Edexcel A Level Chemistry Paper 3: General and Practical Principles in Chemistry June 2022 question paper 9CH0/03

Relative atomic mass is an important concept in chemistry.

Define the term relative atomic mass.

A sample of neon consisted of three isotopes.

Calculate the relative atomic mass of neon in this sample. Give your answer to three significant figures.

Ammonium cobalt(II) sulfate is made by mixing aqueous solutions of ammonium sulfate and excess cobalt(II) sulfate.

The colour of the cobalt(II) sulfate solution used is pink due to the complex cobalt(II) ion, [Co(H₂O)₆]²⁺.

Explain why the solution is coloured.

Explain the shape of the cobalt(II) ion, [Co(H₂O)₆]²⁺, using electron‑pair repulsion theory.

Give the reasons for carrying out Steps 3 and 4 of the procedure, referring particularly to the words in bold.

The percentage yield of this reaction is 70.0 %.

Give two possible reasons, other than an incomplete reaction, why the yield is less than 100 %.

A group of students design and carry out experiments to deduce the formulae of two salts, X and Y.

X contains one cation and one anion.

Y contains water of crystallisation.

A flame test is carried out on X.

Describe how to carry out a flame test.

The colour of the flame is yellow.

Give the formula of the metal ion present in salt X.

A sample of X is placed in a test tube and dissolved in deionised water. The solution is acidified with hydrochloric acid and barium chloride solution is added.

A white precipitate forms.

Give the formula of the anion present in X.

Deduce the formula of X, using your answers to (a)(ii) and (b)(i).

Y is identified as hydrated potassium carbonate, K₂CO₃·nH₂O.

Two of the students were asked to determine the number of moles of water of crystallisation, n, in Y using the procedure shown:

• weigh a sample of hydrated Y into a pre‑weighed crucible

• place a lid loosely on the crucible and heat it for five minutes to remove the water of crystallisation

• allow the crucible and lid to cool, remove the lid and then reweigh the crucible with its contents.

The first student carried out the experiment but forgot to use the lid.

Explain how this mistake would affect the calculated value of n.

The second student carried out the experiment but heated the apparatus for only one minute.

Explain how this mistake would affect the calculated value of n.

In an accurate experiment, Y is found to consist of 71.9 % K₂CO₃ by mass.

Calculate the value of n.

This question is about the white solid barium carbonate.

Barium carbonate decomposes under suitable conditions to form barium oxide and carbon dioxide.

Standard molar entropy data related to this reaction are shown.

Substance	Standard molar entropy, S / J K–1mol–1
BaCO3(s)	112.1
BaO(s)	70.4
CO2(g)	213.6

Show that barium carbonate is thermally stable at 298 K, using the data in the equation and in the table.

Calculate the lowest temperature, in C, at which it is thermodynamically feasible for barium carbonate to decompose. Give your answer to three significant figures.

Explain whether magnesium carbonate is more or less thermally stable than barium carbonate.

A white solid was thought to be barium carbonate. A student suggested that the presence of the carbonate ion could be tested for by adding a small amount of sulfuric acid.

Explain whether or not this suggestion is valid.

Explain the difference in the reactivity of bromine with benzene and with phenol.

Include the type of reaction, the products that form, and any conditions required. Mechanisms for the reactions are not required.

An ester Q has the molecular formula C₈H₁₆O₂.

When burned in excess oxygen, 1.879 g of Q formed 4.594 g of carbon dioxide and 1.879 g of water.

Show that the empirical formula of Q is C₄H₈O.

Data from the high resolution¹H (proton) NMR spectrum of the ester Q are shown in the table.

Part of the structure of Q is shown.

Complete the structure of Q. Justify your answer by linking the proton environments in your structure to the relative peak areas and the splitting pattern of the peaks.

This question is about some reactions of carbonyl compounds.

Methyl methacrylate is the monomer used to make the polymer perspex. It can be synthesised from propanone using the reaction scheme shown.

Draw the mechanism for the reaction in Step 1. Include curly arrows and any relevant lone pairs and dipoles.

