O-LevelChemistryChemistry of the Environment5070/21 · Oct/Nov 2016 · Q66 Marks

Dry air contains nitrogen, oxygen, argon and other gases.

(a)State the percentage compositions by volume of nitrogen and oxygen present in dry air. nitrogen .................................................. % oxygen .................................................. %[1]
(b)(i)The formula for oxygen gas is O2. Draw a 'dot-and-cross' diagram of an oxygen molecule. Show only the outer shell electrons.[1]
(b)(ii)What is the formula of argon gas?[1]
(c)Titanium is extracted from titanium(IV) chloride by reduction with molten sodium in an argon atmosphere and not in air. Suggest why this reaction is carried out in an argon atmosphere and not in air.[2]
(d)State one other use of argon.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2016). The mark or mark type is shown in brackets.

[1] nitrogen 78% AND oxygen 21% (1)
[1] correct 'dot-and-cross' diagram with two pairs of bonding electrons and four non-bonding electrons on each of the two oxygen atoms (1)
[1] Ar (1)
[1] argon is unreactive/argon is inert/argon does not react (1)
[1] air would react with sodium/air would react with titanium/air would react with the reaction mixture/argon prevents sodium reacting with air/argon stops titanium reacting with air/to exclude air (1)
[1] any suitable e.g. lamps/bulbs/lasers (1)

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.