O Level Chemistry 5070 Paper 41, May/June 2025

A student determines the solubility of solid ammonium chloride in water at $60{}^{\circ }\text{C}$. The student: step 1 puts $100{\text{ cm}}^3$ of water into a beaker step 2 heats the water step 3 measures the mass of a weighing bottle containing ammonium chloride step 4 adds some of this ammonium chloride to the water step 5 stirs the mixture to dissolve the solid step 6 repeats steps 4 and 5 until a small amount of undissolved solid remains in the beaker step 7 measures the mass of the weighing bottle and unused ammonium chloride step 8 calculates the mass of ammonium chloride added to the water. State one extra measurement the student must make to calculate the solubility.

The student’s results are shown in Table 1.1. Table 1.1

Use the results to calculate the solubility of ammonium chloride at $60{}^{\circ }\text{C}$ in $\text{g}\mathrm{/}{\text{dm}}^3$.

Explain why the method the student uses gives a higher value for the solubility than the true value.

The student repeats the experiment at different temperatures. The results are shown in Table 1.2. Table 1.2

Estimate the solubility of ammonium chloride at $20{}^{\circ }\text{C}$.

The student also wants to measure the solubility of ammonia. Suggest why the method used for ammonium chloride is not suitable for determining the solubility of ammonia.

The student: • places the sample of $\text{A}$ into a beaker • uses a measuring cylinder to add $100{\text{ cm}}^3$ of $0.250\text{ mol}\mathrm{/}{\text{dm}}^3$ hydrochloric acid, $\text{HCl}$, to the beaker • stirs the mixture until no further effervescence is observed • labels the mixture $\text{B}$. Calculate the number of moles of $\text{HCl}$ added to the beaker.

The student: • rinses a burette with water and then with $0.100\text{ mol}\mathrm{/}{\text{dm}}^3$ sodium hydroxide, $\text{NaOH}$ • fills the burette with $0.100\text{ mol}\mathrm{/}{\text{dm}}^3\text{NaOH}$ • adds $25.0{\text{ cm}}^3$ of $\text{B}$ and five drops of methyl orange indicator to a conical flask • adds $\text{NaOH}$ from the burette to the conical flask until the methyl orange indicator just changes colour • repeats this titration two more times. Fig. 2.1 shows the initial and final burette readings for titration 1. Table 2.1 shows some of the student’s results. Complete Table 2.1 by: • writing the initial and final readings for titration 1 • calculating the volume of $\text{NaOH}$ used in each titration • ticking ($\mathrm{✓}$) the best titration results.

Use the best titration results ($\mathrm{✓}$) to calculate the average volume of $\text{NaOH}$ used.

The acid used in (a) to prepare mixture $\text{B}$ is in excess. Use your answer to (b)(ii) to calculate the number of moles of $0.100\text{ mol}\mathrm{/}{\text{dm}}^3\text{NaOH}$ that react with $25.0{\text{ cm}}^3$ of $\text{B}$.

Calculate the number of moles of $\text{NaOH}$ that react with $100{\text{ cm}}^3$ of $\text{B}$.

The answer to (d) is equal to the number of moles of $\text{HCl}$ that remain in the beaker after the acid reacts with the calcium carbonate in the sample of $\text{A}$. Use your answers to (a) and (d) to calculate the number of moles of $\text{HCl}$ that react with the calcium carbonate in the sample of $\text{A}$.

The equation for the reaction between hydrochloric acid and calcium carbonate is shown. $$2\text{HCl}+{\text{CaCO}}_3\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$$ Calculate the number of moles of calcium carbonate in the sample of $\text{A}$.

In (a) the mixture of $\text{A}$ and acid is stirred until effervescence stops. Give a reason for mixing the reactants by stirring.

Give a reason for waiting for the effervescence to stop.

In (a) a measuring cylinder is used to add $100{\text{ cm}}^3$ of $0.250\text{ mol}\mathrm{/}{\text{dm}}^3\text{HCl}$ to the beaker. Explain why using the measuring cylinder makes the volume of $\text{HCl}$ used inaccurate. Suggest an improvement.

In (b) the burette is rinsed with water and then with $0.100\text{ mol}\mathrm{/}{\text{dm}}^3\text{NaOH}$. Explain why the burette is rinsed with sodium hydroxide after rinsing with water.

The student adds aqueous chlorine to $\text{W}$ and concludes that $\text{W}$ contains iodide ions. Describe the observation that the student makes that leads to this conclusion. Explain how this shows that $\text{W}$ contains iodide ions.

Describe another test and the observation that confirms that $\text{W}$ contains iodide ions.

The student does a flame test on $\text{W}$ and observes a lilac flame. Describe how the student does the flame test on $\text{W}$.

Identify $\text{W}$.

$\text{X}$ contains one anion and one cation. The student: • adds aqueous sodium hydroxide to $\text{X}$ in a test-tube • warms the mixture • holds a piece of damp red litmus paper above the test-tube. A white precipitate, soluble in excess aqueous sodium hydroxide, is formed. The litmus paper does not change colour. State three conclusions that are made from these observations.

The conclusions in (d) identify cations that may be present in $\text{X}$. Describe another test that the student does and its observations to identify the cation in $\text{X}$.

The student adds excess aqueous sodium hydroxide to $\text{X}$, then adds a piece of aluminium foil and warms the mixture. Ammonia gas is given off and tested with damp red litmus paper. Describe what happens to the litmus paper.

Identify the anion in $\text{X}$.