OCR A Level Chemistry B (Salters) Practical skills in chemistry June 2018 question paper H433/03

A student decides to use a microscale method to synthesise an azo dye and dye a fabric.

Naphthalen-2-ol, shown below, is used to make the dye. A piece of cotton is dipped into naphthalen-2-ol dissolved in sodium hydroxide. The diazonium solution is then added to dye the cotton red.

The coupling reaction involves the carbon atom marked with an asterisk, *.

Complete the structure of the azo dye formed in this coupling reaction.

The azo dye formed above has an extended delocalised system.

Describe the bonding occurring in a delocalised system and explain why this can lead to organic compounds being coloured.

Other functional groups can be attached to dye molecules and these can modify the properties of the dye.

Give one property of the dye that might be affected if nitro, NO₂, groups are attached.

Attractions between dye molecules and polymer molecules in fabric fibres can be ionic, covalent or intermolecular bonds.

Use your knowledge of molecular interactions to fill in the empty boxes in the following table.

Suggest one advantage and one disadvantage of using a calomel electrode over a standard hydrogen electrode.

Give the oxidation state of mercury in Hg₂Cl₂.

A 25.0 g sample of Hg₂Cl₂ is vaporised at 400 °C and a pressure of 101 kPa. A student assumes that the formula of the gaseous mercury chloride molecules is Hg₂Cl₂

Calculate the volume of gas, in dm³, that would be expected under these conditions.

* A student investigating the rusting of iron is given a calomel electrode.

The diagram shows a simplified calomel electrode.

The student wants to use this electrode to measure the standard electrode potential of a Fe²⁺(aq)IFe(s) half-cell.

Give instructions on how to do this, justifying the uses of the pieces of apparatus you name. You may add to the diagram above to illustrate your answer.

An equilibrium, represented by equation 2.1, exists between the solid Hg₂Cl₂ and its ions in solution.

The solubility of the solid Hg₂Cl₂ in a saturated solution at 298 K is 3.5 × 10⁻⁴ g dm⁻³.

Calculate the solubility product, K_sp, for Hg₂Cl₂ at 298 K. Include the units.

Give your answer to an appropriate number of significant figures.

solubility product, K_sp =........ units........

Iodine, I₂, is an essential dietary element. The recommended maximum daily intake of iodine for an adult is 1.5 × 10⁻⁴ g (150 g).

A group of chemistry students read that fish is a good source of iodine in the form of iodide ions. They decide to extract the iodine from 600 g of fish.

The students blend the fish in a food processor with 100 cm³ of water, leave it to stand overnight and then filter the mixture into a beaker.

One of the students suggests that if they add silver nitrate solution they can confirm the presence of iodide ions in the solution.

Describe what the students would observe if the only halide ion present in the solution was the iodide.

Write an ionic equation for this reaction. Include state symbols.

The students pour the filtered mixture into a separating funnel containing 20 cm³ of hexane, (b) 5 cm³ of dilute sulfuric acid and 5 cm³ of hydrogen peroxide solution.

Iodine is formed and dissolves in the hexane layer which goes purple. The purple layer is separated from the aqueous layer and transferred to a conical flask.

The purple coloured solution is titrated with standard 0.0010 mol dm⁻³ sodium thiosulfate solution. The end point is indicated by the disappearance of the purple colour.

The hydrogen peroxide oxidises the iodide ions in the fish to iodine.

Write a half equation for this oxidation reaction. Explain why this reaction is classified as oxidation.

The equation for the titration reaction is given below.

Name the element oxidised in this reaction. Give its oxidation state before and after the reaction.

Oxidation state before reaction........ oxidation state after reaction........

The students obtained an average titre of 5.30 cm³ of 0.0010 mol dm⁻³ sodium thiosulfate. (iii)

Calculate the mass of iodine in g in a 120 g portion of fish.

Give your answer to two significant figures.

mass of iodine =........ g

One of the students suggests that the titre value is too small and will lead to an unacceptably high percentage error.

Calculate the percentage error based on the students’ titre value.

Suggest how the experiment could be modified to improve the accuracy of the result.

* The mass of the iodine in the hexane solution could also have been determined using colorimetry.

Suggest a suitable method that could be used to measure the mass of iodine in the hexane solution using a colorimeter or visible spectrophotometer.

Show how you would process the results.

This question refers to the Practical Insert that is provided as an insert to this paper.

The equation for the reaction producing phenyl benzoate is as follows:

Draw a structural formula for phenyl benzoate, showing the bonding in the ester group.

Use the student results to calculate the percentage yield of phenyl benzoate obtained from the practical.

In step 8 of the procedure the water reacts with any remaining benzoyl chloride.

Write the equation for this reaction.

Suggest and explain the reason for step 13 in the procedure.

Describe the practical procedure used to measure the melting point of an organic solid. You do not need to discuss the type of melting point apparatus you use.

What information can the students get from their melting point?

The recrystallisation procedure uses ethanol as the solvent.

Give the key properties needed by a solvent to be effective in recrystallisation.

The students carry out thin layer chromatography of the phenyl benzoate formed. One student states that this will enable them to assess the purity of their product.

Comment on the validity of this statement. You should describe any possible observations to back up your comments.