12.1 Experimental Techniques

The diagram shows two bottles of liquid oven cleaner. 

The oven cleaners contain sodium hydroxide solution. Plan an investigation to show which oven cleaner contains the highest concentration of sodium hydroxide. 

A student investigated the reaction between aqueous sodium hydroxide and acid K.
Two experiments were carried out. 

Experiment 1

Using a measuring cylinder, 25 cm³ of acid K was poured into a conical flask.
Phenolphthalein indicator was added to the flask. A burette was filled with aqueous sodium hydroxide to the 0.0 cm³ mark. Aqueous sodium hydroxide was added from the burette to the flask and the mixture shaken until the solution showed a permanent colour change.
The final volume was measured. Use the burette diagram to record the final volume in the table and complete the table.

Experiment 2
The solution was poured away and the conical flask rinsed.
Using a measuring cylinder, 50 cm³ of acid K was poured into the conical flask. 0.3 g of powdered calcium carbonate was added to the flask and the flask shaken until no further reaction was observed.
Phenolphthalein was added to the mixture in the flask.
A burette was filled with the same aqueous sodium hydroxide and the initial volume measured. Aqueous sodium hydroxide was added from the burette to the flask and the mixture shaken until the solution showed a permanent colour change.
Use the burette diagrams to record the initial and final volumes in the table and complete the table.

What type of chemical reaction occurred when acid K reacted with sodium hydroxide?

If Experiment 1 were repeated using 50 cm³ of acid K, what volume of sodium hydroxide would be required to change the colour of the indicator?

[2]

[2]

[1]

What would be the effect on the results if the solutions of acid K were warmed before adding the sodium hydroxide? Give a reason for your answer.

Separate: Chemistry Only

Suggest the advantage, if any, of 

[2]

[2]

A student investigated what happened when sodium thiosulphate 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 thiosulphate 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 thiosulphate. 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 thiosulphate respectively. Record the temperatures in the table.

What type of chemical reaction occurs when sodium thiosulphate 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.

Ammonia is produced when aqueous sodium hydroxide is warmed with ammonium sulfate. Ammonia is less dense than air and very soluble in water. The apparatus below was used to prepare a sample of dry ammonia gas.

Name substance C.

Name substance D.

What necessary piece of equipment is missing in the diagram?

Suggest why concentrated sulfuric acid should not be used to dry ammonia.

There are two other mistakes in the apparatus shown in the diagram. Identify and explain these mistakes.

mistake 1 ..........................................................................................................................
explanation .......................................................................................................................
mistake 2 ..........................................................................................................................
explanation .......................................................................................................................

A student investigated the reaction between two different solutions of dilute hydrochloric acid, A and B, and solution C which is alkaline.

Two experiments were carried out

Experiment 1

A burette was filled with solution A of dilute hydrochloric acid to the 0.0 cm³ mark. Using a measuring cylinder, 20 cm³ of solution C was poured into a conical flask. A few drops of methyl orange were added to the flask.

Solution A was added to the flask, with shaking, until the mixture just changed colour. Use the burette diagram to record the burette reading in the table and complete the table.

Experiment 2

The burette was emptied and rinsed, first with distilled water, and then with a little of solution B. The burette was filled with solution B of dilute hydrochloric acid to the 0.0 cm³ mark.

Experiment 1 was repeated using solution B.

Use the burette diagram to record the burette reading in the table and complete the table.

[1]

[1]

Why was the burette rinsed, first with distilled water and then with solution B, before starting Experiment 2?

Separate: Chemistry and Extended Only 

Separate: Chemistry and Extended Only

If Experiment 2 was repeated using 10 cm³ of solution C, what volume of dilute hydrochloric acid would be used? Explain your answer. 

Separate: Chemistry Only

Give one advantage and one disadvantage of using a measuring cylinder for solution C.

Describe a method other than titration, using a different reactant, that could be used to compare the concentrations of the two solutions of dilute hydrochloric acid, A and B.

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:

From the products formed in part a) identify:

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.

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 chemist used a volumetric pipette to add 25.0 cm³ of a solution of sodium hydroxide of unknown concentration to a conical flask.

The chemist then carried out a titration to find out the volume of 0.150 mol / dm³ sulfuric acid needed to neutralise the sodium hydroxide.

Describe how the chemist would complete the titration.

You should state a suitable indicator and give the colour change that would be observed.

Explain why a pipette would be used to measure the sodium hydroxide solution but a burette would be used to measure the sulfuric acid solution.