Experimental Design (Cambridge (CIE) O Level Chemistry): Theory Questions

A student investigated the effect of temperature on the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid:

Na₂S₂O₃ (aq) + 2HCl (aq)  → 2NaCl (aq) + S (s) + SO₂ (g) + H₂O (l)

The reaction between these two substances produces a precipitate which makes the mixture turn cloudy. 

A student timed how long it took until the cross could no longer be seen through the precipitate, shown in Figure 1.

They then calculated the rate of reaction and repeated the experiment at different temperatures. 

Figure 1 

Name the product that made the mixture go cloudy.

Suggest two control variables for the investigation that would ensure valid results are obtained.

Define the terms:

i) Solution 

[1]

ii) Solvent

[1]

iii) Solute 

[1]

From the products formed in part a) identify:

i) The solute

[1]

ii) The solvent 

[1]

Rock salt is a mixture of sand and salt. Salt dissolves in water while sand does not. 

A group of students separated rock salt. 

They used the following method.

1. Add the rock salt to a beaker.

2. Add 250 cm³ of water.

3. Leave without disturbing to allow the sand to settle to the bottom.

4. After a while, filter the salt water into an evaporating dish.

5. Heat the evaporating dish with a Bunsen burner until salt crystals begin to form.

i) Suggest one improvement to step 2 that the students could have done to make sure all the salt is dissolved. 

[1]

ii) What name is given to the solution when no more salt can dissolve at this specific temperature? 

[1]

Identify the residue and filtrate in step 4. 

Suggest one safety precaution the students should take in step 5.

Another student separated water from salty water using the apparatus in the diagram below. 

 By referring to points 1 and 2 in the diagram, describe how this technique works.

Identify the residue in the experiment being carried out in part (d).

A student investigates the energy of reaction when different alcohols are burned. The alcohols belong to a homologous series.

The student uses the apparatus shown in Fig. 2.1

Fig 2.1

The student does five experiments.

Experiment 1

• Using a measuring cylinder, 10 cm³ of methanol, CH₃OH, is placed into the spirit burner.

• 100 cm³ of water is poured into a 250 cm³ beaker.

• The initial temperature of the water is recorded.

• The wick is lit and the alcohol is allowed to burn until it is all used up.

• The final temperature of the water is recorded.

Experiment 2

• Experiment 1 is repeated using ethanol, C₂H₅OH.

Experiments 3, 4 and 5

• Experiment 1 is repeated using propanol (C₃H₇OH), butanol (C₄H₉OH) and pentanol (C₅H₁₁OH).

Use the thermometer diagrams for each experiment to complete Table 2.1.

Table 2.1

Experiment 1

Experiment 2

Experiment 3

Experiment 4

Experiment 5

The student wants to study the relationship between the number of carbon atoms in the alcohol molecule and the energy released.

Plot the results from Experiments 1 to 5 on the grid in Fig 2.2. The x-axis is provided. Add a suitable scale for the y-axis. Draw a line of best fit that extends through all the grid.

Fig 2.2

Use your graph to deduce the temperature increase expected if hexanol (6 carbon atoms) is used. Show clearly on the graph how you worked out your answer.

............. °C

Calculate the thermal energy, Q, absorbed by the water during the combustion of hexanol. Use your predicted temperature increase for hexanol from the graph in Fig 2.2.

$\mathrm{Q}=\mathrm{m}\mathrm{c}∆\mathrm{T}$

where:

• c = 4.2 J / g °C

• m = 100 g (mass of water used)

• ΔT is the temperature increase.

Include the unit for Q in your answer.

Describe the relationship between the number of carbon atoms in the alcohol molecule and the temperature change of the water.

Explain why the volume of alcohol used must be kept constant in all five experiments.

The student repeats the experiment but uses a pipette rather than a measuring cylinder to measure the volume of alcohol. Suggest how this change affects the accuracy of the results. Explain your answer.

The student’s calculated value for the energy released was much lower than the theoretical value found in a data book.

Suggest two reasons for this difference, other than student error in reading the thermometer.

A student needs to measure several different volumes of liquid during an investigation.

The student measures 100 cm³ of water by filling a beaker to the mark labelled 100 cm³ on its side.

Explain why this is not a suitable way to measure the volume.

Later in the investigation the student needs to add 25.0 cm³ of dilute acid to each of five conical flasks.

