(i) Describe the appearance of zinc. \_\_\_\_\_\_ (ii) Zinc oxide can be made from zinc by heating in air. Construct the equation for this reaction. \_\_\_\_\_\_ (iii) Which compound may be used to convert zinc oxide into zinc nitrate? \_\_\_\_\_\_ (b) When zinc nitrate is heated in a fume cupboard the following reaction takes place. $$ 2\text{Zn(NO}_3)_2\text{(s)} \rightarrow 2\text{ZnO(s)} + 4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)} $$ (i) Suggest why the heating is…

(a)(i) Silver/grey solid · (a)(ii) 2Zn + O2 -> 2ZnO · (a)(iii) Nitric acid (HNO3) · (b)(i) Toxic/poisonous gas is evolved · (b)(ii) 0.02 moles · (b)(iii) NO2 = 960 cm3; O2 = 240 cm3

Chemistry 5070/41 — May/June 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 · Observations and Measurements · Qualitative Analysis · Use of Techniques, Apparatus and Materials

Q17MObservations and MeasurementsExperimental ContextsUse of Techniques, Apparatus and MaterialsAnalysis, Conclusions and EvaluationFree sample

(a)

(i)

Describe the appearance of zinc.

______

DifficultyEasy

Worked solution

Answer

Silver/grey solid

Final answer

Silver/grey solid

Detailed explanation

Walkthrough

Zinc is a metal, and most metals have a characteristic silvery/grey appearance. The question only asks for appearance, so state the colour and the physical state.

Key Takeaways

Metals often have a characteristic appearance; zinc is silver/grey and solid at room temperature.
Observation questions need the physical state as well as the colour.

Common Mistakes

Writing grey powder; zinc is a solid metal, and the mark scheme accepts silver/grey solid.
Omitting solid; the mark scheme includes it.

Things to Be Careful About

The answer should be a simple observation; no need to mention reactivity or uses.

Techniques used

recognise zinc as a metalstate its colour and physical state

(ii)

Zinc oxide can be made from zinc by heating in air.

Construct the equation for this reaction.

______

DifficultyMedium-Easy

Worked solution

Answer

2\text{Zn} + \text{O}_2 \rightarrow 2\text{ZnO}

Final answer

2Zn + O2 -> 2ZnO

Detailed explanation

Walkthrough

Zinc burns in air to form zinc oxide. Write the formulae: zinc is Zn, oxygen is O2, zinc oxide is ZnO. Balance the equation: there are two oxygen atoms on the left but only one on the right, so put a 2 before ZnO. That gives two zinc atoms on the right, so put a 2 before Zn on the left.

Key Takeaways

Metal + oxygen gives a metal oxide.
Balance oxygen atoms first, then balance the metal.

Common Mistakes

Writing ZnO2 instead of ZnO.
Forgetting to balance the equation.
Writing oxygen as O instead of O2.

Things to Be Careful About

Oxygen is diatomic, so it is O2.
The mark scheme accepts the balanced equation without state symbols; adding them is not required.

Techniques used

write correct formulae for zinc and oxygenbalance the symbol equationidentify the product of heating a metal in air

(iii)

Which compound may be used to convert zinc oxide into zinc nitrate?

______

DifficultyEasy

Worked solution

Answer

Nitric acid, $\text{HNO}_3$

Final answer

Nitric acid (HNO3)

Detailed explanation

Walkthrough

To make a nitrate salt from a metal oxide, react the oxide with nitric acid. Metal oxide + acid gives salt + water. Zinc oxide + nitric acid gives zinc nitrate + water, so the compound needed is nitric acid.

Key Takeaways

Soluble salts are made by reacting an acid with a base (metal oxide, hydroxide or carbonate).
The acid determines the anion in the salt: nitric acid gives a nitrate.

Common Mistakes

Choosing zinc nitrate itself; the question asks what converts the oxide into the nitrate, so the reactant is the acid.
Writing only the formula without the name; either is accepted.

Things to Be Careful About

The salt is zinc nitrate, so the acid must be nitric acid, not hydrochloric or sulfuric acid.

Techniques used

recognise acid + metal oxide reactionidentify nitric acid as the acid needed for a nitrate salt

(b)

When zinc nitrate is heated in a fume cupboard the following reaction takes place.

2\text{Zn(NO}_3)_2\text{(s)} \rightarrow 2\text{ZnO(s)} + 4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)}

(i)

Suggest why the heating is done in a fume cupboard.

______

DifficultyEasy

Worked solution

Answer

Toxic/poisonous gas (nitrogen dioxide) is given off.

