Chemistry 5070/42 — October/November 2015

Cambridge O-Level · Alternative to Practical · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Experimental Contexts · Analysis, Conclusions and Evaluation · Qualitative Analysis · Use of Techniques, Apparatus and Materials · Observations and Measurements · Planning Experiments and Investigations

Q18MUse of Techniques, Apparatus and MaterialsQualitative AnalysisExperimental ContextsFree sample

A student determines the mass of copper in mixture A which is composed of only copper and zinc.

Zinc reacts with dilute sulfuric acid. Copper does not.

Dilute sulfuric acid is mixed with A in the apparatus below. A gas is given off which is collected in B.

Fig. 1.1

(a)

Name apparatus B.

______

DifficultyEasy

Worked solution

Answer

Gas syringe

Final answer

Gas syringe

Detailed explanation

Walkthrough

The diagram shows a horizontal tube with a plunger used to collect and measure the volume of gas produced. In a chemistry laboratory, this apparatus is a gas syringe.

Key Takeaways

Candidates must be able to recognise standard laboratory apparatus from diagrams, particularly those used for collecting and measuring gases.

Common Mistakes

Calling it a 'test tube', 'burette', or 'measuring cylinder'. A gas syringe has a distinctive plunger and barrel.

Things to Be Careful About

Ensure the answer matches the exact name accepted by the mark scheme: 'gas syringe'.

Techniques used

identify apparatus from diagram

(b)

(i)

Name the gas collected in B. Give a test and observation to identify the gas.

name of gas ______
test and observation ______

DifficultyEasy

Worked solution

Answer

name of gas: hydrogen
test and observation: lighted (burning) splint; gas burns with a 'pop' (or 'squeaky pop')

Final answer

Hydrogen; burning splint pops

Detailed explanation

Walkthrough

Zinc is a metal above hydrogen in the reactivity series, so it reacts with dilute acids to produce hydrogen gas. Copper is below hydrogen and does not react. The standard test for hydrogen is to bring a lighted splint near the gas; hydrogen is flammable and burns rapidly with oxygen in the air, causing a characteristic 'pop'.

Key Takeaways

Metals above hydrogen in the reactivity series displace hydrogen from dilute acids. The test for hydrogen is a burning splint causing a pop.

Common Mistakes

Saying the gas 'explodes' (it just pops). Forgetting to specify that the splint must be lit/burning. Naming the gas as 'oxygen' or 'steam'.

Things to Be Careful About

The mark scheme requires both the test (burning splint) and the observation (pops). Writing only one will lose a mark.

Techniques used

identify gas from reaction of metal with aciddescribe standard test for hydrogen

(ii)

Construct an equation for the reaction between zinc and dilute sulfuric acid.

______

DifficultyEasy

Worked solution

Answer

$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$

Final answer

Zn + H2SO4 → ZnSO4 + H2

Detailed explanation

Walkthrough

Zinc displaces hydrogen from dilute sulfuric acid. The reactants are solid zinc ( $\text{Zn}$ ) and aqueous sulfuric acid ( $\text{H}_2\text{SO}_4$ ). The products are aqueous zinc sulfate ( $\text{ZnSO}_4$ ) and hydrogen gas ( $\text{H}_2$ ). The equation is already balanced with a 1:1:1:1 ratio.

Key Takeaways

Metals react with dilute acids to form a salt and hydrogen gas. Zinc forms zinc sulfate with sulfuric acid.

Common Mistakes

Writing $\text{H}_2\text{SO}_4$ as $\text{H}_2\text{S}$ or $\text{H}_2\text{SO}_3$ . Forgetting to write $\text{H}_2$ as a diatomic molecule. Not balancing the equation (though it is naturally balanced here).

Things to Be Careful About

State symbols are not strictly required by this mark scheme, but if included, they should be correct: $\text{Zn}(\text{s}) + \text{H}_2\text{SO}_4(\text{aq}) \rightarrow \text{ZnSO}_4(\text{aq}) + \text{H}_2(\text{g})$ .

Techniques used

write correct formulae for reactants and productsbalance the symbol equation

(c)

When all the zinc has reacted, the volume of gas collected in B is $96.0\text{ cm}^3$ when measured at room temperature and pressure.

[ $1\text{ mole of any gas occupies } 24\,000\text{ cm}^3\text{ at room temperature and pressure.}$ ]

(i)

Calculate the number of moles of gas in $96.0\text{ cm}^3$ .

______ moles

DifficultyEasy

Worked solution

Working

\text{moles} = \frac{\text{volume of gas}}{\text{molar volume}} = \frac{96.0}{24000} = 0.004 \text{ mol}

Answer

0.004

Final answer

0.004

Detailed explanation

Walkthrough

The molar volume of any gas at room temperature and pressure (r.t.p.) is given as $24000 \text{ cm}^3 \text{ / mol}$ . To find the number of moles, divide the measured volume by the molar volume: $96.0 / 24000 = 0.004$ .

