Chemistry 5070/21 — May/June 2012

Cambridge O-Level · Theory · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Stoichiometry · Acids, Bases and Salts · Atoms, Elements and Compounds · Organic Chemistry · Chemical Reactions · Chemistry of the Environment · +6 more

Q17MExperimental Techniques and Chemical AnalysisOrganic ChemistryChemical ReactionsMetalsThe Periodic TableAcids, Bases and SaltsChemistry of the EnvironmentAtoms, Elements and CompoundsFree sample

Choose from the following gases to answer the questions below.

ammonia
carbon monoxide
chlorine
ethane
fluorine
methane
neon
nitrogen
nitrogen monoxide
oxygen
propane
propene
sulfur dioxide
sulfur trioxide

Each gas can be used once, more than once or not at all.

Which gas

(a)

turns moist red litmus paper blue,

______

DifficultyEasy

Worked solution

Answer

Ammonia

Final answer

Ammonia

Detailed explanation

Walkthrough

Moist red litmus paper turns blue in the presence of an alkaline gas. From the bank, ammonia ( $\text{NH}_3$ ) is the only gas that dissolves in water to give an alkaline solution, so it turns red litmus blue. The other gases are neutral, acidic or unreactive.

Key Takeaways

Ammonia is an alkaline gas; red litmus turns blue with alkalis. This is a standard gas test.

Common Mistakes

Writing ammonium or ammonium hydroxide instead of ammonia. Confusing litmus colours: red to blue is alkaline, blue to red is acidic.

Things to Be Careful About

The litmus is moist because ammonia dissolves in the water on the paper to form an alkaline solution. Write the gas name ammonia, not just its formula.

Techniques used

recall that ammonia is the only common alkaline gasrelate moist red litmus turning blue to an alkaline gas

(b)

decolourises bromine water,

______

DifficultyEasy

Worked solution

Answer

Propene (sulfur dioxide is also accepted)

Final answer

Propene

Detailed explanation

Walkthrough

Bromine water is orange-brown. It is decolourised by alkenes because the C=C double bond undergoes an addition reaction with bromine. In the bank, propene is an alkene, so it decolourises bromine water. Sulfur dioxide is also accepted because it reduces bromine to colourless bromide ions. The mark scheme allows either answer.

Key Takeaways

Alkenes contain a C=C double bond and decolourise bromine water; this is the test for unsaturation. Sulfur dioxide is a reducing agent and also decolourises bromine water.

Common Mistakes

Choosing ethane or methane: alkanes do not decolourise bromine water in the dark. Choosing chlorine or fluorine: halogens do not decolourise bromine water.

Things to Be Careful About

The blank asks for a gas from the bank, so write propene, not alkene. If you choose sulfur dioxide, it is also accepted, but make sure not to choose an alkane.

Techniques used

recall that alkenes decolourise bromine wateridentify propene as the alkene in the bankrecognise sulfur dioxide as an alternative reducing agent

(c)

is used in the manufacture of steel,

______

DifficultyEasy

Worked solution

Answer

Oxygen

Final answer

Oxygen

Detailed explanation

Walkthrough

In steel manufacture, oxygen is blown through molten iron to remove impurities such as carbon and sulfur by oxidising them. From the bank, oxygen is the gas used for this purpose.

Key Takeaways

Oxygen supports combustion and is used in steelmaking to remove impurities. It is also used in medicine and welding.

Common Mistakes

Choosing nitrogen because it is used as an inert atmosphere in some processes; the gas used in steel manufacture is oxygen.

Things to Be Careful About

Write oxygen, not air or O2 alone, if the blank expects the gas name. The bank includes both oxygen and nitrogen; oxygen is the reactive one used in steelmaking.

Techniques used

recall oxygen is used in steel manufactureidentify oxygen from the gas bank

(d)

is a monatomic element,

______

DifficultyEasy

Worked solution

Answer

Neon

Final answer

Neon

Detailed explanation

Walkthrough

A monatomic element is one that exists as single atoms rather than molecules. The noble gases are monatomic. From the bank, neon is the only noble gas, so it is monatomic. The other gases are diatomic molecules such as $\text{N}_2$ , $\text{O}_2$ and $\text{Cl}_2$ , or polyatomic molecules such as $\text{NH}_3$ and $\text{CH}_4$ .

Key Takeaways

Noble gases are monatomic because they have full outer shells and do not need to bond. Most non-metal gases exist as diatomic molecules.

