Crude Oil
6 marks · NormalCrude oil is separated into fractions by fractional distillation in an oil refinery. The lowest boiling point fraction is 'refinery gases', which contains a mixture of small hydrocarbons. Some of these gases are used domestically as liquefied petroleum gas (LPG).
Gasoline, another fraction, is mainly used as a fuel in cars.
A refinery processes 50 tonnes of crude oil and produces 3 tonnes of refinery gases. Calculate the percentage by mass of refinery gases obtained from the crude oil. Give your answer to two significant figures.
A house uses 18 kg of refinery gases (LPG) per week for heating and cooking.
Assuming the refinery gases produced in part (a) are supplied entirely for use as LPG, calculate the maximum number of houses that could be supplied for 1 year. (Take 1 year = 52 weeks).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 6.0% · must show Correct division and multiplication by 100
(b) About 3 houses · must show Correct use of mass of refinery gases from (a), Correct calculation for 1 year per house, Rounding down to nearest whole house
Common mistakes: Dividing refinery gases by crude oil the wrong way round; Using 1 year as 365 days instead of 52 weeks; Incorrect unit conversion (tonnes to kg)
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct method for percentage: (mass refinery gases / mass crude oil) × 100
A1 (a) · 1 mark · Correct calculation to two significant figures (follow-through)
M2 (b) · 1 mark · Convert refinery gases tonnes to kg (1 tonne = 1000 kg)
M3 (b) · 1 mark · Calculate yearly use for one house (18 kg × 52)
M4 (b) · 1 mark · Divide total available refinery gases (in kg) by yearly use per house (follow-through)
A2 (b) · 1 mark · Final answer is correct and rounded down to nearest whole house (follow-through)
Worked solution
a) Percentage by mass of refinery gases = (mass of refinery gases / mass of crude oil) × 100
b) 3 tonnes of refinery gases = 3000 kg
Amount used per house per year = 18 kg × 52 = 936 kg
Maximum number of houses supplied = 3000 kg / 936 kg = 3.205...
Number of houses (rounded down) = 3