Acids, alkalis & titrations (Edexcel IGCSE Chemistry (Modular): Unit 1)

A student added some sulfuric acid gradually to a solution of ammonia until it was in excess.

What happens to the pH during this reaction?

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

He uses this method:

Step 1 add 50 cm³ of dilute hydrochloric acid to a conical flask

Step 2 add a 5 cm³ portion of sodium hydroxide solution to the conical flask

Step 3 test the pH of the mixture using both universal indicator paper and a pH meter

The student repeats step 2 and step 3 until a total of 50 cm³ of sodium hydroxide solution has been added.

i) State the piece of apparatus that should be used to measure 50 cm³ of hydrochloric acid.

(1)

 ii) Name the type of reaction that occurs between hydrochloric acid and sodium hydroxide.

(1)

Graph 1 shows how the pH of the mixture changes as the sodium hydroxide solution is added.

i) Determine the pH after 40 cm³ of sodium hydroxide solution has been added.

(1)

ii) Suggest the colour of the universal indicator paper when these volumes of sodium hydroxide solution have been added.

(2)

15 cm³: ............................................................
30 cm³: ............................................................

iii) Give the formula of the ion that causes sodium hydroxide to be alkaline.

(1)

Another student investigates how the temperature changes when the sodium hydroxide solution is added to the hydrochloric acid. The hydrochloric acid and the sodium hydroxide solution are at the same temperature at the start of the investigation. The student records the temperature of the mixture after adding each 5 cm³ portion of sodium hydroxide solution. 

Graph 2 shows her results.

Explain the shape of graph 2.

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A student carried out a titration to investigate the volume of sulfuric acid needed to neutralise 25.0 cm³ of potassium hydroxide.  

The diagram shows the equipment they used. 

Which piece of apparatus would the student use to measure out 25.0 cm³ of potassium hydroxide? 

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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.

from ........................................................... to ................................................................

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

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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

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

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

Separate: Chemistry Only

The steps below briefly describe how to carry out a titration with sodium hydroxide and hydrochloric acid.

1. Add a few drops of a suitable indicator to the solution

2. Record the final volume of HCl and repeat to obtain concordant results

3. Fill the burette with HCl and record the volume

4. Measure 25 cm³ sodium hydroxide solution into a conical flask using a pipette

5. Add the HCl to sodium hydroxide until the end point is reached

What is the correct order of steps?

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A student does a titration to find the concentration of a solution of phosphoric acid. He uses these pieces of apparatus X, Y and Z in his titration.

Diagrams are not to scale.

Give the names of X, Y and Z.

X ....................................................................................

Y ....................................................................................

Z ....................................................................................

Separate: Chemistry Only 

What is the colour of phenolphthalein in phosphoric acid?

Separate: Chemistry Only 

The student titrates 25.0 cm³ of phosphoric acid with a solution of sodium hydroxide (NaOH). Table 1 shows the student’s results.

Table 1

Concordant results are those within 0.20 cm³ of each other.

i) Add ticks (✔) to table 1 to show the concordant results.

(1)

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

(2)

mean volume = ............................................................cm³

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Table 2 shows the titration results of another student.

Table 2

   The equation for the reaction is:

3NaOH + H₃PO₄ → Na₃PO₄ + 3H₂O

i) Calculate the amount, in moles, of NaOH in 30.40 cm³ of sodium hydroxide solution.

(2)

amount = .............................................................................. mol

ii) Calculate the amount, in moles, of H₃PO₄ in 25.0 cm³ of phosphoric acid.

(1)

amount = .............................................................................. mol

iii) Calculate the concentration, in mol/dm³, of the phosphoric acid.

(2)

concentration = .............................................................................. mol/dm³

Separate: Chemistry Only 

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

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

(1)

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

(1)

iii) State why she swirls the flask continuously.

(1)

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

(1)

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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.

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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.

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

(1)

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

(2)

Separate: Chemistry Only 

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.

i) 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.

(2)

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

(1)

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

(2)

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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.

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

(1)

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

(1)

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The table shows the student’s results.

Concordant results are those within 0.20 cm³ of each other.

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

(1)

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

(2)

mean volume = .............................................................. cm³

Separate: Chemistry Only 

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

The equation for the reaction is

2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

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

(2)

amount of KOH = .............................................................. mol

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

(1)

amount of H₂SO₄ = .............................................................. mol

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

(2)

concentration of sulfuric acid = .............................................................. mol/dm³

Separate: Chemistry Only 

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.

Separate: Chemistry Only 

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.

2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

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

(2)

amount of KOH = .............................................................. mol

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

(1)

amount of H₂SO₄ = .............................................................. mol

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

(2)

concentration of sulfuric acid = .............................................................. mol/dm³

This question is about titrations.

An experiment was completed by titrating 25.0 cm³ of hydrochloric acid with 0.100 mol / dm³ sodium hydroxide solution.

Phenolphthalein was used as an indicator. 

Describe the colour change the student would observe during this titration. 

Suggest why an indicator is required for the titration of hydrochloric acid with sodium hydroxide.

Explain why litmus is not a suitable indicator to use in this titration. 

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The titration results are shown in Table 1.

 Table 1

 Calculate the concentration of the hydrochloric acid. 

Use only the student’s concordant results. 

Concordant results are those within 0.10 cm³ of each other.