AS & A Level Chemistry - 9701 Paper 3 2013 Winter Zone 4

FB 1 is 0.125 mol dm⁻³ hydrochloric acid, HCl. FB 2 is an aqueous solution containing sodium hydroxide, NaOH, and sodium carbonate, Na₂CO₃. bromophenol blue acid-base indicator By carrying out titrations, you are to determine the percentage by mass of sodium carbonate in the mixture of sodium hydroxide and sodium carbonate in solution FB 2. (a) Titration • Fill a burette with FB 1. • Pipette 25.0 cm³ of FB 2 into a conical flask. • Add a few drops of bromophenol blue indicator. • Titrate the mixture in the flask with FB 1 until the blue-violet colour of the solution changes to yellow. • Perform a rough titration and record your burette readings in the space below. The rough titre is ……………… cm³. • Carry out as many accurate titrations as you think necessary to obtain consistent results. • Make certain any recorded results show the precision of your practical work. • Record in a suitable form below all of your burette readings and the volume of FB 1 added in each accurate titration. (b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value. 25.0 cm³ of FB 2 required …………….. cm³ of FB 1 [1]

(c) Calculations When the titrations were repeated using phenolphthalein as the indicator, 25.0 cm³ of FB 2 required 23.25 cm³ of FB 1. The following explains why different results are obtained using two different indicators. • When phenolphthalein is used as the indicator, the following reactions have taken place at the end-point of the titration. 1. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) 2. Na₂CO₃(aq) + HCl(aq) → NaCl(aq) + NaHCO₃(aq) • When bromophenol blue is used as the indicator in (a), the following reactions have taken place at the end-point of the titration. 1. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) 2. Na₂CO₃(aq) + HCl(aq) → NaCl(aq) + NaHCO₃(aq) 3. NaHCO₃(aq) + HCl(aq) → NaCl(aq) + CO₂(g) + H₂O(l) Show your working and use appropriate significant figures in the final answer to all steps of your calculations. (i) Calculate the number of moles of hydrochloric acid in the volume of FB 1 calculated in (b). moles of HCl in volume in (b) = …………….. mol (ii) Calculate the number of moles of hydrochloric acid in 23.25 cm³ of FB 1. moles of HCl in 23.25 cm³ = …………….. mol (iii) Use the following formula to calculate the number of moles of hydrochloric acid that react with the Na₂CO₃ in the titration using phenolphthalein indicator. moles HCl = answer (i) – answer (ii) = …………….. mol (iv) Use your answer to (iii) to calculate the mass of sodium carbonate present in 25.0 cm³ of FB 2. [Aᵣ : C, 12.0; O, 16.0; Na, 23.0] mass of Na₂CO₃ in 25.0 cm³ FB 2 = …………….. g (v) The overall equation for the reaction of Na₂CO₃ with HCl when bromophenol blue is used as indicator is given below. Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + CO₂(g) + H₂O(l) Calculate the number of moles of HCl that reacted with the Na₂CO₃ in the above equation in 25.0 cm³ of FB 2. moles of HCl = …………….. mol (vi) Use your answers to (i) and (v) to calculate the mass of sodium hydroxide in 25.0 cm³ of FB 2. [Aᵣ : H, 1.0; O, 16.0; Na, 23.0] mass of NaOH = …………….. g (vii) Calculate the percentage by mass of sodium carbonate in the mixture of sodium hydroxide and sodium carbonate in FB 2. FB 2 contains …………….. % by mass Na₂CO₃ [6]

The percentage by mass of sodium carbonate in a mixture with sodium chloride can be estimated by adding a weighed sample of the mixture to a weighed excess of hydrochloric acid and measuring the mass of carbon dioxide evolved. Sodium chloride does not react with hydrochloric acid.

FB 3 is 2.00 mol dm^-3 hydrochloric acid, HCl.
FB 4 is a mixture of solid sodium carbonate, Na₂CO₃, and solid sodium chloride, NaCl.
You are to determine the mass of carbon dioxide given off when the sodium carbonate in the mixture, FB 4, reacts with excess hydrochloric acid.

(a) Method
Record all weighings, in an appropriate form, in the space below.
● Use the measuring cylinder to transfer 75 cm³ of FB 3 into a 250 cm³ conical flask.
● Weigh the flask and acid, and record the mass.
● Weigh the labelled tube containing FB 4 and record the mass.
● Tip the FB 4 into the acid in the flask, a little at a time.
● When the reaction slows down, swirl the flask for 2 to 3 minutes. Reweigh the flask and its contents, and record the mass.
● Reweigh the tube labelled FB 4 with its stopper and any residual mixture not added to the acid, and record the mass.
● Calculate the mass of the mixture, FB 4, added to the acid.
● Record the mass of carbon dioxide given off in the reaction. This may be calculated using the following formula.

mass of CO₂ = (mass of flask + acid) + (mass FB 4 added) – (final mass of flask + contents)

(b) The reaction of sodium carbonate with hydrochloric acid is shown in the equation.
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + CO₂(g) + H₂O(l)

(i) Calculate the mass of sodium carbonate that reacts with the hydrochloric acid to give the mass of carbon dioxide recorded in (a).
[A_r: C, 12.0; O, 16.0; Na, 23.0]

mass of Na₂CO₃ = ...................... g

(ii) Calculate the percentage by mass of sodium carbonate in FB 4.

FB 4 contains .................. % by mass Na₂CO₃

(c) Mixtures of solids containing sodium carbonate can be analysed either by the procedure you used in Question 1 or the procedure you used in Question 2.

The procedure used in Question 2 is likely to give a less accurate value for the percentage of sodium carbonate.

(i) Suggest a significant source of error in the experimental method used in Question 2.
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(ii) State whether the error identified above would increase or decrease the calculated percentage by mass of Na₂CO₃ in the mixture. Explain your answer.
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(iii) Suggest an improvement to the experimental method or apparatus used in Question 2 that would reduce the error given in (i).
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(a) You are provided with two solids in boiling tubes labelled FB 5 and FB 6. Each solid contains one cation and one anion from those listed on pages 10 and 11.
(i) Add dilute nitric acid slowly to each boiling tube until the tube is approximately one third full. Record your observations in the space below. Keep these solutions for use in (a)(ii).
(ii) Use the solutions from (i) in the following tests.