A standard solution of anhydrous sodium carbonate has been prepared for you by dissolving 2.65 g of the solid in distilled water and making the solution up ...
Question 1 Report
A standard solution of anhydrous sodium carbonate has been prepared for you by dissolving 2.65 g of the solid in distilled water and making the solution up to 250 cm3. This is solution C. Solution D is a dilute sulfuric acid of unknown concentration.
Rinse and fill the burette with solution D. Pipette 25.0 cm3 of solution C into a conical flask standing on a white tile and add three drops of methyl orange. Run solution D from the burette until the indicator changes from yellow to the first permanent orange colour. Repeat the experiment until you have two titres that agree closely. Write your burette readings and each titre clearly in the spaces provided before you begin the calculation.
(a) Record your initial and final burette readings for each titration, and the titre calculated from them. [3]
(b) Calculate your mean titre, in cm3. [1]
(c) Show that the concentration of solution C is 0.100 mol/dm3. The relative formula mass of Na2CO3 is 106. [2]
(d) Calculate the number of moles of sodium carbonate in the 25.0 cm3 portion in the flask. [2]
(e) The equation for the reaction is H2SO4 + Na2CO3 → Na2SO4 + H2O + CO2. Deduce the reacting ratio of acid to carbonate. [1]
(f) Deduce the number of moles of sulfuric acid in your mean titre. [1]
(g) Calculate the concentration of solution D in mol/dm3. [2]
(h) Calculate the concentration of solution D in g/dm3. The relative formula mass of H2SO4 is 98. [2]
(i) When the acid is run in, a gas is given off in the flask. Name this gas and describe the test that would confirm it. [2]
(j) Plan how you would use the same apparatus to check that the 2.65 g of solid supplied to you really was anhydrous sodium carbonate and not the hydrated solid. [3]
Answer Details
This is a titration to find the concentration of a sulfuric acid using a standard sodium carbonate solution. The mark rewards precise burette technique and then a clear mole calculation from the standard solution, through the reacting ratio, to the acid concentration. A representative set of results is used below.
| rough | run 1 | run 2 | |
|---|---|---|---|
| final reading / cm3 | 20.40 | 20.00 | 20.00 |
| initial reading / cm3 | 0.30 | 0.00 | 0.00 |
| titre / cm3 | 20.10 | 20.00 | 20.00 |
(a) Burette readings and titres [3]. Give initial and final readings for each titration, all to 0.05 cm3 [1]; set them out clearly with units [1]; obtain each titre by subtraction [1].
(b) Mean titre [1]. Average the concordant titres: \((20.00 + 20.00)/2 = 20.00\ \text{cm}^3\) [1].
(c) Show that solution C is 0.100 mol/dm3 [2]. Moles of \(\text{Na}_2\text{CO}_3 = 2.65 / 106 = 0.0250\ \text{mol}\) [1]; concentration \(= 0.0250 / 0.250\ \text{dm}^3 = 0.100\ \text{mol/dm}^3\) [1].
(d) Moles in the 25.0 cm3 portion [2]. \(\dfrac{25.0}{1000} \times 0.100\) [1] \(= 2.50 \times 10^{-3}\ \text{mol}\) [1].
(e) Reacting ratio [1]. From \(\text{H}_2\text{SO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} + \text{CO}_2\), the ratio is 1 mol acid : 1 mol carbonate, that is 1 : 1 [1].
(f) Moles of sulfuric acid in the mean titre [1]. By the 1 : 1 ratio, \(2.50 \times 10^{-3}\ \text{mol}\) [1].
(g) Concentration of solution D in mol/dm3 [2]. \(2.50 \times 10^{-3} / 0.02000\ \text{dm}^3\) [1] \(= 0.125\ \text{mol/dm}^3\) (consequential on your mean titre) [1].
(h) Concentration of solution D in g/dm3 [2]. \(0.125 \times 98\) [1] \(= 12.25\ \text{g/dm}^3\), accept 12.3 g/dm3 [1].
(i) The gas and its test [2]. Carbon dioxide [1]; bubble it through limewater, which turns milky or cloudy [1].
(j) Plan: confirm the solid was anhydrous [3]. Weigh out the same mass of the supplied solid and make up 250 cm3 of solution in the same way [1]; titrate 25.0 cm3 portions against the acid of now-known concentration [1]; a hydrated solid contains water of crystallisation, so the same mass would give fewer moles of carbonate and hence a smaller titre, whereas the anhydrous solid gives the same titre as before [1].
Download The App On Google Playstore
Everything you need to excel in your exams
