Concentration of solutions

A student does a titration to find the volume of phosphoric acid solution (H₃PO₄) needed to neutralise a sample of potassium hydroxide solution (KOH).

One of the products of the reaction is potassium phosphate.

This is the student’s method.

• use a measuring cylinder to transfer 25 cm³ of potassium hydroxide solution to a conical flask

• add two drops of an indicator

• fill a burette with phosphoric acid solution and record the initial reading on the burette

• add the acid to the conical flask

• record the final burette reading when the indicator changes colour

This is the equation for the reaction.

Explain two improvements to the student’s method that would give more accurate results.

The student repeats the titration correctly.

The potassium hydroxide solution has a concentration of 0.150 mol / dm³.

The phosphoric acid solution has a concentration of 0.0400 mol / dm³.

Calculate the volume, in cm³, of phosphoric acid solution needed to neutralise 25.0 cm³ of 0.150 mol / dm³ potassium hydroxide solution.

Give your answer to three significant figures.

Potassium phosphate solution reacts with calcium chloride solution.

One of the products is the insoluble solid, calcium phosphate.

Complete the equation for the reaction.

The student reacts a solution containing 0.0400 mol of potassium phosphate with an excess of calcium chloride solution.

The student obtains 5.70 g of calcium phosphate.

Show that the percentage yield for the reaction is about 90%

[for Ca₃(PO₄)₂ M_r = 310]

Describe how the student can obtain a pure, dry sample of calcium phosphate from the mixture at the end of the reaction.

This question is about alcohols and carboxylic acids.

These are the structural formulae of alcohol A and carboxylic acid B.

Alcohol A
CH₃CH₂CH₂OH

Carboxylic acid B CH₃(CH₂)_nCOOH

Name alcohol A.

Draw the displayed formula of alcohol A.

Carboxylic acid B has a chain of carbon atoms with no branches.

The number of CH₂ units is represented by the letter n.

Calculate the value of n.

[for carboxylic acid B, M_r = 242]

Alcohol A and carboxylic acid B react together to form an ester.

Give the other product of the reaction.

These are the structural formulae of dicarboxylic acid C and diol D.

Dicarboxylic acid C HOOCCH₂CH₂COOH

Diol D
HOCH₂CH₂OH

This is the equation for the reaction between dicarboxylic acid C and sodium hydroxide solution.

HOOCCH₂CH₂COOH + 2NaOH NaOOCH₂CH₂COONa + 2H₂O

25.0 cm³ of 0.150 mol/dm³ sodium hydroxide solution is completely neutralised by 17.5 cm³ of a solution of dicarboxylic acid C.

Calculate the concentration, in mol/dm³, of the solution of dicarboxylic acid C.

Give your answer to three significant figures.

concentration =_........ mol/dm

Dicarboxylic acid C and diol D react to form a polyester.

Draw the displayed formula of the repeat unit of this polyester.

A scientist reacts lead(II) nitrate solution with sodium bromide solution.

This is the equation for the reaction.

Pb(NO₃)₂(aq) + 2NaBr(aq) PbBr₂(s) + 2NaNO₃(aq)

Describe how the scientist could obtain a pure dry sample of lead(II) bromide (PbBr₂) from the reaction mixture.

The scientist reacts an excess of lead(II) nitrate solution with 25 cm³ of sodium bromide solution of concentration 2.0 mol/dm³.

Show that the amount of sodium bromide used is 0.050 mol.

Show that the maximum theoretical mass of lead(II) bromide is approximately 9 g.

[for PbBr₂ M_r = 367]

The scientist electrolyses molten lead(II) bromide using this apparatus.

Explain why lead(II) bromide needs to be molten rather than solid for electrolysis to occur.

The electrolyte is at a temperature of 400C.

Explain a suitable material for the electrodes.

Give the half‑equation that occurs at the negative electrode.

Bromine forms at the positive electrode.

This is the half‑equation for the reaction at the positive electrode.

State what is observed at the positive electrode.

State why the half‑equation represents an oxidation reaction.

The diagram shows two pieces of apparatus used in a titration.

Give the names of these pieces of apparatus.

Give the name of a suitable indicator that can be used in an acid-alkali titration.

A student does a titration using sodium carbonate solution and dilute nitric acid.

This is the equation for the reaction.

