What is meant by the theoretical mass of a product?

It is the maximum mass of product that could be made if all the reactant were converted and none of the product were lost.

Calcium carbonate, relative formula mass 100, decomposes to calcium oxide, relative formula mass 56, in a 1:1 ratio. Calculate the theoretical mass of calcium oxide from 50.0 g of calcium carbonate.

50.0 ÷ 100 = 0.500 mol, giving 0.500 mol of calcium oxide, so 0.500 × 56 = 28.0 g, given to 3 significant figures.

Magnesium, relative atomic mass 24, burns to magnesium oxide, relative formula mass 40, one mole of metal giving one mole of oxide. Calculate the theoretical mass of oxide from 4.8 g of magnesium.

4.8 ÷ 24 = 0.20 mol, giving 0.20 mol of magnesium oxide, so 0.20 × 40 = 8.0 g, given to 2 significant figures.

One mole of nitrogen, relative formula mass 28, gives two moles of ammonia, relative formula mass 17. Calculate the theoretical mass of ammonia from 28 g of nitrogen.

28 ÷ 28 = 1.0 mol of nitrogen gives 2.0 mol of ammonia, so 2.0 × 17 = 34 g, given to 2 significant figures.

Two moles of hydrogen, relative formula mass 2, give two moles of water, relative formula mass 18. Calculate the theoretical mass of water from 4.0 g of hydrogen.

4.0 ÷ 2 = 2.0 mol of hydrogen gives 2.0 mol of water, so 2.0 × 18 = 36 g, given to 2 significant figures.

OCR GCSE Chemistry A J248

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions Ionic equations, and state symbols The mole and the Avogadro constant Conservation of mass, and changes of mass in open systems Stoichiometry, and calculating reacting masses Exothermic and endothermic reactions, and reaction profiles Activation energy, and bond energy calculations Oxidation and reduction, by oxygen and by electrons Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids Dilute and concentrated, weak and strong The pH scale, and hydrogen ion concentration pH as a factor of ten, and measuring pH Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures Nanoparticles: size and surface area to volume Uses and risks of nanoparticulate materials

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Biological methods of metal extraction Rate against yield in industry, and reaction conditions The Haber process and fertiliser production Nitrogen, phosphorus and potassium in agriculture Life cycle assessment Recycling, and the decisions behind it Alloys, and the corrosion of metals Comparing glass, ceramics, polymers, composites and metals Functional groups and homologous series Predicting the products of organic reactions Addition polymerisation Condensation polymerisation, and making a polymer DNA and other natural polymers Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking Chemical cells and fuel cells The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Concentration in mol/dm³, and titration Titration calculations, and the mass of a solute Molar gas volume and the volumes of gases Theoretical mass and percentage yield Atom economy, and choosing a reaction pathway Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Changing the conditions of an equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state Limitations of the particle model How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Transition metals Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine Tests for aqueous cations and anions Flame tests, and identifying metal ions Identifying a species from test results Instrumental methods of analysis

Monitoring and Controlling Chemical Reactions

Theoretical mass and percentage yield

3 things to know
Paper 4

Overview

What a theoretical mass is and the three steps to calculate it, worked examples from calcium carbonate, magnesium, nitrogen and hydrogen, the limiting reactant, percentage yield calculations, and why yields fall short of 100%.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

State the three steps used to calculate the theoretical mass of a product from a given mass of reactant.

Key term

If one reactant is present in excess, which reactant must the theoretical mass be calculated from?

Key term

How is the percentage yield of a reaction calculated?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by the theoretical mass of a product?
Model answer
It is the maximum mass of product that could be made if all the reactant were converted and none of the product were lost.
2
State the three steps used to calculate the theoretical mass of a product from a given mass of reactant.
Model answer
Convert the mass of reactant to moles, use the balanced equation to find the moles of product, then multiply by the relative formula mass of the product.
3
Calcium carbonate, relative formula mass 100, decomposes to calcium oxide, relative formula mass 56, in a 1:1 ratio. Calculate the theoretical mass of calcium oxide from 50.0 g of calcium carbonate.
Model answer
50.0 ÷ 100 = 0.500 mol, giving 0.500 mol of calcium oxide, so 0.500 × 56 = 28.0 g, given to 3 significant figures.
4
Magnesium, relative atomic mass 24, burns to magnesium oxide, relative formula mass 40, one mole of metal giving one mole of oxide. Calculate the theoretical mass of oxide from 4.8 g of magnesium.
Model answer
4.8 ÷ 24 = 0.20 mol, giving 0.20 mol of magnesium oxide, so 0.20 × 40 = 8.0 g, given to 2 significant figures.
5
One mole of nitrogen, relative formula mass 28, gives two moles of ammonia, relative formula mass 17. Calculate the theoretical mass of ammonia from 28 g of nitrogen.
Model answer
28 ÷ 28 = 1.0 mol of nitrogen gives 2.0 mol of ammonia, so 2.0 × 17 = 34 g, given to 2 significant figures.
6
Two moles of hydrogen, relative formula mass 2, give two moles of water, relative formula mass 18. Calculate the theoretical mass of water from 4.0 g of hydrogen.
Model answer
4.0 ÷ 2 = 2.0 mol of hydrogen gives 2.0 mol of water, so 2.0 × 18 = 36 g, given to 2 significant figures.

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