2019 O Level Pure Chemistry Practical Paper

Foreword: Comments on the 2019 O Level Chemistry Practical

The 2019 O Level Chemistry practical was more quantitative than qualitative. In simple terms, there were more calculations. There were three parts, consisting of two experiments and one planning question.

Part	Content Needed	Lab Skills Needed
1. Neutralisation 16 marks, 45 mins	Mole ratio Limiting reactant Solution concentration Acids and bases	Use burette Use measuring cylinder Measure temperature Plot graph
2. Decomposition 20 marks, 50 mins	Mass and moles Mole ratio Qualitative analysis	Heat using Bunsen burner Measure mass Record data in a table Test for anions
3. Salt Preparation 4 marks, 15 mins	Salt preparation Titration Separation techniques	None, as you are not required to carry out your plan

The first question involves the measurement of temperature change during acid-base neutralisation. By finding the ratio of acid and base that gives the sharpest increase in temperature, you will calculate the concentration of acid.

The second involves the thermal decomposition of an impure sample of sodium hydrogenbicarbonate. By measuring the loss of mass, you will calculate the percentage purity of sodium hydrogenbicarbonate. There is also a qualitative analysis to identify the impurity present.

In the final part, you will draw out a plan for the preparation of sodium hydrogensulfate salt by titration. This is a pen-and-paper exercise, with no accompanying experiment to do.

While you cannot practise lab techniques on Chem Not Cheem, you may try out the questions in the 2019 O Level Chemistry practical below.

Part 1: Experiment on Neutralisation

H⁺(aq) + OH^–(aq) ⟶ H₂O(l)

The reaction between a strong acid and an alkali is exothermic. In this experiment, you will measure the increase in temperature when different volumes of acids and alkalis are mixed.

Reagents and Apparatus

Solution A, 0.5 mol/dm³ of a strong acid
Solution B, 1.0 mol/dm³ of sodium hydroxide
Styrofoam cup, glass beaker, measuring cylinder, burette, and thermometer

Procedure

Place a Styrofoam cup into a 250 cm³ beaker.
Use a burette to transfer 40.00 cm³ of A into the Styrofoam cup. Measure the temperature of A and record the value in the table below.
Use a measuring cylinder to transfer 10 cm³ of B into the Styrofoam cup. Use the thermometer to stir and measure the temperature throughout the reaction. Record the highest temperature reached in the table below.
Empty the Styrofoam cup and rinse it with water.
Repeat steps 1 to 4, but with different volumes of A and B as given in the table below.
Complete the table by measuring and recording the initial and maximum temperatures for each set-up.

Volume of A/ cm³	Volume of B/ cm³	Initial temperature of A/ °C	Maximum temperature/ °C	Temperature increase/ °C
40.00	10	27.5	30.0	2.5
35.00	15	27.5	31.0	3.5
30.00	20	27.5	32.5	5.0
25.00	25	27.5	34.0	6.5
20.00	30	27.5	33.0	5.5
15.00	35	27.5	31.5	4.0
10.00	40	27.5	29.5	2.0

Note: The measurements written in blue are hypothetical student answers. However, they are theoretically valid readings that you might get, if you were to conduct the experiment on your own.

QUESTION 1
Plot a graph of temperature increase against volume of B on the grid below. Use these points to draw and extrapolate two intersecting straight lines of best fit.

Your graph should have:

Two extrapolated best-fit lines, drawn with a ruler
Labelled axes, with units
Title

QUESTION 2
From the graph, read the volume of B where the two lines intersect.

25.5 cm³

Marker’s note: leave your answer to 1 d.p., as the uncertainty of the graph is ±0.5, which is half the smallest scale division.

QUESTION 3
Calculate the volume of A required to neutralise the volume of B in Question 2.

As the total volume is 50 cm³, volume of A = 50.0 – 25.5 = 24.5 cm³

QUESTION 4
Calculate the concentration of hydrogen ions, H⁺, in A.

