A-LevelChemistryAnalytical techniques9701/31 · Oct/Nov 2024 · Q215 Marks
2 Acids react with carbonates to produce carbon dioxide gas. Na2CO3(s) + H2SO4(aq) → Na2SO4(aq) + CO2(g) + H2O(l) This reaction can be used to determine the concentration of acid, using the mass of carbon dioxide released. You will determine the concentration of sulfuric acid in FA 2. FA 2 is the solution used in Question 1. FA 3 is sodium carbonate, Na2CO3.
- (a)Method • Use the 25 cm³ measuring cylinder to transfer 25.0 cm³ of FA 2 into the conical flask. • Weigh the flask with the acid. Record the mass. • Weigh the container with FA 3. Record the mass. • Carefully tip all of FA 3 into the acid in the conical flask. Swirl the contents of the flask and leave the flask to stand with occasional swirling until the fizzing stops. • Weigh the container with any residual FA 3. Record the mass. • Calculate and record the mass of FA 3 added to the flask. • Calculate and record the total mass of flask + acid + FA 3. • Weigh the flask and contents when the fizzing has stopped. Record the mass. • Calculate and record the mass of carbon dioxide given off during the experiment. Results[5]
- (b)(i)Calculate the amount, in mol, of carbon dioxide given off in the reaction. amount of CO2 = .............................................. mol[1]
- (b)(ii)The sodium carbonate, FA 3, was in excess in the reaction with sulfuric acid. Show by calculation that the sodium carbonate was in excess. Use your answer to (b)(i).[2]
- (b)(iii)Calculate the concentration, in moldm⁻³, of sulfuric acid in FA 2. concentration of H2SO4 = .............................................. mol dm⁻³[1]
- (c)(i)A student does not have a conical flask and uses a small beaker for the reaction. Explain why a conical flask is better.[1]
- (c)(ii)Two students made suggestions of how they thought the experiment in (a) could be adapted to determine the concentration of sulfuric acid, FA 2, by using other reactions. In each case their teacher told them that this method was not suitable. Explain, in each case, why the method is not suitable. Do not consider factors based on quantities of any reagent. Student 1 suggested using magnesium in place of sodium carbonate. Student 2 suggested using calcium carbonate in place of sodium carbonate.[3]
- (d)State the uncertainty in a single reading of your balance. uncertainty = ± ..................................... g Calculate the maximum percentage error in the mass of FA 3 that you weighed out in (a). maximum percentage error = ...................................... %[2]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.
- [5] I Table with correct headings and units all shown in space given. Headings (Mass of) flask + acid / FA 2 1 (Mass of) container + FA 3 2 (Mass of) container (empty/with residue) 3 (Mass of) FA 3 added (Mass of) flask + acid + FA 3 (added) (Mass of) flask and contents after reaction (owtte) 4 (Mass of) carbon dioxide / mass loss Units: /g, (g), in g or g next to each value.
- [5] II All four specified balance readings shown AND All balance readings given either to 2 or to 3 dp
- [5] III Correctly calculates mass of FA 3 (2 – 3) AND Correctly calculates total ('initial') mass (1 + [2-3])
- [5] IV Correctly calculates mass CO2 (1 + 2 – 3-4) Calculate supervisor and candidate values to 2 dp for mass of FA 3/mass loss
- [5] V awarded if δ ± 15% between supervisor and candidate.
- [1] Correctly calculates amount of CO2 = mass CO2/44 mol AND answer given to 2 – 4 sf
- [2] M1 Correct use of 106 e.g. mass of FA 3 from (a)/106 OR (b)(i) × 106
- [2] M2 Comparison of moles / masses of Na2CO3 used versus needed OR statement that moles Na2CO3 > CO2 from correct M1 OR calculation of excess moles of Na2CO3
- [1] Correctly uses (amount H2SO4 needed (in 25 cm³) = amount of CO2 = (b)(i) mol) Concentration = (b)(i) × 1000/25 mol dm⁻³ AND answer given to 2 – 4 sf
- [1] Reaction / mixture froths / undergoes (rapid) effervescence AND more likely to overflow / some (reaction mixture) is lost from beaker.
- [3] Student 1 M1 Hydrogen has very low density M2 Error in loss of small mass gives large % error
- [3] Student 2 M3 Calcium sulfate is insoluble / sparingly soluble OR Calcium sulfate (formed) coats solid (preventing further reaction)
- [1] 2 dp balance uncertainty, U = 0.01 g or 0.005 g 3 dp balance uncertainty, U = 0.001 g or 0.0005 g
- [1] AND Percentage error = (2 x U) x 100/mass FA 3
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