Preventing rusting by galvanising
3 marks · NormalA section of iron gutter is to be protected from rusting by galvanising. The gutter is 2.5 m long and each square metre of iron requires 80 g of zinc for an even, protective coating.
State the two conditions required for rusting of iron to occur.
The gutter has a surface area of 0.30 m². Calculate the minimum mass of zinc needed to galvanise the entire surface. Show your working and give your answer in grams.
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(a) Presence of water and oxygen · must show Both water and oxygen must be mentioned
(b) 24\,g · must show Calculation of zinc mass: 0.3 × 80 = 24 g
Common mistakes: Using incorrect units for area; Multiplying instead of dividing coating rate
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Marking points
B1 (a) · 1 mark · States both water and oxygen are required for rusting
M1 (b) · 1 mark · Correct method: mass of zinc = area × zinc per m²
A1 (b) · 1 mark · Correct answer: 24 g (accept answers within rounding tolerance) (follow-through)
Worked solution
a) The two conditions required for rusting of iron are the presence of water and oxygen.
b) Surface area of gutter = 0.30 m²
Mass of zinc needed = surface area × zinc needed per m²