ScienceGCSE chemistry: quantitative chemistry and chemical change30 min★★★ difficulty

Quantitative chemistry: masses, moles and yield

Quantitative chemistry turns equations into predictions: how much product you should get, and how much you actually did.

Lesson overview

What you'll learn in this lesson

Carry out quantitative chemistry calculations and explain chemical changes.

Key learning points

• Relative formula mass and moles
• Using equations
• Yield and atom economy

This lesson at a glance

30 minutes
15 parts to scroll through
4 quick checks
Marked quiz at the end
Gentle pace: short sittings with pauses

Words to know

quantitativechemistrycalculationschemicalchanges

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Part 1 of 15 · Discover7%

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Part 1 of 15

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Quantitative chemistry: masses, moles and yield

An English teaching video on this topic from Brainstorm - Maths and Science. Watch it, then scroll down for the lesson.

In a nutshell

Carry out quantitative chemistry calculations and explain chemical changes.

Learning cycle

Part 2 of 15

Learning cycle 1 of 2

Part 1 · Relative formula mass and moles

A short piece of teaching, then a check to make sure it has landed.

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Part 3 of 15

Learn

Relative formula mass and moles

Relative formula mass, Mr, is the sum of the relative atomic masses in a formula. Moles = mass ÷ Mr, so 44 g of CO₂ (Mr = 44) is one mole. Setting out mass, Mr and moles in a small table before calculating prevents most errors.

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Part 4 of 15

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Using equations

The balancing numbers give the ratio of moles reacting. Convert the known mass to moles, apply the ratio, then convert back to a mass. A limiting reactant is the one that runs out first and therefore determines how much product forms; the other is in excess.

Quick check

Part 5 of 15

Quick check

Moles equals

Quick check

Part 6 of 15

Quick check

The limiting reactant is the one that

Reset break

Part 7 of 15

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

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Learning cycle

Part 8 of 15

Learning cycle 2 of 2

Part 2 · Yield and atom economy

A short piece of teaching, then a check to make sure it has landed.

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Yield and atom economy

Percentage yield = actual ÷ theoretical × 100, and is below 100% because of incomplete reactions, losses during transfer and side reactions. Atom economy = Mr of the desired product ÷ total Mr of reactants × 100, and measures how much of the starting material becomes something useful.

Quick check

Part 10 of 15

Quick check

Percentage yield is

Quick check

Part 11 of 15

Quick check

Atom economy measures

Explore the idea

Part 12 of 15

Worked example

Worked answer: 10 g of calcium carbonate decomposes to calcium oxide and carbon dioxide. Find the theoretical mass of CaO (4 marks)

Mr of CaCO₃ = 40 + 12 + 48 = 100, and Mr of CaO = 40 + 16 = 56 (1). Moles of CaCO₃ = 10 ÷ 100 = 0.1 mol (1). The equation CaCO₃ → CaO + CO₂ is a 1 : 1 ratio, so 0.1 mol of CaO forms (1). Mass = 0.1 × 56 = 5.6 g (1). If only 4.8 g were collected, the percentage yield would be 4.8 ÷ 5.6 × 100 = 86%.

Challenge round

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Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

Mastery quiz

Part 14 of 15

Marked quiz

End of lesson quiz: Quantitative chemistry: masses, moles and yield

4 questions, marked with the reasoning shown. No timer.

1. Moles equals
2. The limiting reactant is the one that
3. Percentage yield is
4. Atom economy measures

Lesson round-up

Part 15 of 15

Lesson round-up

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Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

Name the variable the club changed the second time, and one variable they kept the same.
Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
Write a conclusion for the club's report, saying how far the evidence supports it.

Method and results
Writing
Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.
1 page + results table
Conclusion
Writing
Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.
6–8 sentences
Labelled diagram
Diagram
Draw the apparatus or the process in pencil, label it fully, and photograph it.
1 labelled diagram
Reading and notes
Reading
Read the lesson text again and write the key ideas in your own words.
Half a page of notes

Write your answer, notes or working here

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Photos of paper work, diagrams or working outHow many minutes did the written work take?

Part 1 of 15 · Watch & discover

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