Explain why the buffer expression uses the ratio of acid to salt rather than their absolute concentrations.

Both concentrations appear in the same volume of solution, so the volume cancels and only their ratio, or equivalently the ratio of moles, determines the pH.

Calculate the pH of a buffer containing 0.100 mol dm⁻³ ethanoic acid and 0.100 mol dm⁻³ sodium ethanoate, given Ka = 1.74 × 10⁻⁵ mol dm⁻³.

The ratio is one, so the hydrogen ion concentration equals Ka at 1.74 × 10⁻⁵ mol dm⁻³ and the pH is 4.76.

Calculate the pH of a buffer containing 0.200 mol dm⁻³ ethanoic acid and 0.100 mol dm⁻³ sodium ethanoate, given Ka = 1.74 × 10⁻⁵ mol dm⁻³.

The hydrogen ion concentration is 1.74 × 10⁻⁵ × 2 = 3.48 × 10⁻⁵ mol dm⁻³, giving a pH of 4.46.

50.0 cm³ of 0.200 mol dm⁻³ ethanoic acid is mixed with 20.0 cm³ of 0.200 mol dm⁻³ sodium hydroxide. Calculate the moles of ethanoic acid remaining.

There are 0.0100 mol of acid and 0.00400 mol of alkali, so 0.00600 mol of ethanoic acid remains unreacted.

For that mixture of 0.00600 mol ethanoic acid and 0.00400 mol ethanoate, calculate the pH given Ka = 1.74 × 10⁻⁵ mol dm⁻³.

The hydrogen ion concentration is 1.74 × 10⁻⁵ × (0.00600 ÷ 0.00400) = 2.61 × 10⁻⁵ mol dm⁻³, giving a pH of 4.58.

CCEA A-Level Chemistry 1110

Formulae, Equations and Amounts of Substance

Topic overview Ionic charges and the formulae of compound ions Writing and balancing full and ionic equations The mole, molar mass and Avogadro's constant Reacting masses and reactants in excess Hydrated salts and water of crystallisation

Atomic Structure

Topic overview Protons, neutrons and electrons Atomic number, mass number and the composition of ions Isotopes and relative isotopic mass Relative atomic, molecular and formula mass Mass spectra and relative atomic mass calculations Mass spectra of diatomic elements Electronic configuration and the building up principle Orbitals, electron spin and orbital shapes The s-, p-, d- and f-blocks of the Periodic Table Ionisation energies and their equations Trends in ionisation energy as evidence for shells and subshells

Bonding

Topic overview Ionic bonding and electron transfer Dot and cross diagrams for ionic compounds Covalent bonding and the shared pair Lone pairs and co-ordinate bonding Dot and cross diagrams for covalent molecules and ions The octet rule and its limitations Electronegativity and bond polarity Metallic bonding

Intermolecular Forces and Structure

Topic overview Van der Waals' forces and permanent dipole-dipole attractions Hydrogen bonding, boiling points and solubility Giant ionic, metallic and giant covalent structures Molecular covalent structures and melting points across Period 3

Shapes of Molecules and Ions

Topic overview Electron pair repulsion and VSEPR theory Shapes and bond angles up to four electron pairs Shapes with five and six electron pairs Polar bonds and polar molecules

Redox

Topic overview Rules for assigning oxidation states Oxidation states in peroxides, hydrides and Roman numeral names Oxidation, reduction and electron transfer Oxidising agents, reducing agents and disproportionation Half-equations and combining them into redox equations

The Halogens

Topic overview Colours, physical states and solubility of the halogens Chlorine with water and the treatment of drinking water Disproportionation of the halogens with sodium hydroxide Oxidising ability and displacement reactions Solid halides with concentrated sulfuric and phosphoric acid

Acid-Base Titrations

Topic overview Strong and weak acids and bases Concentration, molarity and units Preparing a standard solution Titration technique, mean titres and uncertainty Indicators and the end point Titration calculations

Qualitative Tests

Topic overview Chemical tests for the gases Flame tests for metal cations Testing for the ammonium ion, sulfate and carbonate Distinguishing the halides, and the starch test for iodine

