Chemical Formulae, Equations and Calculations Part 2
7 marks · HardCalcium reacts with hydrochloric acid to produce hydrogen gas according to the following equation:
A student wants to produce exactly 600 cm³ of hydrogen gas at room temperature and pressure (rtp). The concentration of hydrochloric acid available is 0.500 mol/dm³.
Calculate the number of moles of hydrogen gas required to produce 600 cm³ of gas at rtp.
Calculate the number of moles of hydrochloric acid (HCl) needed to react to produce this amount of hydrogen gas.
Calculate the volume (in cm³) of 0.500 mol/dm³ hydrochloric acid solution required to produce 600 cm³ of hydrogen gas.
If the same volume of hydrogen gas (600 cm³) is to be produced, what is the corresponding volume (in cm³) at rtp if measured in dm³?
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) 0.025 mol · must show Conversion using molar volume (24 000 cm³ = 1 mol)
(b) 0.050 mol · must show Stoichiometric use of 2:1 ratio between HCl and H2
(c) 100 cm³ · must show Rearrangement of concentration equation; answer in cm³; correct unit conversion
(d) 0.600 dm³ · must show Conversion of cm³ to dm³
Common mistakes: Incorrect cm³/dm³ conversions; Incorrect stoichiometric ratio use; Using volume instead of moles in calculations
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct conversion of 600 cm³ to moles using 24,000 cm³/mol.
M2 (b) · 1 mark · Correct use of stoichiometric ratio (2 mol HCl per 1 mol H₂) to find moles of HCl needed. (follow-through)
M3 (c) · 1 mark · Rearrange conc. equation: volume = moles / conc.; correct units. (follow-through)
M4 (c) · 1 mark · Convert calculated volume in dm³ to cm³ correctly. (follow-through)
A1 (c) · 1 mark · Final answer for HCl volume required: 100 cm³ (accept rounding to nearest cm³). (follow-through)
M5 (d) · 1 mark · Recognise that 1 dm³ = 1000 cm³ and perform the appropriate conversion.
A2 (d) · 1 mark · Final answer for volume in dm³: 0.600 dm³ (accept 0.6 dm³). (follow-through)
Worked solution
(a) Number of moles of hydrogen gas:
(b) From equation, 2 mol HCl produces 1 mol H₂: moles of HCl = 2 × 0.025 = 0.050 mol.
(c) Volume of 0.500 mol/dm³ HCl needed for 0.050 mol:
Convert dm³ to cm³: 0.100 dm³ × 1000 = 100 cm³.
(d) 600 cm³ in dm³: