Practising calculations in chemistry | 14–16 years

Use this resource to consolidate and to check on learning. As different types of calculation are included, you can revisit it over a period of time.

Learning objectives

1. Calculate the relative formula mass of a compound given its formula and appropriate relative atomic masses.
2. Calculate the percentage by mass of an element from a given formula.
3. Calculate masses of reactants and products from balanced equations using the mole equation.

Teaching sequence

Introduction

Introduce learners to quantitative chemistry with this resource, focusing on the use of mathematics to understand and predict chemical reactions. While learners may already be familiar with identifying reactants and products, this topic dvelops their ability to determine how much of each substance is involved in a reaction.

Learners will calculate the relative formula mass (Mr) of compounds using relative atomic masses from the periodic table, and detmine the percentage by mass of an element with a compound in order to understand chemical composition. They will then apply this knowledge using balanced equations and the mole equation to calculate the masses of reactants and prducts in chemical reactions.

By developing these skills, learners will build their confidence in apply mathematical techniques in chemistry and gain a clearer understanding of how quantitative methods make chemistry precise, reliable and relevant to real-world applications.

Activies

The teaching sequence is the same for each learning objective:

• Give a brief introduction to the topic.
• Provide the workshed example. You can give this to learners in full afterwars, so they have an example, or ask them to copy this down as you are modelling it.
• Get learners to complete the two checkpoint questions in pairs on mini whiteboards.
• After each question, ask for a show of boards before going through the questions with cold-call questioning to the whole class.
• Ask learners to work through the final four calculations independently.

Commentary

Use a checkpoint quiz to remove some of the pressure of moving straight onto independent work, to allow you to determine who needs more support during the independent task, and to determine whether to use the scaffolded or unscaffolded sheet with your learners.

Scaffolding

• Two versions of the worksheet are available: scaffolded (one star) and unscaffolded (two stars). The different sheets are indicated by the number of stars in the header.
• The scaffolded worksheet provides support to learners through backwards fading for calculating relative formula mass and percentage by mass calculations. This is where examples are partially completed and the amount of completion is lowered through each example.
• For calculations of masses of reactants and products from balanced equations using the moles equation, step by step instructions are given.

Answers

Answers can be found in the teacher notes, with full working shown in the lesson slides for self-assessment.