AS & A Level Chemistry - 9701 Paper 3 2010 Winter Zone 5

(a) Method

• Fill a burette with FA 2.
• Pipette 10.0 cm³ of FA 1 into a conical flask.
• Add to the flask a few drops of the acid-base indicator provided.
• Place the flask on a white tile.
• Titrate the acid in the flask with FA 2.

You should perform a rough titration.
In the space below record your burette readings for this rough titration.

The rough titre is .................................................. cm³.

• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Record in a suitable form below all of your burette readings and the volume of FA 2 added in each accurate titration.
• Make certain any recorded results show the precision of your practical work.

(b) From your titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value.

10.0 cm³ of FA 1 required ................................ cm³ of FA 2.

(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i) Calculate the concentration, in mol dm^-3, of the sodium hydroxide in FA 2. FA 2 contains 10.00 g dm^-3 NaOH.
[A_r: H, 1.0; O, 16.0; Na, 23.0]

The concentration of sodium hydroxide in FA 2 is ........................................ mol dm^-3.

(ii) Calculate how many moles of sodium hydroxide are contained in the volume recorded in (b).

........................................................... mol of NaOH.

(iii) Deduce how many moles of hydrochloric acid were pipetted into the conical flask and calculate the concentration, in mol dm^-3, of the hydrochloric acid in FA 1.

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

The concentration of the hydrochloric acid in FA 1 is ................................. mol dm^-3.

You are to determine the percentage purity of calcium carbonate by dissolving a measured mass of FA 3 in a known volume of hydrochloric acid, which is in excess. The hydrochloric acid remaining after all the calcium carbonate has dissolved can be determined by titration with aqueous sodium hydroxide, FA 4. You may assume that any impurity present in the calcium carbonate does not react with hydrochloric acid.

(a) Method - Read through the instructions before starting any practical work.

• Weigh and record the mass of an empty boiling-tube.
• Add to the boiling-tube between 2.60 g and 2.80 g of FA 3.
• Reweigh the tube and its contents.
• In part (b) of the method you will tip the FA 3 into hydrochloric acid, then re-weigh the tube and any residual FA 3.

• Pour approximately 150 cm$^3$ of FA 4 into a 250 cm$^3$ beaker.
• Add, a little at a time with constant stirring, the weighed FA 3 to the acid in the beaker.
• After each small addition stir until the effervescence has ceased and all the solid has dissolved.
• Reweigh the tube and any residual FA 3. Record the mass in (a).
• Transfer the solution in the beaker to the 250 cm$^3$ graduated (volumetric) flask labelled FA 6.
• Rinse the beaker several times with a small amount of FA 4 and add the rinsings to the graduated flask.
• Make up the solution to the 250 cm$^3$ mark by adding FA 4, not water.
• Shake the flask to obtain a uniform solution.

• Remember - FA 4 is 0.500 mol dm$^{-3}$ hydrochloric acid FA 5 is 0.280 mol dm$^{-3}$ sodium hydroxide.
• (i) Calculate how many moles of sodium hydroxide are contained in the volume recorded in (c).
• (ii) Deduce how many moles of hydrochloric acid reacted with the sodium hydroxide in (i) and calculate how many moles of hydrochloric acid were present in the 250 cm$^3$ graduated flask labelled FA 6.
• NaOH(aq) + HCl(aq) ➔ NaCl(aq) + H$_2$O(l)
  
• (iii) Calculate how many moles of hydrochloric acid were present in 250 cm$^3$ of FA 4.
• 250 cm$^3$ of FA 4 contained ................................... mol HCl.
  
• (iv) Calculate the following.
  (answer to (d)(iii) – answer to (d)(ii))
• This is the amount of hydrochloric acid that reacted with the calcium carbonate in the weighed sample of FA 3.
• ................ mol of HCl reacted with the calcium carbonate in ............ g FA 3.
• (v) Use your answer to (iv) to calculate the mass of calcium carbonate that reacted with hydrochloric acid.
  This is the mass of pure CaCO$_3$ in the weighed sample of FA 3.
  CaCO$_3$(s) + 2HCl(aq) ➔ CaCl$_2$(aq) + CO$_2$(g) + H$_2$O(l)
  [A$_r$: Ca, 40.0; C, 12.0; O, 16.0]
• The weighed sample of FA 3 contains .................................. g of CaCO$_3$.
• (vi) Calculate the percentage of calcium carbonate, CaCO$_3$, in FA 3 by evaluating the following expression.
  
  mass of CaCO$_3$ from (d)(v)
  mass of FA 3 used, from (a) * 100
  
  Complete your evaluation even if your answer is greater than 100%
  FA 3 contains ..................... % calcium carbonate.

FA 7, FA 8 and FA 9 are aqueous solutions, each containing one cation and one anion from those listed on pages 13 and 14 in the Qualitative Analysis Notes.
At each stage of any test you are to record details of the following.
• colour changes seen
• the formation of any precipitate
• the solubility of such precipitates in an excess of the reagent added
Where gases are released they should be identified by a test, described in the appropriate place in your observations.
You should indicate clearly at what stage in a test a change occurs.
Marks are not given for chemical equations.
No additional tests for ions present should be attempted.
If any solution is warmed, a boiling-tube MUST be used.
Rinse and reuse test-tubes wherever possible.

(a) Use aqueous sodium hydroxide and aqueous ammonia, in separate tests, to identify the cation present in FA 7, FA 8 and FA 9.
Present your results for each of the solutions in a suitable form below.

Conclusion
Complete the following table.

| solution | cation | supporting evidence |
|----------|--------|--------------------|
| FA 7 | | |
| FA 8 | | |
| FA 9 | | |

[6 marks]

(b)(i) FA 7, FA 8 and FA 9 each contain a single anion which may be $\text{Cl}^-$, $\text{I}^-$ or $\text{SO}_4^{2-}$.
Suggest a reagent that would enable you to identify any solutions containing $\text{SO}_4^{2-}$.
Reagent ................................................
Use this reagent to test each of the solutions. Record your observations in the table below. Indicate, with a tick in the final column, any solution containing $\text{SO}_4^{2-}$.

| solution | observation | $\text{SO}_4^{2-}$ present |
|----------|-------------|----------------------------|
| FA 7 | | |
| FA 8 | | |
| FA 9 | | |

(ii) Select a further reagent that will enable you to identify the halide ion present in any remaining solution(s).
Reagent ................................................
Use this reagent to test the remaining solution(s). Record your observations and the identity of the halide in a suitable form in the space below.

[5 marks]

(c) FA 10 is a white crystalline solid which turns into another white solid, FA 11, when heated strongly.
Carry out the tests on FA 10 and FA 11 in the table below.
Observe carefully at each stage and record all of your observations in the table.

| test | observations |
|------|--------------|
| (i) | |
Place 1 spatula measure of FA 10 in a hard glass test-tube.
Heat the solid very strongly until no further change is seen.

| (ii) | |
Place 1 small spatula measure of FA 11 in a test-tube and add 1 cm depth of dilute hydrochloric acid.

As soon as you have completed your observation in (ii), fill the test-tube with water.

[5 marks]