O-LevelChemistryChemistry of the Environment5070/22 · May/June 2023 · Q98 Marks

A sample of clean, dry air contains 0.0400% carbon dioxide by volume.

(a)Calculate the number of molecules of carbon dioxide in 480dm³ of clean, dry air at room temperature and pressure. One mole of any gas contains 6.02 × 10²³ molecules. number of molecules[3]
(b)Complete combustion of fuels such as gasoline makes carbon dioxide. One of the hydrocarbons in gasoline has the molecular formula C₉H₂₀. Construct the equation for the complete combustion of C₉H₂₀.[1]
(c)(i)Higher levels of atmospheric carbon dioxide lead to increased global warming. Give one adverse effect of global warming.[1]
(c)(ii)Describe how the presence of gases such as carbon dioxide in the atmosphere causes global warming.[2]
(d)Carbon dioxide is removed from the atmosphere by photosynthesis. State the word equation for photosynthesis.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

9(a) (volume of CO2) = 480 × 0.04 ÷ 100 OR 0.192 (1)
(moles of CO2) = 0.192 ÷ 24 OR 0.008 (1)
(number of molecules = 0.008 × 6.02 × 1023) = 4.816 × 1021 (1)
9(b) C9H20 + 14O2 → 9CO2 + 10H2O (1)
9(c)(i) climate change / sea-level rising / ice caps melting (1)
9(c)(ii) Any two from:
absorbs thermal energy (from the Earth) (1)
reflection of thermal energy (back to the Earth) / emission of thermal energy (towards the Earth) (1)
reduces or stops loss of thermal energy into space (1)
9(d) carbon dioxide + water → glucose + oxygen (1)

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