Formulae and Amounts of a Substance

Topic overview Empirical and molecular formulae Calculating formulae from composition data Molar gas volume and reacting gas volumes Percentage yield Atom economy

Nomenclature and Isomerism in Organic Chemistry

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Alkanes

Topic overview Alkanes as saturated hydrocarbons Boiling points of alkanes and van der Waals' forces Boiling points of branched isomers Complete and incomplete combustion of alkanes Pollutants from alkane fuels and the atmosphere Catalytic converters Radicals, homolytic fission and heterolytic fission Free radical substitution of alkanes

Alkenes

Topic overview Unsaturated hydrocarbons and the bromine water test Sigma and pi bonds in the carbon–carbon double bond Electron density at C=C, reactivity and catalytic hydrogenation Addition of chlorine and bromine to alkenes Addition of hydrogen chloride and hydrogen bromide Electrophiles, heterolytic fission and the addition mechanism Carbocation stability and Markovnikov addition Addition polymerisation of alkenes

Halogenoalkanes

Topic overview Classification and boiling points of halogenoalkanes Laboratory preparation of a halogenoalkane from an alcohol Nucleophiles and hydrolysis with aqueous alkali Reactions with ammonia and with potassium cyanide Relative rates of hydrolysis and carbon–halogen bond enthalpy Elimination using ethanolic potassium hydroxide CFCs and the ozone layer

Alcohols

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Infrared Spectroscopy

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Kinetics

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Equilibrium

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Lattice Enthalpy, Entropy and Free Energy

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Equilibrium and Acid-Base Equilibria

Topic overview Equilibrium concentrations from initial amounts Kc expressions, values and units Brønsted–Lowry acids, bases and conjugate pairs Kw, pH and the pH of strong acids and bases Ka, pKa and the pH of weak acids Buffer solutions and how they resist pH change Calculating the pH of a buffer solution Titration curves, indicators and the pH of salt solutions

Isomerism

Topic overview Structural isomerism between different families Chiral centres and optical isomers Representing optical isomers in three dimensions Plane polarised light, optical activity and racemic mixtures Stereochemistry, drug action and receptor sites

Aldehydes and Ketones

Topic overview Structure and naming of aldehydes and ketones Boiling points and solubility of carbonyl compounds Preparation from primary and secondary alcohols Reaction with hydrogen cyanide and the nucleophilic addition mechanism Racemic mixtures from nucleophilic addition 2,4-dinitrophenylhydrazine and the identification of carbonyls Distinguishing aldehydes from ketones, and reduction with lithal

Carboxylic Acids

Topic overview Structure, naming and physical properties of carboxylic acids Preparation from primary alcohols and aldehydes Formation by hydrolysis of esters and nitriles Salt formation with carbonates, hydroxides and ammonia Reactions with alcohols, phosphorus pentachloride and lithal

Derivatives of Carboxylic Acids

Topic overview Esters and acyl chlorides: structure and naming Boiling points and solubility of monoesters Making esters from alcohols with carboxylic acids or acyl chlorides The laboratory preparation of a liquid ester Fats and oils as esters of glycerol Transesterification, biodiesel and margarine Reactions of acyl chlorides with water and alcohols

Aromatic Chemistry

Topic overview The structure of benzene and delocalised π electrons The stability of benzene compared with alkenes Electrophilic substitution: bromination and nitration of benzene Friedel–Crafts reactions and the nitration of methyl benzoate

Spectroscopy and Chromatography

Topic overview The mass spectrum: molecular ion and base peak Fragmentation and the formulae of fragment ions High resolution mass spectrometry Nuclear magnetic resonance and the TMS standard Chemically equivalent hydrogens and chemical shift Integration curves Spin–spin splitting and the n+1 rule Deducing a structure from an nmr spectrum Paper and thin-layer chromatography Gas–liquid chromatography

Volumetric Analysis

Topic overview Iodine–thiosulfate titrations Manganate(VII) titrations Deducing titration equations from half-equations Back titration

