Chemical Formulae, Equations and Calculations Part 2
6 marks · NormalA student reacts excess magnesium ribbon with 100 cm³ of 2.0 mol/dm³ hydrochloric acid in a flask at room temperature and pressure (rtp). Magnesium reacts with hydrochloric acid as follows:
Calculate the amount, in moles, of hydrochloric acid present in 100 cm³ of 2.0 mol/dm³ HCl.
Calculate the maximum volume, in cm³, of hydrogen gas produced at rtp.
Suggest one reason why the actual volume of hydrogen collected might be less than this calculated value.
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) 0.20 mol
(b) 2400 cm³
(c) Hydrogen may escape before being measured or the reaction may not go to completion.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses correct relationship: moles = concentration × volume (in dm³)
A1 (a) · 1 mark · Correctly calculates number of moles as 0.2 mol
M2 (b) · 1 mark · Determines mole ratio HCl:H2 (2:1) from balanced equation and finds moles of H2 produced (follow-through)
M3 (b) · 1 mark · Uses volume of gas = moles × molar volume (in cm³: 24,000 cm³ per mol) (follow-through)
A2 (b) · 1 mark · Correct final answer: 2400 cm³ (to 2–3 sf, with correct units) (follow-through)
B1 (c) · 1 mark · Gives a sensible reason: gas escapes, incomplete reaction, impure magnesium, etc.
Worked solution
a) First, convert 100 cm³ to dm³: 100 cm³ = 0.100 dm³.
Moles of HCl = concentration × volume (in dm³):
b) From the equation, 2 mol HCl produces 1 mol H2.
So number of moles of H2 = 0.20 / 2 = 0.10 mol.
At rtp, 1 mol of H2 occupies 24,000 cm³. So:
c) Reason: Some hydrogen gas may escape before measuring, or the magnesium may not react completely.