Key Takeaways

At r.t.p., 1 mole of any gas occupies $24000 \text{ cm}^3$ (or $24 \text{ dm}^3$ ). Moles = Volume / Molar Volume.

Common Mistakes

Dividing 24000 by 96.0 instead of the other way around. Forgetting to convert $\text{dm}^3$ to $\text{cm}^3$ if using $24 \text{ dm}^3 \text{ / mol}$ (i.e., using 24 instead of 24000).

Things to Be Careful About

Ensure the volume units match the molar volume units. Here, both are in $\text{cm}^3$ , so no conversion is needed.

Techniques used

convert gas volume to moles using molar volume at r.t.p.

(ii)

Using your answers to (b)(ii) and (c)(i) calculate the mass of zinc in A.
[ $A_r\text{: Zn, } 65$ ]

______ $\text{g}$

DifficultyMedium-Easy

Worked solution

Working

From the equation in (b)(ii), 1 mole of $\text{Zn}$ produces 1 mole of $\text{H}_2$ .
Moles of $\text{Zn} = \text{moles of } \text{H}_2 = 0.004 \text{ mol}$ .

\text{mass of Zn} = \text{moles} \times A_r = 0.004 \times 65 = 0.26 \text{ g}

Answer

0.26

Final answer

0.26

Detailed explanation

Walkthrough

The balanced equation shows a 1:1 molar ratio between zinc and hydrogen gas. Therefore, the moles of zinc that reacted equal the moles of hydrogen gas collected, which is $0.004 \text{ mol}$ . Using the relative atomic mass of zinc ( $A_r = 65$ ), the mass is calculated as $\text{moles} \times A_r = 0.004 \times 65 = 0.26 \text{ g}$ .

Key Takeaways

Use the mole ratio from the balanced equation to find the moles of the reactant, then use $\text{mass} = \text{moles} \times M_r \text{ (or } A_r\text{)}$ to find the mass.

Common Mistakes

Using the wrong mole ratio (e.g., assuming 1:2). Multiplying by the molar mass of the gas instead of zinc. Forgetting to multiply by $A_r$ .

Things to Be Careful About

The question asks for the mass of zinc, not the mass of the salt. Use the correct $A_r$ value given in the question (65).

Techniques used

use stoichiometry to find moles of zincconvert moles of zinc to mass using relative atomic mass

(iii)

The mass of mixture A is $1.20\text{ g}$ . Calculate the mass of copper in mixture A.

______ $\text{g}$

DifficultyEasy

Worked solution

Working

Mixture A contains only copper and zinc.

\text{mass of Cu} = \text{total mass of A} - \text{mass of Zn} = 1.20 - 0.26 = 0.94 \text{ g}

Answer

0.94

Final answer

0.94

Detailed explanation

Walkthrough

The mixture is composed solely of copper and zinc. Since we have calculated the mass of zinc ( $0.26 \text{ g}$ ) and know the total mass of the mixture ( $1.20 \text{ g}$ ), the mass of copper is simply the difference: $1.20 - 0.26 = 0.94 \text{ g}$ .

Key Takeaways

In a mixture of two substances where one reacts and the other does not, the mass of the unreacted substance can be found by subtracting the mass of the reacted substance from the total initial mass.

Common Mistakes

Adding the masses instead of subtracting. Using the wrong total mass value.

Things to Be Careful About

Ensure significant figures are consistent. $1.20 - 0.26 = 0.94$ , which is correct to 2 decimal places.

Techniques used

subtract mass of zinc from total mass of mixture

(d)

When the reaction has finished, the student separates the copper from the solution remaining in the conical flask by filtration, using a previously weighed filter paper. As soon as the filtration finishes the student weighs the filter paper containing the copper residue and finds that its mass is greater than he expected. Explain why.

______

DifficultyMedium-Easy

Worked solution

Answer

The copper residue is wet (not dried) / still contains water or solution.

Final answer

The copper is wet / not dried / contains solution.

Detailed explanation

Walkthrough

After filtration, the solid residue on the filter paper will be coated with the filtrate (the liquid that passed through). If the student weighs it immediately without drying it (e.g., in an oven or by leaving it to dry), the mass of the water or remaining solution will add to the mass of the copper, making it greater than expected.

Key Takeaways

Filtration separates an insoluble solid from a liquid. To obtain the dry mass of the solid, it must be dried after filtration. Weighing it wet will give an erroneously high mass.

Common Mistakes

Saying the filter paper absorbed water (it doesn't, it's already wet). Saying the copper reacted further (it doesn't react with the dilute acid or water).

Things to Be Careful About

The explanation must relate to the mass being greater than expected. Mentioning that the copper is 'wet', 'not dried', or 'has solution on it' are all acceptable answers. Simply saying 'it is wet' is sufficient.

Techniques used

explain sources of error in filtration

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