Complete the table to show the information missing from the reaction scheme.

Complete the equation for the formation of the polymer from methyl methacrylate.

Propanone can be formed from the fermentation of polysaccharides such as starch. The propanone can be separated from the fermentation mixture by distillation.

Draw the apparatus used in the laboratory for distillation of propanone from the reaction mixture.

Carbonyl compounds, such as propanone, react with 2,4‑dinitrophenylhydrazine in solution (Brady’s reagent) to form a precipitate which can be used to identify the compound.

The precipitate can be purified by recrystallisation.

Details of the recrystallisation process are shown.

Step 1 Dissolve the precipitate in the minimum volume of hot ethanol.

Step 2 Warm a filter paper and funnel in an oven for use in Step 3.

Step 3 Filter the solution whilst still warm to remove any undissolved solids, using gravity filtration.

Step 4 Allow the filtrate to cool and recrystallise.

Step 5 Filter the crystals under reduced pressure.

Step 6 Rinse the crystals with a small amount of ice‑cold ethanol.

Step 7 Dry the crystals between filter papers and leave in a desiccator.

Explain why the filter paper and funnel are warmed in an oven before Step 3.

Explain how Steps 4 and 5 remove impurities from the crystalline product.

State how the purified crystals can be used to identify the carbonyl compound that reacts with 2,4‑dinitrophenylhydrazine.

Detailed descriptions of practical procedures are not required.

This question is about acids and bases.

Devise an experiment to determine the acid dissociation constant, K_a, for a solution of ethanoic acid, CH₃COOH, of unknown concentration.

Assume you have access to a pH meter and a solution of sodium hydroxide of similar concentration to the acid. Include how to determine K_a from your results.

Calculate the pH of the solution formed when 51.2 cm³ of 0.927 mol dm^–3 NaOH(aq) is mixed with 40.4 cm³ of 0.370 mol dm^–3 H₂SO₄(aq).

[Ionic product of water K_w = 1.0010^–14mol²dm^–6]

Pineapple juice contains the weak acids citric acid (C₆H₈O₇) and ascorbic acid (C₆H₈O₆). The amount of each compound in a sample of 150 cm³ of pineapple juice can be determined by titration.

Experiment 1 is designed to determine the total amount of acid. 10.0 cm³ samples of pineapple juice are transferred to separate conical flasks and titrated with a solution of sodium hydroxide of known concentration.

The total amount of acid in the 150 cm³ sample of pineapple juice is 8.0010^–3mol.

Give a reason why methyl orange would not be a suitable indicator to use in this titration.

A student did not notice an air bubble in the tip of the burette before carrying out one of their accurate titrations. During this titration, the air bubble escaped.

Explain the effect this mistake would have on the value of this titre.

The end-point of the titration is when the starch changes colour.

Explain how this occurs, including the colour change.

The total amount of acid in the 150 cm³ sample is 8.0010^–3mol.

The mean titre in Experiment 2 using 5.00 cm³ of pineapple juice is 9.50 cm³.

Calculate the mass of citric acid in the 150 cm³ sample.

While doing background research for the experiment, a student found that three other compounds, D, E and F, are often present in pineapple juice.

Predict which one of these compounds is most likely to affect the result of Experiment 1 and hence predict the effect on the mass of citric acid calculated in (b)(ii). Justify your answer.

Explain why sodium hydrogencarbonate is added in Step 4.

Write the ionic equation for the reaction that takes place during Step 4. State symbols are not required.

Some data from the experiment are shown.

Plot a graph of the volume of sodium thiosulfate against the time the sodium hydrogencarbonate is added.

Explain how the graph of volume of thiosulfate against time confirms the reaction is zero order with respect to iodine, I₂.

Predict which of the three steps is the rate‑determining step. Justify your answer.

The student stated that

‘The hydrogen ions cannot be acting as a catalyst. One hydrogen ion is a reactant in Step 1 but two hydrogen ions are formed as products in Steps 1 and 3.’

Explain whether or not this statement is valid.