Other alkaline solutions
3 marks · NormalA student adds hydrochloric acid (HCl), at a concentration of 0.020 mol/dm³, to water. Hydrochloric acid completely dissociates into ions:
Calculate the concentration of hydrogen ions (H⁺) in the solution after the HCl fully dissociates. Explain your reasoning.
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.
Show answer
0.020 mol/dm³ H^+ mol/dm³ (±0.001)
Also accept: 2.0 × 10^{-2} mol/dm³; 2.0 × 10^{-2} mol L^{-1}; 0.02 mol L^{-1}
Common mistakes: 0.010 mol/dm³ by dividing by 2
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Marking points
M1 · 1 mark · States or uses the balanced equation for HCl dissociation (shows 1:1 ratio between HCl and H^+).
M2 · 1 mark · Correctly relates concentration of HCl to H^+ produced per mole, using the stoichiometry (1 mol HCl gives 1 mol H^+). (follow-through)
A1 · 1 mark · Calculates and states the final concentration of H^+ as 0.020 mol/dm³ (accept: 2.0 × 10^{-2} mol/dm³). (follow-through)
Worked solution
When HCl dissolves in water, it dissociates completely, forming one hydrogen ion (H^+) per molecule of HCl.
Therefore, the number of moles of H^+ produced equals the number of moles of HCl added.
Given concentration of HCl = 0.020 mol/dm³.
Thus, [H^+] = 0.020 mol/dm³.
Final answer: the concentration of hydrogen ions is 0.020 mol/dm³.