the group.
3. Check with the teacher that their answers are correct.

Activity, stage 2: demonstrating knowledge

At this point, form the groups into two pairs. Task each pair to devise and ask questions of the other pair, so that they can demonstrate that they can correctly calculate relative formula masses.

Activity, stage 3: designating ‘Expert’ groups

1. When a group of four students believes that they can all correctly calculate relative formula masses, validate this.
2. Select students randomly, asking them to calculate the relative formula mass of a compound, with five correct answers as the standard to achieve.
3. Designate the group as an ’Expert’ group.
4. Tick, date and sign the relevant box on their ‘Calculations in chemistry student sheet’.
5. Authorise the pairs in the ‘Expert’ group to:
   • Take on the validation role.
   • Use questions to validate other groups.
   • Designate other groups as an ‘Expert’ group.
   • Tick, date and sign their ‘Calculations in chemistry student sheet’.

Repeating the activity

Repeat the above sequence for the calculation of:

• The percentage of an element from a given formula.
• The masses of reactants and products from balanced equations.

Commentary

Using a quick assessment with ’traffic light’ cards instantly opens the learning objective for the students.

Because group performance is the focal point, it softens the focus on individuals. Furthermore, the group supports weaker students to complete the tasks successfully. The process of setting questions for their peers is as much a learning and assessment opportunity for the questioner as it is for the responder. This is the case at the early stage as well as when students take on the mantle of validating other groups.

Equipment

For each student:

• Set of ‘traffic light’ cards

Answers

Relative formula mass

What is the relative formula mass of:

1. Methane CH₄
   • Answer: 16
2. Sodium hydroxide NaOH
   • Answer: 40
3. Sulfuric acid H₂SO₄
   • Answer: 98
4. Zinc nitrate Zn(NO₃)₂
   • Answer: 189

Percentages of elements

What is the percentage of:

1. Carbon in methane CH₄
   • Answer: 75
2. Calcium in calcium carbonate CaCO₃
   • Answer: 40
3. Oxygen in sulfur dioxide SO₂
   • Answer: 50
4. Nitrogen in ammonium sulfate (NH₄)₂SO₄
   • Answer: 21

Masses of reactants and products

1. What mass of calcium oxide is formed when 10 g of calcium carbonate is completely decomposed?
   CaCO₃ CaO + CO₂
   • Answer: 5.6 g
2. What mass of sulfur dioxide is produced when 2.4 g of sulfur is burnt?
   S + O₂ SO₂
   • Answer: 4.8 g
3. What mass of carbon is needed to react with 8 g of copper(II) oxide?
   2CuO + C 2Cu + CO₂
   • Answer: 0.6 g
4. What mass of iron(III) oxide is needed to react with carbon monoxide to produce 112 g of iron?
   Fe₂O₃ +3CO 2Fe + 3CO₂
   • Answer: 160 g