A-LevelChemistryAtoms, molecules and stoichiometry9701/52 · Oct/Nov 2020 · Q110 Marks
Calcium hydroxide, Ca(OH)₂, is slightly soluble in water. Aqueous calcium hydroxide is also known as limewater. A student measures the concentration of saturated aqueous calcium hydroxide using the following method. step 1 Add a spatula measure of solid calcium hydroxide to 250 cm³ of distilled water in a conical flask. Stopper the flask and shake the flask thoroughly. step 2 Leave the flask stoppered for 24 hours. step 3 Filter the aqueous calcium hydroxide into a clean, dry beaker. step 4 Transfer 25.00 cm³ of the aqueous calcium hydroxide to a clean conical flask. step 5 Carry out a titration with 0.100 mol dm⁻³ hydrochloric acid. step 6 Record the volume of hydrochloric acid required to react with the aqueous calcium hydroxide.
- (a)(i)Suggest why it is necessary for the flask to remain stoppered in step 2.[1]
- (a)(ii)Suggest why the student leaves the solution for 24 hours in step 2.[1]
- (b)(i)The student finds that the mean titre is 10.65 cm³. The equation for the reaction is shown. Ca(OH)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l) Calculate the number of moles of calcium hydroxide that reacted. moles of Ca(OH)₂ = ................................................. mol [1][1]
- (b)(ii)Calculate the concentration of the aqueous calcium hydroxide in g dm⁻³. [Aᵣ: Ca, 40.1; O, 16.0; H; 1.0] concentration of Ca(OH)₂(aq) = ................................................. g dm⁻³ [2][2]
- (b)(iii)In a chemistry data book, the student found the value for the concentration of a saturated solution of calcium hydroxide at 293K to be higher than the value calculated in (b)(ii). Give two reasons why the data book value is higher than the value calculated in (b)(ii). (Assume the student recorded all volumes correctly, carried out the titration accurately and recorded concordant titres.) [2][2]
- (c)A second student carried out the experiment but did not filter their solution correctly in step 3, leaving some undissolved calcium hydroxide suspended in the solution. Predict and explain how this would affect the titre obtained in their experiment. [1][1]
- (d)Describe how to prepare exactly 250.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid from 1.00 mol dm⁻³ hydrochloric acid. Include the names and capacities of any suitable apparatus used. [2][2]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.
- [1] 1(a)(i) To prevent carbon dioxide / CO2 from entering the flask
- [1] 1(a)(ii) To ensure that the solution is saturated
- [1] 1(b)(i) (10.65 / 1000 × 0.100) / 2 = 0.0005325 OR 5.325 × 10⁻⁴ (mol)
- [2] 1(b)(ii) M1: Conversion of mol to g (multiplication by 74.1) 0.0005325 × 74.1 = 0.03945825 (mol)
- M2: Scaling up to 1000 cm³ (multiplication by 1000 / 25) = 0.03945825 × 1000 / 25 = 1.57833 (g dm⁻³)
- [2] 1(b)(iii) M1: The temperature (of solution) was different (from 293 K) OR Temperature (of solution) was unknown
- M2: 24 hours was insufficient time (to reach saturation)
- [1] 1(c) Increase (in titre volume) AND greater number of moles of (calcium) hydroxide present in flask
- [2] 1(d) M1: Pipette AND 25(.00) cm³ (1.0 mol dm⁻³) HCl (Into...)
- M2: 250 cm³ volumetric flask AND top up to mark with distilled water
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