STEP 1: State the chemical equation
H⁺(aq) + OH^–(aq) ⟶ H₂O(l)

STEP 2: Find the number of moles of OH^– from NaOH
No of moles of OH^– = conc × vol = 1.0 × (25.5/1000) = 0.0255 mol

STEP 3: State the mole ratio of H⁺ to OH^–
Mole ratio of H⁺ to OH^– = 1:1

STEP 4: Use the mole ratio to find number of moles of H⁺
No of moles of H⁺ = (1/1) × 0.025 = 0.0255 mol

STEP 5: Find the concentration of H⁺
Concentration of H⁺ = no of moles ÷ vol = 0.0255 ÷ (24.5/1000) = 1.04 mol/dm³

QUESTION 5
Suggest the identity of the strong acid in A and explain your suggestion.

A is sulfuric acid. 0.5 mol/dm³ of A gives around 1 mol/dm³ of hydrogen ions. This suggests that the acid is dibasic, which releases 2 hydrogen ions for every molecule of acid.

QUESTION 6
Suggest two changes that could be made to this experiment to improve the accuracy to the results.

Use a burette to measure the volume of B, as it has a higher precision than a measuring cylinder
Record both the initial temperatures of A and B, and take the average as the initial temperature of the reaction mixture
Double or triple the starting concentration of A and B, such that the increase in temperature will be more pronounced to reduce the percentage error

Part 2: Experiment on Thermal Decomposition

2NaHCO₃ ⟶ Na₂CO₃ + H₂O + CO₂

Sodium hydrogencarbonate undergoes thermal decomposition when it is heated.

A sample of impure sodium hydrogencarbonate, S, is contaminated by another sodium compound that does not react upon heating. In this experiment, you will first determine the percentage by mass of sodium hydrogencarbonate present in the impure sample and then identify the impurity.

Procedure to Find Percentage by Mass

Use the balance to measure the mass of an empty test tube.
Add about 1.5 g of S to the test tube and then measure the combined mass of the test tube and its content.
Gently heat S for 3 minutes to decompose the sodium hydrogencarbonate present.
Upon cooling, measure the mass of the test tube and the remaining content.

QUESTION 7
Assume that you have conducted the experiment and the three measurements you have taken are:

Mass of empty test tube = 10.21 g
Mass of test tube and S = 11.74 g
Mass of test tube and residual solid after heating = 11.35 g

Record the above measurements in an appropriate format. From your results, determine the mass of S before heating and the change in mass upon heating.

	Mass/ g
Empty test tube/ g	10.21
Test tube and S/ g	11.74
Test tube and residue/ g	11.35
S before heating/ g	11.74 – 10.21 = 1.53
Change in mass/ g	11.74 – 11.35 = 0.39

Marker’s note: While it looks trivial, this question tests you on the presentation of experimental data. Your table will need to be drawn neatly with a ruler, with headings that include units. The masses should be recorded according to the accuracy of the balance, which is usually at 0.01 g (2 d.p.).

QUESTION 8
Using your answers to QUESTION 7 and the chemical equation for the thermal decomposition of sodium hydrogencarbonate, calculate the mass of sodium hydrogencarbonate present in the mass of R heated.

The M_r of NaHCO₃, H₂O and CO₂ are 84, 18 and 44 respectively.

The decrease in mass is due to the loss of carbon dioxide gas and water in the form of gaseous steam. From the chemical equation, water and carbon dioxide gas are lost in a 1:1 ratio.

STEP 1: Convert mass data to number of moles
Combined mass of carbon dioxide and steam = change in mass = 0.39 g
No of moles of carbon dioxide-steam pair = 0.39 ÷ (18 + 44) = 0.0063 mol

STEP 2: Use mole ratio to find the number of moles of NaHCO₃
Mole ratio of NaHCO₃ to H₂O-CO₂ = 2:1
No of moles of NaHCO₃ = (2/1) × 0.0063 = 0.013 mol

STEP 3: Convert number of moles to mass
Mass of NaHCO₃ = 0.013 × 84 = 1.1 g

Marker’s note: We leave our answers in 2 s.f. as we keep to the lowest s.f. when we multiply or divide.

QUESTION 9
Calculate the percentage by mass of sodium hydrogencarbonate in S.