Final answer

Toxic/poisonous gas is evolved

Detailed explanation

Walkthrough

The equation shows that nitrogen dioxide, NO2, is produced when zinc nitrate is heated. NO2 is a toxic/poisonous gas. A fume cupboard removes toxic gases from the laboratory air and prevents them being breathed in.

Key Takeaways

Fume cupboards are used when a reaction gives off toxic or poisonous gases.
Read the equation to identify the gas product.

Common Mistakes

Saying to prevent an explosion; this is not correct here.
Saying harmful fumes without using toxic or poisonous; the mark scheme wants toxic/poisonous gas.

Things to Be Careful About

Mentioning nitrogen dioxide is good, but the key idea is that the gas is toxic/poisonous.

Techniques used

identify toxic gas product from equationrelate fume cupboard to preventing toxic gas inhalation

(ii)

Calculate the number of moles of zinc nitrate in $3.78\text{ g}$ of zinc nitrate.
[ $A_r$ : Zn, 65; N, 14; O, 16]

______ moles

DifficultyMedium-Easy

Worked solution

Working

$M_r(\text{Zn(NO}_3\text{)}_2) = 65 + 2(14 + 3 \times 16) = 65 + 124 = 189$

$\text{moles} = \frac{3.78}{189} = 0.02$

Answer

0.02 moles

Final answer

0.02 moles

Detailed explanation

Walkthrough

Calculate the relative molecular mass of zinc nitrate. Zn = 65, N = 14, O = 16. Each nitrate group is N + 3O = 14 + 48 = 62. There are two nitrate groups, so 2 x 62 = 124. Add Zn: 65 + 124 = 189. Then use moles = mass / Mr = 3.78 / 189 = 0.02 mol.

Key Takeaways

Moles = mass / Mr.
Mr must include every atom in the formula, including the subscript 2 outside the nitrate bracket.

Common Mistakes

Forgetting to multiply the nitrate group by 2.
Using the wrong Ar values.
Arithmetic slip: 3.78 / 189 = 0.02.

Things to Be Careful About

The answer is in moles.
This value is used in part (b)(iii), so keep it as 0.02.

Techniques used

calculate relative molecular mass of zinc nitrateconvert mass to moles using moles = mass / Mr

(iii)

Using the equation for the reaction and your answer to (b)(ii) calculate the volume of each gas produced when $3.78\text{ g}$ of zinc nitrate is heated.
[ $1\text{ mole}$ of a gas occupies a volume of $24\,000\text{ cm}^3$ at room temperature and pressure.]

volume of $\text{NO}_2$ = ______ $\text{cm}^3$
volume of $\text{O}_2$ = ______ $\text{cm}^3$

DifficultyMedium

Worked solution

Working

From the equation, 2 mol $\text{Zn(NO}_3\text{)}_2$ gives 4 mol $\text{NO}_2$ and 1 mol $\text{O}_2$ .

So 0.02 mol $\text{Zn(NO}_3\text{)}_2$ gives:

$\text{NO}_2: 0.02 \times 2 = 0.04\text{ mol}$
$\text{O}_2: 0.02 \times \frac{1}{2} = 0.01\text{ mol}$

Volume = moles $\times 24\,000\text{ cm}^3$ :

$\text{NO}_2: 0.04 \times 24\,000 = 960\text{ cm}^3$
$\text{O}_2: 0.01 \times 24\,000 = 240\text{ cm}^3$

Answer

volume of $\text{NO}_2$ = 960 $\text{cm}^3$
volume of $\text{O}_2$ = 240 $\text{cm}^3$

Final answer

NO2 = 960 cm3; O2 = 240 cm3

Detailed explanation

Walkthrough

The balanced equation shows the mole ratio: 2 mol zinc nitrate gives 4 mol nitrogen dioxide and 1 mol oxygen. So 0.02 mol zinc nitrate gives 0.04 mol NO2 and 0.01 mol O2. At room temperature and pressure, 1 mol of any gas occupies 24000 cm3. Multiply each amount of gas by 24000: NO2 = 960 cm3, O2 = 240 cm3.

Key Takeaways

Use the mole ratio from the balanced equation.
Volume of gas = moles x molar gas volume (24000 cm3/mol at r.t.p.).

Common Mistakes

Using 0.02 mol for both gases instead of applying the ratio.
Mixing up which gas has the larger volume: 4 mol NO2 compared with 1 mol O2.
Using 24 dm3 instead of 24000 cm3, or forgetting the unit.

Things to Be Careful About

The answer must use the moles from part (b)(ii); if that value was wrong, carry the error forward.
The question asks for volumes in cm3, so keep the answer in cm3.

Techniques used

use mole ratio from balanced equationmultiply moles by molar gas volumecalculate gas volumes

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