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Mass of copper in an alloy
A 5.25 g sample of an alloy of copper and zinc contains 13.5% zinc by mass.
Calculate the mass of copper in the 5.25 g sample. Give your answer to 3 significant figures.
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Marking points
- 1 Uses a valid route based on 86.5% copper, or equivalently subtracts the calculated zinc mass from 5.25 g.
- 2 Preserves a suitable unrounded intermediate value.
- 3 Gives the final mass as 4.54 g to 3 significant figures.
Full-mark answer
Percentage of copper = 100 − 13.5 = 86.5%. Mass of copper = (86.5 ÷ 100) × 5.25 = 4.54125 g, which is 4.54 g to 3 significant figures.
Why this answer loses marks
I multiplied the sample mass by the zinc percentage.
That finds the other component. First find copper's remaining percentage, then apply it to the sample mass.