36 
Calculations in chemistry 
1416 

This activity enables students to check and practice simple calculations in chemistry. They have to demonstrate their ability to their peers and their teacher. As groups of students become ‘expert’, 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

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

Share the first of the three learning objectives with students and explain that they will be working in groups to achieve this.
Repeat this process at an appropriate stage to share the second and third learning objectives with students. 

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:
 do the four practice calculations on their Calculations in chemistry sheet
 help each other within the group
 check with the teacher that their answers are correct.


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.


When a group of four students believes that they can all correctly calculate relative formula masses, validate this.
Select students randomly, asking them to calculate the relative formula mass of a compound, with five correct answers as the standard to achieve.
Designate the group as an Expert group.
Tick date and sign the relevant box on their Calculations in chemistry sheets.
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 sheets.


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.

Assessment for learning 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.
Resources
For each student


Calculations in chemistry sheet 
 Set of traffic light cards
Answers
Practice questions
What is the relative formula mass of 
methane 
CH_{4} 
Answer 16 
sodium hydroxide 
NaOH 
Answer 40 
sulfuric acid 
H_{2}SO_{4} 
Answer 98 
zinc nitrate 
Zn(NO_{3})_{2} 
Answer 189 
What is the percentage of: 
carbon 
in methane CH_{4} 
Answer 75 
calcium 
in calcium carbonate CaCO_{3} 
Answer 40 
oxygen 
in sulfur dioxide SO_{2} 
Answer 50 
nitrogen 
in ammonium sulfate (NH_{4})_{2}SO_{4} 
Answer 21 
What mass of calcium oxide is formed when 10 g of calcium carbonate is completely decomposed? 
CaCO_{3} CaO + CO_{2} 

Answer 5.6 g 
What mass of sulfur dioxide is produced when 2.4 g of sulfur is burnt? 
S + O_{2} SO_{2} 

Answer 4.8 g 
What mass of carbon is needed to react with 8 g of copper(II) oxide? 
2CuO + C 2Cu + CO_{2} 

Answer 0.6 g 
What mass of iron(III) oxide is needed to react with carbon monoxide to produce 112 g of iron? 
Fe_{2}O_{3} +3CO 2Fe + 3CO_{2} 

Answer 160 g 