Percentage by mass = 1.1 ÷ 1.53 × 100% = 72%

QUESTION 10
Suggest why heating the sample once for 3 minutes may not give an accurate value for the percentage by mass of sodium hydrogencarbonate in S.

Not all the sodium hydrogencarbonate may have fully reacted in 3 minutes, hence causing the change in mass to be smaller than expected.

QUESTION 11
Suggest one way to improve the experiment to obtain a more accurate value of the percentage by mass of sodium hydrogencarbonate in S.

Heat the test tube for another 3 minutes and measure its mass. Repeat this until the mass does not decrease further, which suggests that the reaction is complete.

Procedure to Identify the Unknown Impurity

T is the residual solid left behind after heating S. Use a fresh sample of T provided in a separate test tube.
Place the test tube containing T in a test-tube rack. Then add dilute nitric acid, a little at a time, until no further reaction occurs. Record your observations.
Transfer a small portion of the solution from step 2 into a test tube. Add a few drops of aqueous silver nitrate.
Finally, transfer another portion of the solution from step 2 into another test tube. Add an equal volume of aqueous barium nitrate.

QUESTION 12
Assume that you have added dilute nitric acid to T. You observed an effervescence of colourless, odourless gas. The gas was bubbled into limewater, causing white precipitate to form.

Name the gas evolved.

Carbon dioxide gas

Marker’s note: In the actual practical, you would write your own observations and test for the gas without any guidance. As you know that heating sodium hydrogencarbonate in S forms sodium carbonate in T, it is sensible to test for carbon dioxide gas first.

QUESTION 13
When aqueous silver nitrate was added to the solution from step 2, white precipitate was formed. However, when aqueous barium nitrate was added to it, no precipitate was formed.

Identify the impurity in S.

Sodium chloride

Marker’s note: Firstly, the question tells us that the impurity is a sodium compound. Secondly, the white precipitate formed is silver chloride, hence confirming the presence of chloride anions.

QUESTION 14
Why was excess acid added to T before carrying out steps 3 and 4?

To react away the carbonate anions, which would also form a white precipitate with both aqueous silver nitrate and aqueous barium nitrate. This would lead to false positive.

QUESTION 15
Why was nitric acid used in step 2 rather than hydrochloric or sulfuric acid?

As all nitrate compounds are soluble, the nitrate anions from nitric acid will not form any precipitate to affect the results in steps 3 and 4.

Part 3: Planning Question

H₂SO₄ + 2NaOH ⟶ Na₂SO₄ + 2H₂O
H₂SO₄ + NaOH ⟶ NaHSO₄ + H₂O

The complete neutralisation of sulfuric acid by sodium hydroxide forms the salt sodium sulfate, Na₂SO₄ .

However, sodium hydrogensulfate, NaHSO₄, can also form when sulfuric acid reacts with sodium hydroxide.

QUESTION 16
Outline a method to obtain crystals of sodium hydrogensulfate using 1.0 mol/dm³ sulfuric acid and aqueous sodium hydroxide of unknown concentration.

Use a pipette to transfer 25.00 cm³ of sulfuric acid into a conical flask.
Add a few drops of methyl orange indicator into the conical flask.
Use a burette to add aqueous sodium hydroxide into the conical flask, until the indicator changes from red to yellow. Record the volume.
Repeat the titration, using 25.00 cm³ of sulfuric acid and half the recorded volume of aqueous sodium hydroxide. Do not add any indicator.
Heat the resulting solution to obtain a hot saturated solution.
Cool the solution to form sodium hydrogensulfate crystals.
Wash the crystals with cold, deionised water. Dry them between sheets of filter paper.

Marker’s note: Note that 1:2 ratio of sulfuric acid to sodium hydroxide yields sodium sulfate, but 1:1 ratio yields sodium hydrogensulfate. However, there is no indicator we can use to find out the exact volume of sodium hydroxide that gives a 1:1 ratio. Instead, we can use methyl orange to indicate the complete neutralisation of sulfuric acid, which corresponds with the 1:2 ratio. We then halve the volume.

End of 2019 O Level Chemistry Practical

VIRTUAL LAB: TEST FOR CARBONATE