Common Mistakes

Choosing nitrogen or oxygen because they are elements; they are diatomic, not monatomic. Choosing chlorine or fluorine is also wrong for the same reason.

Things to Be Careful About

Monatomic means one atom per particle. Neon is the only monatomic element in the list.

Techniques used

recall noble gases exist as single atomsidentify neon as a noble gas in the bank

(e)

is used as a food preservative,

______

DifficultyEasy

Worked solution

Answer

Sulfur dioxide (nitrogen is also accepted)

Final answer

Sulfur dioxide

Detailed explanation

Walkthrough

Sulfur dioxide is used as a food preservative, for example in dried fruit and fruit juices, because it kills microorganisms and prevents oxidation. From the bank, sulfur dioxide fits. Nitrogen is also accepted by the mark scheme because it is used in food packaging to exclude oxygen and keep food fresh.

Key Takeaways

Sulfur dioxide is a preservative and also a reducing agent. Nitrogen is an unreactive gas used in food packaging.

Common Mistakes

Choosing oxygen because it is present in food; oxygen causes food to spoil. Choosing carbon monoxide, which is toxic and not used as a preservative.

Things to Be Careful About

The mark scheme accepts both nitrogen and sulfur dioxide. If you choose sulfur dioxide, write the full name sulfur dioxide, not sulfur oxide.

Techniques used

recall sulfur dioxide is used as a food preservativeidentify sulfur dioxide from the gas bank

(f)

is used to disinfect water,

______

DifficultyEasy

Worked solution

Answer

Chlorine

Final answer

Chlorine

Detailed explanation

Walkthrough

Chlorine is added to drinking water and swimming pools to kill bacteria and other microorganisms. From the bank, chlorine is the gas used to disinfect water.

Key Takeaways

Chlorine is a disinfectant and is used in water treatment. It is also a halogen.

Common Mistakes

Choosing oxygen because it supports life; oxygen does not disinfect. Choosing fluorine, which is too reactive and toxic to be used for water disinfection.

Things to Be Careful About

Write chlorine, not chloride or chlorate. The question asks for a gas, and chlorine is the element.

Techniques used

recall chlorine is used to disinfect wateridentify chlorine from the gas bank

(g)

is a molecule with 14 protons?

______

DifficultyMedium-Easy

Worked solution

Answer

Nitrogen (carbon monoxide is also accepted)

Final answer

Nitrogen

Detailed explanation

Walkthrough

The number of protons in a molecule is the sum of the proton numbers of its atoms. Nitrogen has proton number 7, so a nitrogen molecule $\text{N}_2$ has $7 + 7 = 14$ protons. Carbon monoxide has proton numbers 6 (carbon) and 8 (oxygen), so CO also has 14 protons. The mark scheme accepts either. Other gases have different totals: methane has $6 + 4(1) = 10$ protons, and oxygen has $8 + 8 = 16$ protons.

Key Takeaways

Proton number is the same as atomic number. For a molecule, add the proton numbers of all atoms. Different molecules can have the same total number of protons.

Common Mistakes

Choosing oxygen because its mass number is 16; the question asks for protons, not mass. Forgetting to multiply by the number of atoms in a diatomic molecule, for example writing 7 for nitrogen instead of 14.

Things to Be Careful About

Use proton numbers, not mass numbers. Nitrogen is $\text{N}_2$ , so count two nitrogen atoms. Carbon monoxide has one carbon and one oxygen. The mark scheme accepts both nitrogen and carbon monoxide.

Techniques used

recall proton numbers of elementsadd the protons in a moleculecompare total proton counts with 14

The rest of this paper

8 more questions

Q2Experimental Techniques and Chemical Analysis · Stoichiometry · Chemistry of the Environment8M
Q3Atoms, Elements and Compounds · States of Matter · The Periodic Table11M
Q4Atoms, Elements and Compounds · Acids, Bases and Salts10M
Q5Metals · Chemical Energetics · Stoichiometry9M
Q6Stoichiometry · Chemistry of the Environment · Acids, Bases and Salts10M
Q7Organic Chemistry · Acids, Bases and Salts10M
Q8Atoms, Elements and Compounds · Electrochemistry · Chemical Reactions · Chemical Energetics · Stoichiometry10M
Q9Chemical Reactions · Chemical Energetics · Stoichiometry · Organic Chemistry10M

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