The table shows the concentrations of the two solutions and the volume of sodium carbonate used in the titration.

Use the equation and the data in the table to answer these questions.

Calculate the volume of dilute nitric acid that the student would need to neutralise the sodium carbonate solution.

Calculate the volume, in cm³, of carbon dioxide gas at rtp that would be produced from the 25.0 cm³ of the sodium carbonate solution.

[at rtp, molar volume = 24 000 cm³]

Describe a test to show that sodium carbonate solution contains carbonate ions.

A student does a titration to find the concentration of potassium hydroxide solution.

This is the student’s method.

• add 25.0 cm³ of the potassium hydroxide solution to a conical flask

• add a few drops of methyl orange indicator to the conical flask

• fill a burette with dilute sulfuric acid and record the initial burette reading

• place the conical flask on a white tile

• add the acid from the burette to the mixture in the conical flask, swirling the flask continuously

• when the indicator changes colour at the end point, record the final burette reading

Repeat the titration to obtain concordant results.

Give the name of the most suitable piece of apparatus to measure out 25.0 cm³ of potassium hydroxide solution.

Give the colour of methyl orange in potassium hydroxide solution and in dilute sulfuric acid.

colour in potassium hydroxide solution

colour in dilute sulfuric acid

State why the student places the conical flask on a white tile.

State why the student swirls the flask continuously.

State what is meant by the term concordant results.

The student finds that 15.00 cm³ of sulfuric acid of concentration 0.180 mol / dm³ neutralises 25.0 cm³ of potassium hydroxide solution.

This is the equation for the reaction.

Calculate the concentration of the potassium hydroxide solution.

concentration =_........ mol / dm

This is the ionic equation for the reaction between an acid and an alkali.

Explain why the OH⁻ ion is a proton acceptor in this reaction.

Describe the forces of attraction in metallic bonding.

When a small piece of potassium is added to water, bubbles of hydrogen gas are observed.

Give the test for hydrogen gas.

Give two other observations that would be made.

Explain why lithium is less reactive than potassium.

Refer to atomic structure in your answer.

This is the equation for the reaction between sodium and water.

A mass of 0.750 g of sodium is reacted with an excess of water.

Calculate the volume of hydrogen gas produced, in cm³, at room temperature.

[molar volume of hydrogen at rtp = 24 000 cm³]

[for Na,A_r = 23]

Give your answer to three significant figures.

This is the equation for the reaction between sodium hydroxide and sulfuric acid.

A volume of 25.0 cm³ of sodium hydroxide solution is completely neutralised by 16.3 cm³ of 0.0500 mol / dm³ sulfuric acid.

Calculate the concentration of the sodium hydroxide solution in mol / dm³.

concentration =_........ mol / dm

A student is asked to find the concentration of a solution of nitric acid by doing a titration.

The student is provided with sodium hydroxide solution with a concentration of 0.350 mol/dm³

These are the first four steps in the student’s method.

• use a pipette to add exactly 25.0 cm³ of the nitric acid solution to a conical flask placed on a white tile

• add a few drops of indicator to the flask

• use a burette to add sodium hydroxide solution to the flask until the indicator has changed colour

• record the volume needed for the indicator to change colour

Give the reason for using the white tile.

Universal indicator is not a suitable indicator to use in a titration.

Complete the table to show the name of a suitable indicator and its final colour in this titration.

The student wants to obtain an accurate value for the volume of sodium hydroxide solution needed to neutralise the 25.0 cm³ of nitric acid.

Describe all the further steps the student should take.

This is a summary of the student’s results.

volume of HNO₃ used = 25.0 cm³

concentration of NaOH solution = 0.350 mol/dm³

volume of NaOH solution needed for neutralisation = 18.80 cm³

The equation for the reaction is

HNO₃ + NaOH NaNO₃ + H₂O

Calculate the amount, in moles, of NaOH that reacts.

Determine the amount, in moles, of HNO₃ that reacts.

amount of HNO₃ =_........

Calculate the concentration, in mol/dm³, of the HNO₃

concentration =_........ mol/dm

This question is about hydrazine, N₂H₄

This is the displayed formula for a molecule of hydrazine.

Complete the dot‑and‑cross diagram for hydrazine.

Use the data in the table to calculate the total energy needed to break all the bonds in the reactants.

