Formulas (Cambridge (CIE) A Level Chemistry): Exam Questions

Cobalt(II) sulfate is used in storage batteries, electroplating baths and as an additive to soils and animal feeds.

A sample of hydrated cobalt sulfate, CoSO₄•xH₂O, has a relative formula mass of 173.0.

Calculate a value for 'x'.

A student completed an experiment to determine the value of 'x' in a hydrated salt.

They heated a solid sample of the salt, CoSO₄.xH₂O, in a crucible for 60 seconds and recorded the following set of results.

Table 2.1

Use the data obtained by the student in Table 2.1 to calculate a value for 'x'.

The actual value for x in CoSO₄.xH₂O is 7.

i) Suggest one reason for the difference between your calculated value in part (b) and the actual value.

[1]

ii) Explain how you could improve the experiment the student did, using the same equipment, to achieve a value closer to the correct value for x.

[2]

Cobalt(II) sulfate undergoes a displacement reaction with magnesium because magnesium is a more reactive metal than cobalt.

Write an ionic equation for this reaction. State symbols are not required.

Chevreul's salt or copper(I, II) sulfite dihydrate, Cu₃(SO₃)₂•2H₂O, is an unusual hydrated salt as it contains copper in both of its common oxidation states.

The first step in the formation of Chevreul's salt is the reaction of aqueous copper sulfate with a solution of potassium metabisulfite, K₂S₂O₅.

Complete the equation for the reaction to form the anhydrous Chevreul's salt.

_CuSO₄ + _K₂S₂O₅ → _Cu₃(SO₃)₂ + 4K₂SO₄ + _SO₂

The chemical formula of Chevreul's salt is sometimes written as Cu₂SO₃•CuSO₃•2H₂O to show that it contains copper(I) sulfite and copper(II) sulfite.

Calculate the percentage by mass of copper(I) ions in copper(I) sulfite

Show your working.

Chevreul's salt can react with ammonia solution to form a deep blue solution of the tetraamminecopper(II) complex, [Cu(NH₃)₄]²⁺ (aq). It can also react with hydrochloric acid to form a white solid of copper(I) chloride, water and sulfur dioxide.

Write an ionic equation for the formation of copper(I) chloride from Chevreul's salt. Include state symbols.

When Chevreul's salt is heated in an inert atmosphere, it is stable to around 200 ^oC. Above this temperature, a variety of compounds can be produced although the final product of sustained heating at high temperatures is copper(II) oxide.

During the heating process, a sample of the mixture is taken. After separation, the most common compound was analysed and found to contain 39.8% copper, 20.1% sulfur and the rest oxygen.

Calculate the empirical formula of the most common compound. Show your working.

A synthetic carbohydrate was isotopically labelled with carbon-13.

Analysis of a 12 g sample of the carbohydrate found it to contain 6.93 g carbon, 0.80 g hydrogen and the remainder was oxygen.

Calculate the empirical formula of the carbohydrate. Show your working.

The fermentation of glucose, C₆H₁₂O₆, produces ethanol and carbon dioxide.

Construct an equation for this reaction.

During the fermentation process in the reaction in part (b), 312 g of ethanol was produced from 1.5 kg of glucose.

Calculate the percentage yield of this reaction. Give your answer to 3 significant figures. Show your working.

Magnesium carbonate is a common antacid that is used to neutralise excess stomach acid, HCl.

MgCO₃ (s) + 2HCl (aq) → MgCl₂ (aq) + CO₂ (g) + H₂O (l)

5.31 g of magnesium carbonate reacts completely with an excess of hydrochloric acid.

Calculate the volume, in dm³, of carbon dioxide produced at room conditions. Show your working.

Volume of CO₂ = .................. dm³

Another magnesium salt, magnesium sulfate, can exist in hydrated or anhydrous forms.

6.31 g of a sample of magnesium sulfate, MgSO₄.xH₂O, was strongly heated until a mass of 3.61 g of solid remained with no further change in mass recorded.

Calculate the value of x. Show your working.

x = .....................

Another sample of hydrated magnesium sulfate, MgSO₄.xH₂O, is found to contain 51.1% water by mass.

Calculate the value of x. Show your working.

x = .....................

State the effect on the calculated value of x if the salt was not heated to constant mass. Explain your answer.

A sample of a caffeinated drink contains 34 mg of caffeine, M_r = 194.0 g mol^-1.

Calculate the amount, in mol, of caffeine in the sample. Show your working.

amount of caffeine = .................... mol

Caffeine contains 49.48% carbon, 5.20% hydrogen and 28.85% nitrogen by mass. The remainder is oxygen.

Calculate the empirical formula of caffeine. Show your working.

Use your answer to (b) and the M_r of caffeine given in part (a) to deduce the molecular formula of caffeine. Show your working.

Apatite is a general term used for a family of various calcium phosphate compounds commonly found in rocks and minerals, bones and teeth.

i) State the formula of calcium phosphate.

[1]

ii) Hydroxylapatite, Ca₅(PO₄)₃OH, is one member of the apatite family and is a major component of bone.

Deduce the oxidation number of phosphorus in hydroxylapatite, Ca₅(PO₄)₃OH. Show your working.

[2]

iii) Another member of the apatite family is chlorapatite.

Suggest the formula of chlorapatite.

[1]

Fluorapatite, Ca₅(PO₄)₃F, reacts with sulfuric acid to form phosphoric acid, calcium sulfate and hydrogen fluoride.

Construct a balanced equation for this reaction.

Fluorapatite, Ca₅(PO₄)₃F, can also react with sulfuric acid to form calcium dihydrogenphosphate, Ca(H₂PO₄)₂, calcium sulfate and hydrogen fluoride.

2Ca₅(PO₄)₃F + 7H₂SO₄ → 3Ca(H₂PO₄)₂ + 7CaSO₄ + 2HF

Calculate the mass of calcium dihydrogenphosphate formed when 50.2 g of Ca₅(PO₄)₃F reacts with 87.9 g H₂SO₄. Show your working.

mass of Ca(H₂PO₄)₂ = ............................ g