Calculating the volume of a certain number of moles
5 marks · HardA student reacts zinc with dilute hydrochloric acid to produce hydrogen gas at room temperature and pressure (rtp), according to the equation:
The student collects exactly 0.85 mol of hydrogen gas.
Calculate the volume, in dm³, of 0.85 mol of hydrogen gas at rtp. Show your working.
Convert your answer from part (a) into cm³.
Explain why a different gas produced under the same conditions and with the same number of moles would occupy the same volume.
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.
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(a) 20.4 dm³
(b) 20,400 cm³
(c) Any gas with the same number of moles at rtp occupies the same volume because the number of molecules is the same (Avogadro's law/molar volume idea). · must show Reference to Avogadro's law OR equal number of molecules implies equal volume at rtp
Common mistakes: Incorrect use of molar volume (22.4 dm³, STP instead of rtp); Incorrect conversion between dm³ and cm³
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Marking points
M1 (a) · 1 mark · Uses correct formula: volume = moles × 24 (dm³)
A1 (a) · 1 mark · Correct calculation: 0.85 × 24 = 20.4 dm³ (follow-through)
M2 (b) · 1 mark · Converts answer from (a) to cm³ by multiplying by 1000. (follow-through)
B1 (c) · 1 mark · States that any gas with the same number of moles occupies same volume at rtp (reference to Avogadro's law or equal number of molecules).
B2 (c) · 1 mark · Explanation linked specifically to same volume at rtp due to same number of molecules OR clear link to 'molar volume' concept.
Worked solution
a) Calculate hydrogen volume in dm³:
b) Convert to cm³: 20.4 dm³ × 1000 = 20,400 cm³
c) Explanation: Any gas with the same number of moles at rtp occupies the same volume because, according to Avogadro's law, equal numbers of gas molecules at the same temperature and pressure occupy equal volumes.