Use the data in the table to calculate the total energy released when all the bonds in the products are made.

Calculate the enthalpy change, Δ H, in kJ/mol, for the reaction.

ΔH =_........ kJ/mol

Explain, in terms of bonds broken and bonds made, why this reaction is exothermic.

A sample of hydrazine is completely decomposed.

This is the equation for the decomposition of hydrazine.

The products of the decomposition are bubbled through 1100 cm³ of water.

The ammonia completely dissolves in the water, but nitrogen is insoluble in water.

The nitrogen has a volume of 1570 cm³ at room temperature and pressure (rtp).

Calculate the concentration, in mol/dm³, of the ammonia solution.

Give your answer to 3 significant figures.

[for a gas at rtp, molar volume = 24 000 cm³]

concentration =_........ mol/dm

The diagram shows two pieces of apparatus, X and Y.

Give the name of each piece of apparatus.

In a titration, a student adds 25.0 cm³ of barium hydroxide solution to a conical flask.

Give a reason why it is better to use Y rather than X.

The student uses methyl orange indicator in the titration.

State the colour of methyl orange in barium hydroxide solution.

Give a reason why universal indicator is not suitable for use in a titration.

The student adds some dilute nitric acid to a burette and does the titration.

The equation for the reaction is

The student finds that 21.50 cm³ of nitric acid of concentration 0.600 mol/dm³ neutralises 25.0 cm³ of barium hydroxide solution.

Calculate the concentration, in mol/dm³, of the barium hydroxide solution.

concentration =_........ mol/dm

State why sulfuric acid would not be a suitable acid to use in this titration.

A student uses this method to do a titration.

• use a measuring cylinder to obtain 25 cm³ of sodium hydroxide solution

• transfer the solution to a conical flask

• add a few drops of universal indicator to the flask

• fill a burette with dilute sulfuric acid and record the initial burette reading

• add the acid to the flask, swirling the flask continuously

• add the acid slowly near the end‑point

• record the final burette reading at the end‑point

The student repeats the titration until at least two concordant results are obtained.

State what is meant by concordant results.

Explain two improvements to the student’s method so that more accurate results are obtained.

The student makes the improvements and repeats the titration.

The sulfuric acid has a concentration of 0.600 mol/dm³.

The sodium hydroxide solution has a concentration of 1.50 mol/dm³.

This is the equation for the reaction.

Calculate the volume, in cm³, of sulfuric acid that the student needs to completely react with 25.0 cm³ of the sodium hydroxide solution.

The student plans to obtain pure dry crystals of hydrated sodium sulfate.

They add the calculated volume of sulfuric acid to 25.0 cm³ of the sodium hydroxide solution to form sodium sulfate solution.

Describe what the student should do to obtain pure dry crystals of hydrated sodium sulfate from the solution.

A student does a titration to find the concentration of a solution of dilute sulfuric acid.

The student uses these solutions and this apparatus.

• dilute sulfuric acid

• potassium hydroxide solution of concentration 0.240 mol / dm³

• methyl orange indicator

The student wants to find the volume of sulfuric acid needed to neutralise 25.0 cm³ of the potassium hydroxide solution.

Describe how the student should do this titration.

Assume that all pieces of apparatus are clean and dry.

The student needs 15.00 cm³ of sulfuric acid to neutralise 25.0 cm³ of the potassium hydroxide solution.

This is the equation for the reaction.

Calculate the amount, in moles, of KOH in 25.0 cm³ of potassium hydroxide solution of concentration 0.240 mol / dm³.

Calculate the amount, in moles, of H₂SO₄ in 15.00 cm³ of the sulfuric acid.

amount of H₂SO₄ =_........

Calculate the concentration, in mol / dm³, of the sulfuric acid.

concentration of sulfuric acid =_........ mol / dm

This question is about esters.

Ester A reacts with water to form ethanoic acid and ethanol.

The displayed formulae of the reactants and products are shown in this equation

ester A

The molar enthalpy change (Δ H) for the reaction is 0 kJ/mol.

Draw a ring around the functional group in ester A.

Give the name of ester A.

Describe a chemical test, other than using an indicator, to show that the reaction mixture contains ethanoic acid.

Explain why the molar enthalpy change (Δ H) for the reaction between ester A and water is 0 kJ/mol.

