Pure substances

A teacher uses this apparatus to separate a mixture of ethanol and water.

Give the name of this method of separation.

Name the pieces of apparatus labelled X, Y and Z.

Give a physical test that the teacher could do to find out if the ethanol produced is pure.

A teacher uses this apparatus to test the products of the combustion of liquid hydrocarbons.

Explain the change in appearance that occurs in the limewater.

The equation represents the complete combustion of an alkene.

C_nH_2n + oxygen carbon dioxide + nH₂O

Complete combustion of 0.0100 mol of the alkene produces 2.16 g of water.

Determine the molecular formula of this alkene.

[for H₂O, M_r = 18]

Give a reason why the mass of pure water that collects in the U-tube is less than 2.16 g.

Give a physical test to show that the water that collects in the U-tube is pure.

The teacher burns 30.0 g of heptane.

This is the equation for the complete combustion of heptane.

Calculate the minimum mass of oxygen needed for the complete combustion of 30.0 g of heptane.

[for C₇H₁₆, M_r = 100 for O₂, M_r = 32]

This question is about different hydrated forms of sodium sulfate.

A compound has the formula Na₂SO₄.7H₂O

How many different elements are there in the formula Na₂SO₄.7H₂O?

What is the total number of atoms in the formula Na₂SO₄.7H₂O?

Another hydrated form of sodium sulfate has the formula Na₂SO₄.xH₂O

A student uses this apparatus to find the value of x.

This is the student’s method.

• find the mass of an empty tube

• add solid hydrated sodium sulfate to the tube

• find the mass of the tube and hydrated sodium sulfate

• heat the tube for several minutes

• allow the tube to cool and find the mass of the tube and contents

Describe what the student should do next to make sure that all the water is removed from the hydrated sodium sulfate.

Explain the role of the ice in the beaker.

Describe how the student could prove that the liquid collected is pure water.

The table gives the student’s results.

Use the student’s results to calculate the value of x.

[for Na₂SO₄, M_r = 142 for H₂O, M_r = 18]

This question is about hydrated compounds.

Crystals of hydrated barium chloride (BaCl₂. xH₂O) contain water of crystallisation.

A student uses this apparatus to remove and collect the water from some crystals.

This is the student’s method.

Step 1 record mass of tube A when empty

Step 2 place a sample of hydrated barium chloride crystals in tube A and record new mass

Step 3 heat tube A

Step 4 allow tube A to cool and record mass

Repeat steps 3 and 4 until the mass recorded in step 4 is constant.

These are the student’s results.

mass of tube A = 10.55 g

mass of tube A and BaCl₂. xH₂O = 16.65 g

final mass of tube A and BaCl₂ = 15.75 g

Give a reason why the student repeats steps 3 and 4 until the mass is constant.

Calculate the mass of BaCl₂ that forms in tube A.

Calculate the mass of water lost.

Determine the value of x in BaCl₂. xH₂O

Show your working.

[for BaCl₂, M_r = 208 for H₂O, M_r = 18]

Describe a physical test to show that the water in tube B is pure.

A sample containing 0.02 mol of hydrated copper(II) sulfate is heated using the same apparatus.

The products of the reaction are anhydrous copper(II) sulfate and water.

This is the equation for the reaction.

Give the meaning of the symbol

Describe how the reaction can be used to show that a liquid contains water.

Calculate the maximum number of water molecules in tube B after the sample of hydrated copper(II) sulfate has completely reacted.

One mole of any substance contains 610²³ particles.

This question is about apparatus used in the laboratory.

Complete the table by giving the name of each piece of apparatus and a unit used for the quantity it measures.

The diagram shows apparatus used to obtain pure water from sodium chloride solution by simple distillation.

Explain why it is necessary for water to flow continuously in and out of the apparatus.

Describe a chemical test to show that the sodium chloride solution contains chloride ions.

Describe a physical test to show that the liquid in the conical flask is pure water.

This question is about the oxides of lead.

Yellow lead oxide (PbO) can be reacted with hydrogen to produce lead.

Complete the equation for the reaction by adding the missing state symbols.

What is the charge on the lead ion in PbO?

Explain why the reaction of yellow lead oxide with hydrogen is a redox reaction.

Describe a physical test to show that the water produced in this reaction is pure.

When red lead oxide (Pb₃O₄) is heated, yellow lead oxide forms.

The equation for the reaction is

A scientist heats a known mass of red lead oxide in a crucible in a fume cupboard.

The scientist leaves the crucible to cool, then records the total mass of the crucible and its contents.

Describe what the scientist should do next to make sure that all the red lead oxide has reacted.

The red lead oxide used in the reaction has a mass of 5.48 g.

Calculate the maximum mass of yellow lead oxide that could form.

[M_r of Pb₃O₄ = 685 M_r of PbO = 223]

A student uses this apparatus to heat crystals of hydrated zinc sulfate and collect the liquid produced.

Describe a chemical test to show that the colourless liquid contains water.

Describe a physical test to show the colourless liquid is pure water.

The equation for the decomposition of hydrated zinc sulfate is

The student records these masses.

mass of boiling tube = 41.64 g

mass of boiling tube + ZnSO₄·7H₂O = 54.46 g

Calculate the maximum volume, in cm³, of pure water that could be produced.

Give your answer to 1 decimal place.

[1.00 cm³ of pure water has a mass of 1.00 g]

[M_r of ZnSO₄·7H₂O = 287 M_r of H₂O = 18]

In an experiment using a different mass of ZnSO₄·7H₂O the maximum volume of pure water that could be produced is 8.5 cm³.

The student collected the pure water and calculated the percentage yield to be 20.3 %.

Calculate the volume, in cm³, of pure water collected.

Explain an improvement to the apparatus that would increase the percentage yield of pure water.

The boxes list changes that may happen in a laboratory and the names of some changes.

Draw one straight line from each change to its correct name.

A student has two solids, X and Y.

One of these solids is a pure substance and the other is a mixture.

Describe how the student could identify which solid is pure and which is a mixture by measuring a physical property of each solid.