25.0 cm³ of sodium hydroxide solution of concentration 0.100 mol/dm³ is exactly neutralised by 20.0 cm³ of hydrochloric acid. The two react in a 1:1 ratio. Calculate the concentration of the acid.

Moles of alkali = 0.100 × 0.0250 = 0.00250 mol, so the acid contains 0.00250 mol in 0.0200 dm³, giving 0.125 mol/dm³ to 3 significant figures.

25.0 cm³ of hydrochloric acid of concentration 0.200 mol/dm³ is neutralised by sodium hydroxide solution of concentration 0.100 mol/dm³, reacting in a 1:1 ratio. Calculate the volume of alkali needed.

Moles of acid = 0.200 × 0.0250 = 0.00500 mol, so the volume of alkali is 0.00500 ÷ 0.100 = 0.0500 dm³, which is 50.0 cm³ to 3 significant figures.

Sulfuric acid reacts with sodium hydroxide in the ratio 1 mol of acid to 2 mol of alkali. 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide is neutralised by 20.0 cm³ of the acid. Calculate the concentration of the acid.

Moles of alkali = 0.100 × 0.0250 = 0.00250 mol, so moles of acid = 0.00125 mol in 0.0200 dm³, giving 0.0625 mol/dm³ to 3 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

Titration calculations, and the mass of a solute

3 things to know
Paper 4

Overview

The three steps from titration results to an unknown concentration, worked titration calculations including a 1:2 ratio, averaging concordant titres, converting mol/dm³ to g/dm³, and mass, volume and concentration calculations.

What you need to know

Answer each one before you reveal it.

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Process

State the three steps used to work out an unknown concentration from titration results.

Numbers

How do you calculate the number of moles in a solution from its concentration in mol/dm³ and its volume in cm³?

Key term

Why must the balanced equation be used in a titration calculation rather than assuming the moles are equal?

Questions from this subtopic

Each one with the model answer underneath.

1
State the three steps used to work out an unknown concentration from titration results.
Model answer
Work out the moles of the solution whose concentration is known, use the balanced equation to convert to moles of the other substance, then divide by the volume of that solution in dm³.
2
How do you calculate the number of moles in a solution from its concentration in mol/dm³ and its volume in cm³?
Model answer
Multiply the concentration by the volume in cm³ and divide by 1000.
3
25.0 cm³ of sodium hydroxide solution of concentration 0.100 mol/dm³ is exactly neutralised by 20.0 cm³ of hydrochloric acid. The two react in a 1:1 ratio. Calculate the concentration of the acid.
Model answer
Moles of alkali = 0.100 × 0.0250 = 0.00250 mol, so the acid contains 0.00250 mol in 0.0200 dm³, giving 0.125 mol/dm³ to 3 significant figures.
4
25.0 cm³ of hydrochloric acid of concentration 0.200 mol/dm³ is neutralised by sodium hydroxide solution of concentration 0.100 mol/dm³, reacting in a 1:1 ratio. Calculate the volume of alkali needed.
Model answer
Moles of acid = 0.200 × 0.0250 = 0.00500 mol, so the volume of alkali is 0.00500 ÷ 0.100 = 0.0500 dm³, which is 50.0 cm³ to 3 significant figures.
5
Sulfuric acid reacts with sodium hydroxide in the ratio 1 mol of acid to 2 mol of alkali. 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide is neutralised by 20.0 cm³ of the acid. Calculate the concentration of the acid.
Model answer
Moles of alkali = 0.100 × 0.0250 = 0.00250 mol, so moles of acid = 0.00125 mol in 0.0200 dm³, giving 0.0625 mol/dm³ to 3 significant figures.
6
Why must the balanced equation be used in a titration calculation rather than assuming the moles are equal?
Model answer
Because the reacting ratio is often not 1:1, and using the wrong ratio scales the answer by that factor.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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