In your answer, refer to the bonds broken and the bonds formed.

A mixture of ester A and water is left in a sealed container until the reaction mixture reaches dynamic equilibrium.

Describe what is meant by dynamic equilibrium.

Explain why adding a catalyst does not change the position of equilibrium.

The ethanoic acid produced in the reaction is completely neutralised by 22.75 cm³ of 0.150 mol/dm³ barium hydroxide solution.

The equation for the neutralisation reaction is

Calculate the amount, in moles, of ethanoic acid neutralised.

Give your answer to 3 significant figures.

The structures of two organic compounds are shown.

These compounds react together to form a polymer.

Give the repeat unit of the polymer formed.

This question is about alcohols, carboxylic acids and esters.

Ethanol can be manufactured by reacting ethene with steam in the presence of a phosphoric acid catalyst.

Which row gives the correct conditions of temperature and pressure for this reaction?

Give the displayed formula of butanol.

Ethanoic acid (CH₃COOH) is a carboxylic acid present in vinegar.

The concentration of CH₃COOH in vinegar can be found by titration with aqueous potassium hydroxide (KOH).

The equation for the reaction is

CH₃COOH + KOH CH₃COOK + H₂O

In a titration, a 25.0 cm³ sample of vinegar is neutralised by 45.00 cm³ of KOH solution of concentration 0.400 mol/dm³.

Calculate the concentration, in mol/dm³, of CH₃COOH in this sample of vinegar.

concentration =_........ mol/dm

A sample of vinegar containing 0.0030 mol of CH₃COOH is poured into a flask.

Calculate the maximum volume, in cm³, of carbon dioxide gas formed at rtp when excess sodium carbonate is added to the flask.

The equation for the reaction is

[Assume that the molar volume of carbon dioxide at rtp is 24 000 cm³]

Alcohols react with carboxylic acids to form esters.

Which alcohol could react to form the ester ethyl propanoate?

Polyesters are formed in condensation polymerisation reactions between dicarboxylic acids and diols.

State one difference between condensation polymerisation and addition polymerisation.

The repeat unit of a polyester is

O O H H

H H

Give the displayed formula of each of the two monomers needed to form this polyester.

Give one advantage of biopolyesters.

A student does a titration using dilute sulfuric acid to find the concentration of a solution of potassium hydroxide.

The student adds 25.0 cm³ of the potassium hydroxide solution to a conical flask. He then adds a few drops of methyl orange indicator.

The student does the titration four times.

Name the piece of apparatus the student should use to add the potassium hydroxide solution.

What is the colour of methyl orange in an alkaline solution?

The table shows the student’s results.

Concordant results are those within 0.20 cm³ of each other.

Place ticks (✓) in the table to show which results are concordant.

Use the concordant results to calculate the mean (average) volume of acid added.

This table shows the student’s results for another titration.

The equation for the reaction is

Calculate the amount, in moles, of KOH in 25.0 cm³ of the potassium hydroxide solution.

Calculate the amount, in moles, of H₂SO₄ in 21.20 cm³ of sulfuric acid.

Calculate the concentration, in mol/dm³, of the sulfuric acid.

concentration of sulfuric acid =_........ mol/dm

This question is about the insoluble salt silver chloride (AgC l).

Silver chloride can be made by the reaction between copper(II) chloride and silver nitrate.

Describe how a student could prepare a pure, dry sample of silver chloride starting with copper(II) chloride solution and silver nitrate solution.

A student investigates the quantity of silver chloride produced when different volumes of silver nitrate solution are added to copper(II) chloride solution.

This is the student’s method. pour 5.0 cm³ of copper(II) chloride solution into a test tube add 1.0 cm³ of silver nitrate solution to the test tube

allow the silver chloride precipitate to settle

measure the height of the precipitate

The student repeats the method using different volumes of silver nitrate solution.

The table shows the student’s results.

Plot the student’s results.

Draw two straight lines of best fit, ignoring the anomalous result.

Volume of silver nitrate added in cm³

Suggest a mistake the student made to cause the anomalous result.

Give a reason why the last three heights are the same.

The equation for the reaction between copper(II) chloride and silver nitrate is

CuCl₂(aq) + 2AgNO₃(aq) 2AgCl(s) + Cu(NO₃)₂(aq)

A student measures 25.0 cm³ of 0.500 mol/dm³ copper(II) chloride solution and reacts it with silver nitrate solution.