Transition Metals

Topic overview The characteristic properties of the transition metals Electronic configurations of the metals and their ions Complexes, ligands and coordination number Oxidation number, redox and disproportionation Monodentate, bidentate and polydentate ligands Ligand strength and the entropy of ligand replacement Ligand replacement in copper(II) complexes The colours of the aqueous transition metal ions Hydroxide precipitates with sodium hydroxide Precipitates with aqueous ammonia and their dissolution Vanadium and the interconversion of its oxidation states

Electrode Potentials

Topic overview Standard electrode potential Types of half-cell Cell representation and calculating emf Non-rechargeable, rechargeable and fuel cells The hydrogen–oxygen fuel cell Electrode potentials and reactivity

Amines

Topic overview Structure and classification of amines Physical properties of amines Preparing aliphatic and aromatic amines Amines as bases Reaction with ethanoyl chloride and identifying amines Diazonium salts and coupling Colour and delocalisation

Amides and Polymers

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Amino Acids and Proteins

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Chemistry in Medicine

Topic overview Indigestion remedies Analysing an indigestion remedy by back titration Treating the skin Silver nitrate and cisplatin Aspirin: synthesis and analysis Chromatography in identifying drugs Haemoglobin, carbon monoxide and edta Drugs, testing and judgement

Equilibrium and Acid-Base Equilibria

Calculating the pH of a buffer solution

3 things to know
Unit A2 1

Overview

Recall questions on the buffer expression and why it uses the ratio of acid to salt, calculating the pH of ethanoic acid and propanoic acid buffers, working from an acid partly neutralised by alkali, why pH equals pKa at half-neutralisation, finding the ratio needed for a target pH, and showing the effect of added alkali.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the expression used to find the hydrogen ion concentration of an acidic buffer.

Key term

Explain why the pH at half-neutralisation of a weak acid equals its pKa.

Key term

A buffer of pH 4.58 rises only to pH 4.76 when a quantity of alkali is added. Explain how this demonstrates buffering

Questions from this subtopic

Each one with the model answer underneath.

1
State the expression used to find the hydrogen ion concentration of an acidic buffer.
Model answer
The hydrogen ion concentration equals Ka multiplied by the concentration of the acid divided by the concentration of its salt.
2
Explain why the buffer expression uses the ratio of acid to salt rather than their absolute concentrations.
Model answer
Both concentrations appear in the same volume of solution, so the volume cancels and only their ratio, or equivalently the ratio of moles, determines the pH.
3
Calculate the pH of a buffer containing 0.100 mol dm⁻³ ethanoic acid and 0.100 mol dm⁻³ sodium ethanoate, given Ka = 1.74 × 10⁻⁵ mol dm⁻³.
Model answer
The ratio is one, so the hydrogen ion concentration equals Ka at 1.74 × 10⁻⁵ mol dm⁻³ and the pH is 4.76.
4
Calculate the pH of a buffer containing 0.200 mol dm⁻³ ethanoic acid and 0.100 mol dm⁻³ sodium ethanoate, given Ka = 1.74 × 10⁻⁵ mol dm⁻³.
Model answer
The hydrogen ion concentration is 1.74 × 10⁻⁵ × 2 = 3.48 × 10⁻⁵ mol dm⁻³, giving a pH of 4.46.
5
50.0 cm³ of 0.200 mol dm⁻³ ethanoic acid is mixed with 20.0 cm³ of 0.200 mol dm⁻³ sodium hydroxide. Calculate the moles of ethanoic acid remaining.
Model answer
There are 0.0100 mol of acid and 0.00400 mol of alkali, so 0.00600 mol of ethanoic acid remains unreacted.
6
For that mixture of 0.00600 mol ethanoic acid and 0.00400 mol ethanoate, calculate the pH given Ka = 1.74 × 10⁻⁵ mol dm⁻³.
Model answer
The hydrogen ion concentration is 1.74 × 10⁻⁵ × (0.00600 ÷ 0.00400) = 2.61 × 10⁻⁵ mol dm⁻³, giving a pH of 4.58.

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