Reversing the calculations

5 marks · Hard

Calcium hydroxide solution reacts with hydrochloric acid according to the following balanced equation:

In a titration, 25.0 cm³ of hydrochloric acid solution of concentration 0.120 mol/dm³ is reacted completely with calcium hydroxide solution.

(a)

Calculate the number of moles of HCl present in 25.0 cm³ of solution.

[1 mark]

(b)

Determine the number of moles of Ca(OH)₂ required to react with this amount of HCl.

[1 mark]

(c)

Given that the concentration of Ca(OH)₂ is 0.0800 mol/dm³, calculate the volume (in cm³) of Ca(OH)₂ solution that supplies this amount of Ca(OH)₂.

[2 marks]

(d)

Explain briefly why it is important to use the balanced equation when carrying out these calculations.

[1 mark]

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) 0.00300 mol

(b) 0.00150 mol

(d) Because the balanced equation gives the correct stoichiometric ratio between reactants, which shows how many moles of each reactant react together. · must show States importance of correct stoichiometric ratio

Common mistakes: Not converting cm³ to dm³; Using wrong ratio in stoichiometry; Incorrect formula rearrangement; Ignoring mole ratios

Show mark scheme and worked solution

Marking points

M1 (a) · 1 mark · Convert 25.0 cm³ to dm³ and multiply by HCl concentration to find moles.

M2 (b) · 1 mark · Use the 2:1 ratio from the balanced equation to find moles of Ca(OH)₂. (follow-through)

M3 (c) · 1 mark · Rearrange concentration formula to solve for volume in dm³. (follow-through)

A1 (c) · 1 mark · Correctly convert to cm³ and give answer to 3 significant figures. (follow-through)

B1 (d) · 1 mark · States that the balanced equation shows the correct ratio in which reactants combine.

Worked solution

(a) Moles of HCl:

(b) According to the equation, 2 mol HCl reacts with 1 mol Ca(OH)₂, so required moles of Ca(OH)₂:

Convert to cm³: (rounded to 3 significant figures)

(d) The balanced equation gives the correct ratio between reactants so you can calculate the right amount of each needed for the reaction.