Name a piece of apparatus suitable for measuring 25.0 cm³ of copper(II) chloride solution.

Calculate the maximum mass, in grams, of silver chloride that could be produced.

[M_r of AgCl = 143.5]

In an experiment using different solutions, the mass of silver chloride produced is 0.744 g.

The maximum mass of silver chloride that could be produced is 0.850 g.

Calculate the percentage yield.

A student is provided with a bottle containing a colourless solution X.

Solution X is thought to be dilute sulfuric acid of concentration 0.10 mol/dm³.

The student does some experiments on samples of solution X to try to show that it is dilute sulfuric acid.

The student adds a few drops of litmus to a sample of solution X.

The litmus turns red.

The student knows that the products of the electrolysis of dilute sulfuric acid are hydrogen and oxygen.

She carries out the electrolysis using this apparatus.

Suggest why the student does not use zinc electrodes in her experiment.

State what is observed at both the anode and the cathode during the electrolysis.

Which of these tests shows that the gas formed at the cathode is hydrogen?

Describe a test to show that solution X contains sulfate ions.

The student then does a titration to see if the concentration of the dilute sulfuric acid is 0.10 mol/dm³.

She measures 25.0 cm³ of potassium hydroxide solution into a conical flask, and then adds a few drops of indicator solution.

Name the piece of apparatus the student should use to measure 25.0 cm³ of the potassium hydroxide solution.

The concentration of potassium hydroxide in the solution is 0.125 mol/dm³.

Calculate the amount, in mol, of KOH in 25.0 cm³ of this solution.

The equation for the reaction in the titration is

Calculate the volume, in cm³, of 0.10 mol/dm³ sulfuric acid needed to neutralise 25.0 cm³ of the potassium hydroxide solution.

A student wants to prepare sodium chloride crystals from sodium hydroxide solution and dilute hydrochloric acid.

He does a titration to find the volume of dilute hydrochloric acid needed to neutralise the sodium hydroxide solution.

This is his method.

● add 25.0 cm³ of sodium hydroxide solution to a conical flask

● add a few drops of phenolphthalein indicator to the conical flask

● titrate the solution with the hydrochloric acid

Name a suitable piece of apparatus that the student should use to measure 25.0 cm³ of sodium hydroxide solution.

Give the colour of the phenolphthalein indicator in sodium hydroxide solution and in hydrochloric acid.

colour in sodium hydroxide solution_........

colour in hydrochloric acid_........

Suggest why universal indicator is never used in a titration.

The student finds that 21.50 cm³ of hydrochloric acid is needed to neutralise 25.0 cm³ of sodium hydroxide solution.

Describe what the student should do next to prepare a pure solution of sodium chloride.

Describe how the student could obtain dry crystals of sodium chloride from the pure sodium chloride solution.

The student needs 21.50 cm³ of hydrochloric acid to neutralise 25.0 cm³ of sodium hydroxide solution of concentration 0.800 mol/dm³.

The equation for the reaction is

Calculate the concentration, in mol/dm³, of the hydrochloric acid.

concentration =_........ mol/dm

Solutions of silver nitrate and potassium chloride react together to make the insoluble salt, silver chloride.

A student uses this method to prepare a sample of silver chloride.

Step 1 add 25 cm³ of silver nitrate solution to a conical flask

Step 2 add potassium chloride solution to the flask

Step 3 filter off the silver chloride

What term is used for this reaction?

Give two more steps that will produce a pure, dry sample of silver chloride.

Acidified silver nitrate solution is used to test for chloride ions.

Give a reason why hydrochloric acid is not used to acidify silver nitrate solution.

The chemical equation for the reaction between solutions of silver nitrate and potassium chloride is

AgNO₃(aq) + KCl(aq) AgCl(s) + KNO₃(aq)

A student adds an excess of potassium chloride solution to 25.0 cm³ of 0.100 mol/dm³ silver nitrate solution.

Calculate the maximum mass of silver chloride, in grams, that can be produced.

[M_r of AgCl = 143.5]

A student investigates the reaction between sodium hydroxide solution and dilute sulfuric acid.

He does a titration to find the concentration of the sulfuric acid.

