Calculations involving gas volumes
3 marks · NormalA student claims that 48 dm³ of hydrogen gas at rtp contains twice as many molecules as 24 dm³ of oxygen gas at rtp.
Is the student correct? Explain your answer.
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Yes, because at rtp equal volumes of gases contain equal numbers of molecules (Avogadro's law), so 48 dm³ will have twice as many molecules as 24 dm³, regardless of the gas.
Must show: State if correct or incorrect, Reference Avogadro's law, Explanation relating to equal volumes/number of molecules
Also accept: Yes, because for gases at the same temperature and pressure, volume is proportional to the number of molecules.
Common mistakes: Comparing masses instead of number of molecules; Ignoring Avogadro's law
Show mark scheme and worked solution
Marking points
B1 · 1 mark · States that the student is correct (or says 'Yes')
B2 · 1 mark · References Avogadro's law or that equal volumes of gases have equal numbers of molecules at rtp
B3 · 1 mark · Links this to 48 dm³ having twice as many molecules as 24 dm³, regardless of gas
Worked solution
Yes, the student is correct.
According to Avogadro's law, equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
So 48 dm³ of any gas at rtp contains twice as many molecules as 24 dm³.