Simple differences between mixtures and compounds
3 marks · NormalA sample contains 8 g of oxygen and 2 g of hydrogen. Describe whether this sample is a compound or a mixture, and state the formula of the compound (if any) that could be formed by these elements. Show your calculation.
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(a) Mixture · must show Identifies that ratio does not match H2O
(b) Water, formula H2O · must show Gives formula for compound
Common mistakes: Assuming any ratio forms a compound; Not giving chemical formula
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Marking points
M1 · 1 mark · Calculates mass ratio H:O = 2:8 = 1:4
A1 · 1 mark · States that the sample is a mixture (not compound) as the ratio is not 2:16 (H:O by mass for water)
B1 · 1 mark · States the formula of the compound formed would be H2O (water)
Worked solution
The ratio by mass of hydrogen to oxygen here is 2:8 = 1:4.
In water (H2O), 2 g of hydrogen combine with 16 g of oxygen (ratio 1:8). Here, not all oxygen will react, so the sample is a mixture.
The chemical formula for water is H2O.