OCR A Level Chemistry A Unified chemistry June 2024 question paper H432/03

This question is about acids, bases and salts.

What is the pH of 1.00 dm³ of 0.400 mol dm^–3 of NaOH(aq) at 298 K?

Water is added to 10.0 cm³ of 0.750 mol dm^–3 HCl (aq) to produce 100 cm³ of diluted HCl (aq).

What is the pH of the diluted HC l (aq)?

Give your answer to 2 decimal places.

A solution has concentrations of 0.300 mol dm^–3 CH₃COOH(aq) and 0.100 mol dm^–3 CH₃COONa(aq).

K_a for CH₃COOH = 1.75 10^–5 mol dm^–3 at 298 K.

What is the pH of the solution at 298 K?

Give your answer to 2 decimal places.

A student is provided with hydrated copper(II) nitrate, Cu(NO₃)₂•3H₂O.

The student needs to prepare a standard solution of Cu(NO₃)₂•3H₂O with a concentration of 0.200 mol dm^–3. The student has access to usual laboratory apparatus and equipment.

Describe how the student would prepare 100.0 cm³ of this solution, giving quantities, apparatus and method.

This question is about different areas of chemistry.

Why do all average bond enthalpies have a positive value?

Determine the C–H bond enthalpy, in kJ mol^–1, using the information above.

C–H bond enthalpy =........ kJ mol^–1

Hydrogen gas is being considered as a household fuel to replace methane.

The enthalpy change of formation, _fH, for H₂O(l) is –285.8 kJ mol^–1.

Determine the energy released when 60.0 m³ of hydrogen is used as a household fuel at RTP.

Give your answer to 3 significant figures and in standard form.

Compound A is a chloride of a Period 3 element.

A student carries out the 2 steps below to find the formula of compound A.

Step 1 The student adds 5.00 10^–4 mol of compound A to water. A colourless solution is formed.

Step 2 The colourless solution reacts with exactly 60.0 cm³ of 2.50 10^–2 mol dm^–3 AgNO₃(aq) to form a white precipitate.

Write an ionic equation, with state symbols, for the reaction in Step 2.

Determine the formula of compound A.

Compound B, shown below, is an antiviral medicine.

What is the molecular formula of compound B ?

How many chiral carbon atoms are there in one molecule of compound B ?

This question is about compounds and ions of iron(II) and iron(III) that contain ethanedioate ions, C₂O₄^2–.

The C₂O₄^2– ion, shown below, is an example of a bidentate ligand.

Explain what is meant by the term bidentate ligand.

A complex ion E contains three C₂O₄^2– ions bonded to an iron(III) ion in an octahedral shape.

Complex ion E exists as a mixture of two optical isomers.

Draw 3D diagrams to show the structures of the optical isomers of E.

Include any overall charge.

A student plans an investigation to find the number of waters of crystallisation, x, in a sample of hydrated iron(II) ethanedioate, FeC₂O₄•xH₂O.

The student decides to carry out a redox titration between solutions of iron(II) ethanedioate and potassium manganate(VII) in acidic conditions.

In the titration, both iron(II) ions and ethanedioate, C₂O₄^2–, ions are oxidised.

Construct half-equations for the oxidation of iron(II) and ethanedioate ions.

Oxidation of iron(II) ions

Oxidation of ethanedioate ions

The uncertainty in each burette reading is ±0.05 cm³.

Calculate the percentage uncertainty for the titre in Titration 1.

* In the titration, 5 mol of iron(II) ethanedioate reacts with 3 mol of manganate(VII) ions.

Analyse the student’s results to find the number of waters of crystallisation, x, in the hydrated

iron(II) ethanedioate, FeC₂O₄•xH₂O.

This question is about the chemistry of compounds containing phosphorus.

Phosphorus forms several acids including H₃PO₄ and H₃PO₃.

H₃PO₄ is a tribasic acid. The equilibria for the dissociations are shown below.

During the equilibria, H₂PO₄^– behaves both as an acid and as a base.

Explain this statement, using the equilibria 1, 2 and 3, as required.

In a H₃PO₃ molecule, the O atoms are covalently bonded to the P atom. The H atoms are bonded to the O atoms.

Draw the structure of a H₃PO₃ molecule, showing all the bonds.

On your diagram, add the values for the O–P–O and P–O–H bond angles.

The systematic name of H₃PO₄ is phosphoric(V) acid.

What is the systematic name of H₃PO₃?

Phosphine, PH₃, is a poisonous gas.

Phosphine reacts with oxygen gas to form phosphorus(V) oxide and water.

Write the equation for this reaction.

Aqueous silver nitrate, AgNO₃, is reduced by PH₃. The unbalanced equation is shown below.

Balance the equation and use oxidation numbers to explain why this is a redox reaction.

Explanation........

When phosphorus(V) chloride, PCl₅, and ammonium chloride are heated together, the compound P₃N₃Cl₆ is formed, together with HCl gas.

P₃N₃Cl₆ has a cyclic structure, like the Kekulé structure of benzene.

Write an equation for the reaction of PCl₅ and ammonium chloride to form P₃N₃Cl₆.

Calculate the percentage by mass of P in P₃N₃Cl₆.

Give your answer to 2 decimal places.

Suggest one example of evidence that could show that P₃N₃Cl₆ has a Kekulé structure rather than a delocalised structure.

In a molecule of P₃N₃Cl₆ all the N and Cl atoms are bonded to P atoms.

Suggest a possible structure for a molecule of P₃N₃Cl₆.

This question is about the analysis of organic compounds.

Determine the R_f value of propan-1-ol.

Show your working.

Write an equation for the alkaline hydrolysis of 1-bromopropane.

Show structures of organic compounds.

A student investigates the alkaline hydrolysis of 1-chloropropane using the same method as for 1-bromopropane.

Predict, with reasons, how the appearance of the reaction mixture in the chromatogram produced after 20 minutes would be different when 1-chloropropane is used instead of 1-bromopropane.

Suggest why propan-1-ol is run alongside the reaction mixture.

Write the formula of the species causing the colours after refluxing with acidified dichromate(VI

Green solution........

Orange solution........

* The student is provided with further information about compounds F– I.

• They all have the molecular formula C₅H₁₀O.

• One of the compounds is alicyclic.

• The other compounds are unbranched.

Use this further information and the student’s observations in Table 5.1 to answer the following.

• How do the observations provide evidence for the possible functional groups in compounds F– I ?

• Suggest a possible structure for each of the compounds F– I.

Show your reasoning.