Chemistry 5070/22 — May/June 2018

Walkthrough

The question gives a bank of gases. We need the gas that burns in air to give only water. Burning means reacting with oxygen. Hydrogen burns in oxygen to form water:

The only product is water. Butane and propane are hydrocarbons; when they burn they produce carbon dioxide and water, so they cannot be the answer. Therefore the gas is hydrogen.

Key Takeaways

• Hydrogen burns in oxygen to produce water only.
• Hydrocarbons burn to produce carbon dioxide and water.
• "Only water" is the key phrase that rules out the hydrocarbons.

Common Mistakes

• Choosing butane or propane because they burn: they also produce carbon dioxide.
• Choosing oxygen: oxygen supports burning but does not itself burn to give water.

Things to Be Careful About

• The equation must be balanced: two molecules of hydrogen react with one molecule of oxygen.
• The phrase "only water" is essential; any gas that also forms carbon dioxide is wrong.

Walkthrough

The question asks which gas is used to kill bacteria in the purification of water for domestic use. In water treatment, chlorine is added in small amounts to kill bacteria and make the water safe to drink. No other gas in the list has this use, so the answer is chlorine.

Key Takeaways

• Chlorine is used as a disinfectant in the treatment of domestic water.
• Water treatment may involve filtration and chlorination.

Common Mistakes

• Choosing oxygen because it is present in water: oxygen is not added to kill bacteria.
• Choosing ammonia: ammonia is not used for disinfection.

Things to Be Careful About

• The question asks for the gas used to kill bacteria, not the gas that makes up water.
• Chlorine is used in controlled amounts; it is toxic in large amounts.

Walkthrough

We need the gas whose molecule contains exactly 11 atoms. Count the atoms in each formula using the subscripts:

• ammonia, $\text{NH}_3$: 1 + 3 = 4 atoms
• carbon dioxide, $\text{CO}_2$: 1 + 2 = 3 atoms
• chlorine, $\text{Cl}_2$: 2 atoms
• butane, $\text{C}_4\text{H}_{10}$: 4 + 10 = 14 atoms
• hydrogen, $\text{H}_2$: 2 atoms
• nitrogen, $\text{N}_2$: 2 atoms
• oxygen, $\text{O}_2$: 2 atoms
• propane, $\text{C}_3\text{H}_8$: 3 + 8 = 11 atoms
• sulfur dioxide, $\text{SO}_2$: 1 + 2 = 3 atoms

Only propane has 11 atoms, so the answer is propane.

Key Takeaways

• A molecular formula shows the number of each type of atom in one molecule.
• A subscript tells you how many atoms of that element are present.
• Propane is $\text{C}_3\text{H}_8$.

Common Mistakes

• Choosing butane: $\text{C}_4\text{H}_{10}$ has 14 atoms, not 11.
• Forgetting to count the subscript: e.g. thinking propane has 3 atoms because of the 3 in $\text{C}_3$.

Things to Be Careful About

• Count every atom: carbon atoms plus hydrogen atoms.
• If there is no subscript, the number of atoms is 1 (e.g. the S in $\text{SO}_2$).

Walkthrough

The composition of clean, dry air is approximately 78% nitrogen, 21% oxygen and 1% other gases (including argon and carbon dioxide). The gas that occupies 78% by volume of dry air is therefore nitrogen.

Key Takeaways

• Dry air is about 78% nitrogen by volume.
• Oxygen is about 21% by volume.

Common Mistakes

• Choosing oxygen: oxygen is about 21%, not 78%.
• Choosing carbon dioxide: carbon dioxide is only a trace gas in air.

Things to Be Careful About

• The question says by volume, so use the volume composition of air, not the mass composition.
• Dry air means water vapour has been removed; the percentages refer to the remaining gases.

Walkthrough

Ammonium nitrate is an ammonium salt. When an ammonium salt is warmed with an alkali such as calcium hydroxide, ammonia gas is released. The reaction is:

So the gas released is ammonia.

Key Takeaways

• Ammonium salts react with alkalis to release ammonia gas.
• This is the basis of the test for the ammonium ion: warm with sodium hydroxide and test the gas with damp red litmus paper, which turns blue.
• Ammonium nitrate is used as a nitrogen fertiliser.

Common Mistakes

• Choosing nitrogen because fertilisers contain nitrogen: the gas released from ammonium salts with alkali is ammonia, not nitrogen.
• Forgetting that calcium hydroxide is an alkali that can release ammonia from ammonium salts.

Things to Be Careful About

• The equation must be balanced.
• Ammonia is an alkaline gas; it turns damp red litmus paper blue.
• The question asks for the gas released, not the fertiliser itself.