Name the most suitable piece of apparatus for this task.

Explain why it suits this task.

The student then needs to add exactly 37.5 cm³ of the dilute acid to a sixth flask, once only.

Name the most suitable piece of apparatus for this.

Explain your choice.

The beaker is still used to hold the reaction mixture during the investigation.

Suggest why a beaker is suitable for holding the mixture, even though it is not suitable for measuring a volume.

A student carries out three titrations, adding dilute acid from a burette to 25.0 cm³ of aqueous sodium hydroxide.

The burette readings are shown in Table 1.1.

Table 1.1

Complete Table 1.1.

A second student records the volume of acid added in titration 2 as 24.9 cm³.

State the volume the second student should have recorded.

Explain the error that has been made.

When taking each burette reading, the student lowers their eye until it is level with the bottom of the curved liquid surface.

Explain why the reading is taken in this way.

Before filling the burette with the dilute acid, the student rinses the burette with a small volume of that acid.

i) Suggest why.

[1]

ii) State the effect on the recorded volume of acid if the burette were filled while still wet with distilled water.

[1]

A student investigated what happened when sodium thiosulfate dissolved in water.

Experiment 1

By using a measuring cylinder, 20 cm³ of distilled water was poured into a polystyrene cup. Use the thermometer diagram to record the temperature of the water in the table.

1 g of powdered sodium thiosulfate was added to the cup and the mixture stirred with a thermometer. Use the thermometer diagram to record the temperature of the solution.

Experiment 2

Experiment 1 was repeated using 2 g of powdered sodium thiosulfate. Record the temperature in the table.

Experiments 3, 4 and 5

Experiment 1 was repeated using 3 g, 4 g and 5 g of powdered sodium thiosulfate respectively. Record the temperatures in the table.

What type of chemical reaction occurs when sodium thiosulfate dissolves in water?

Explain how the temperature changes would differ in the experiments if 40 cm³ of water were used.

Suggest one change you could make to the apparatus used in the experiments to obtain more accurate results.

A student investigates the temperature rise when aqueous sodium hydroxide is neutralised by dilute hydrochloric acid.

The method is shown.

Name the most suitable apparatus for:

i) step 1

[1]

ii) step 2

[1]

iii) the rinsing in step 4.

[1]

A second student suggests measuring the water for the rinsing in step 4 more closely than the apparatus named in part (a)(iii) allows.

Explain why this would not improve the investigation.

In the method, one volume is written as 25.0 cm³ and another is written as about 100 cm³.

Explain what the way each of these volumes is written tells the student about how it should be measured.

The method gives a volume of water for the rinsing in step 4.

Suggest why it does so.

A volumetric pipette is always filled to the same level, whatever liquid it is used with.

Name the feature of a volumetric pipette that fixes the volume it delivers.

Explain why a volumetric pipette can accurately deliver only one volume.

A student has a 25 cm³ volumetric pipette and a 50 cm³ burette.

The student needs to deliver 15.0 cm³ of a solution accurately.

State which piece of apparatus the student should use.

Explain your choice.

A burette has to be read twice to find the volume of liquid it has delivered.

A volumetric pipette needs no reading to find the volume it has delivered.

Explain why.

A student carries out a rough titration, which is not shown, followed by four accurate titrations.

The volumes of acid added are shown in Table 2.1.

Table 2.1

Results are concordant when they lie within 0.10 cm³ of each other.

State which titrations gave concordant results.

Calculate the mean volume of acid added.

Make clear which values you have used.

Another student says that titration results are concordant when they are the same.

Explain why this is not an adequate statement.

Explain why including a discordant result in the mean would make the result less reliable, rather than more reliable.

Two students investigate the same reaction.

Acidified aqueous potassium manganate(VII) is mixed with a solution that reacts with it, and the purple colour slowly disappears.

Each student uses a stop-watch to time how long the purple colour takes to disappear.

Explain why the stop-watch must be started at the moment the two solutions are mixed, rather than when the student has finished stirring them.

The two students record different times for the same reaction.

Suggest two reasons why.

The two students decide to take the mean of their two times.

Explain why taking the mean does not necessarily give a more accurate value.

One of the stop-watches displays the time to the nearest 0.01 s.

The other displays the time to the nearest 1 s.

Explain why the student using the first stop-watch does not necessarily obtain a more accurate time.