This is his plan for the titration. There are some mistakes and omissions in his plan.

● rinse a conical flask with the sodium hydroxide solution

● use a measuring cylinder to measure out 25 cm³ of the sodium hydroxide solution and add it to the conical flask

● add a few drops of methyl orange indicator to the conical flask

● rinse a burette with water and then fill it with the sulfuric acid

● add the acid from the burette to the conical flask until the indicator changes colour at the end‑point of the titration

● record the final burette reading

Give the colour change of the methyl orange indicator at the end‑point.

Describe four changes that the student could make to improve his plan.

The student then does the titration correctly.

He finds that 16.70 cm³ of the dilute sulfuric acid neutralises 25.0 cm³ of sodium hydroxide solution of concentration 0.200 mol/dm³

The equation for the reaction is

Calculate the concentration, in mol/dm³, of the sulfuric acid.

concentration of sulfuric acid =_........ mol/dm

This question is about Group 1 metals and their reactions.

When lithium is added to water, bubbles of hydrogen gas are observed.

Give two other observations that could be made.

Give the test for hydrogen gas.

Give one observation that would be different if potassium is used instead of lithium.

The diagram represents an atom of lithium and an atom of potassium.

Explain why potassium is more reactive than lithium.

The equation for the reaction between lithium and water is

A mass of 0.500 g of lithium reacts with an excess of water.

Calculate the volume, in cm³, of hydrogen gas produced at rtp.

[molar volume of a gas at rtp = 24 000 cm³]

Give your answer to three significant figures.

In a reaction between lithium and water, 150 cm³ of lithium hydroxide solution is formed.

The lithium hydroxide solution is then completely neutralised by 24.85 cm³ of 0.100 mol/dm³ sulfuric acid.

The equation for the neutralisation is

Calculate the concentration, in mol/dm³, of the lithium hydroxide solution.

concentration =_........ mol/dm

When a bottle of wine is left open for several days, some of the ethanol in the wine turns to ethanoic acid, CH₃COOH

A scientist uses a titration method to investigate how much ethanoic acid is formed if a bottle of white wine is left open for one week.

She uses this method.

● fill a burette with the white wine and record the reading

● add 25.0 cm³ of sodium hydroxide solution to a conical flask

● add a few drops of phenolphthalein indicator to the flask

● swirl the flask continuously while adding wine from the burette

● add the wine drop by drop near the end point

● record the reading at the end point

Name the piece of apparatus that would be most suitable for measuring the 25.0 cm³ of sodium hydroxide solution.

Suggest why red wine would not be suitable to use for this investigation.

State why she swirls the flask continuously.

State why she adds the wine drop by drop near the end point.

The diagram shows the burette readings at the start and end of one of the titrations.

Use the readings to complete the table.

Give your values to the nearest 0.05 cm³.

The scientist repeats the titration four more times.

The table shows her results for these four titrations.

Concordant results are those within 0.20 cm³ of each other.

Add ticks () to the table to show the concordant results.

Use your ticked results to calculate the mean (average) volume of wine added.

Another scientist repeats the titration with a different bottle of white wine that has been left open for a week.

The equation for the reaction that occurs in this titration is

CH₃COOH + NaOH CH₃COONa + H₂O

The mean volume of wine added is 19.50 cm³.

The concentration of the sodium hydroxide solution is 0.0500 mol/dm³.

Calculate the amount, in moles, of NaOH in 25.0 cm³ of sodium hydroxide solution.

Deduce the amount, in moles, of CH₃COOH in 19.50 cm³ of the wine.

Calculate the concentration, in mol/dm³, of CH₃COOH in the wine.

concentration of CH₃COOH =_........ mol/dm³

The concentration of NaC l O(aq) in a solution of bleach is found by reacting it with hydrochloric acid.

The equation for the reaction is

An excess of dilute hydrochloric acid is added to 4.00 cm³ of bleach solution.

60.0 cm³ of chlorine gas is produced.

Explain a safety precaution that should be taken when doing this experiment.

Calculate the amount, in moles, of chlorine gas produced. Assume one mole of chlorine gas occupies 24 000 cm³.

Determine the amount, in moles, of NaClO in 4.00 cm³ of bleach.

Calculate the concentration, in mol/dm³, of the bleach solution.

concentration =_........ mol/dm³