NAME: _________________________        TEACHER: _________________________

 

ADELAIDE HIGH SCHOOL

 

CHEMISTRY MID-YEAR EXAMINATION: 2002

 

MONDAY 17/6/2002         1.00 pm-3.10pm

 

INSTRUCTIONS TO CANDIDATES

 

CHECK THAT YOU HAVE 22 PAGES AND THAT THEY ARE COLLATED CORRECTLY

 

1. You have 10 minutes to read the paper. In that tine you may make notes on the scribbling paper provided. You must not write in your question booklet until instructed to do so.

 

2. You will be expected to extract the relative atomic mass from the periodic table supplied.

 

3. Answer ALL parts of ALL questions in the spaces provided in the booklet.

 

4. There is no need to fill all the spaces provided; clear, well-expressed answers are required. Extra space has been allowed for those who require it.

 

5. If you delete part or all of an answer you should clearly indicate your final answer and label it with the appropriate question number. You may write on pages 21-22 to if you need more space.

 

6. The total mark is 130.

 

7. DO NOT USE RED PEN OR A PENCIL when writing answers.

 

 

Question

 

Maximum mark

 

Mark obtained

 

1

 

33

 

 

2

 

28

 

 

3

 

36

 

 

4

 

33

 

 

Total

 

130

 

 

PERCENTAGE: _______________                     GRADE: _____________________
QUESTION 1                                                                                     (33 marks)

 

(a) A solution contains 5.255 g of pure ammonium iron (11) sulfate (molar mass = 392.13 g mol-1)

in a total volume of 250.0 mL.

(1) 50.0 mL of this solution reacts with 26.80 mL of a solution of potassium permanganate.

Given that the equation for the reaction is:

 

 

 

 

 

MnO4-    +    5Fe2+    +    8H+    à    Mn2+    +    5Fe3+    +    4H2O

 

(i) By the use of oxidation numbers show that the above chemical reaction involves reduction.

 

                                                                                                         (3 marks)

(ii) Identify the reducing agent in the above chemical reaction.

 

____________________________________________________                             (1 mark)

 

(iii) Calculate the concentration of the potassium permanganate solution used.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                         (4 marks)

 

(b) 3.90 g of dry sodium carbonate, Na2CO3 was dissolved in a volumetric flask to make a total volume of 200.0 mL.

(i) Calculate the concentration of the sodium carbonate in:

 

(1) g L-1

 

 

 

                                                                                                         (2 marks)

(2) ppb

 

      

                                                                                                         (1 mark)

(3) %w/v

 

 

                                                                                                         (1 mark)

 

(ii) Some solid sodium carbonate was used to neutralise 23.8 mL of a 0.147 mol L-1 dilute hydrochloric acid according to the following balanced chemical equation.

 

Na2CO3      +       2HCl         à     CO2          +       H2O          2NaCl

 

(1) Calculate the mass of sodium carbonate that was used.

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                         (4 marks)

 

(2) In another experiment a student had 100.0 mL of a 0.250 mol L-1 solution of hydrochloric acid.

He needed to dilute all of this to 0.0500 mol L-1 for another experiment.

 

(i) Calculate the new volume of hydrochloric acid solution prepared.

 

 

 

                                                                                                         (2 marks)

 

(ii) Calculate the volume of distilled water that would have to be added to the original solution of hydrochloric acid to obtain the new concentration of 0.0500 mol L-1.

                                                                                                   (1 mark)               
(c) An Atomic Absorption Spectrometer (AAS) was used to determine the amount of iron lost by peas in the cooking process. Using a suitable means of extraction, the iron was first extracted from a 5.13 g sample of uncooked peas to produce 100.0 mL of extract. The AAS was then calibrated and the results are shown below.

 

Iron concentration (ppm)

Absorption

Solution 1                              1.0

0.080

Solution 2                                              2.5

0.200

Solution 3                                              3.5

0.275

Solution 4                                              5.0

0.396

Extract from uncooked peas            ?

0.150

 

(i)    Draw a labelled graph of the above results on the grid below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                   (6 marks)

(ii) Other metals apart from iron, such as manganese are also present, however, this does not affect the determination of iron levels. Explain.

 

___________________________________________________________________________

 

___________________________________________________________________________

 

___________________________________________________________________________

 

______________________________________________________________________  (2 marks)

 

(iii) State the change that would have to be made to the spectrometer in order to measure the concentration of manganese in the peas.

 

___________________________________________________________________________

 

______________________________________________________________________  (1 mark)

 

(iv) When tested in the instrument, the uncooked peas gave an absorbance reading of 0.150.

Use the graph to find the iron concentration in ppm in the diluted sample.

(1ppm = 1 mg L-1 or 1 mg mL-1 or 1 mg g-1)

 

_________________________________________                                          (1 mark)

 

(v) Determine the mass of iron, in mg, in the 100.0 mL of extract.

 

 

 

 

 

 

 

 

                                                                                                   (2 marks)

(vi) Calculate the concentration in ppm of iron in the original 5.13 g sample.

 

 

 

 

 

 

 

 

 

 

 

                                                                                                    (2 marks)

 

QUESTION 2                                                                                    (28 marks)

 

(a) The reaction between nitric acid and ammonia forms the salt ammonium nitrate (NH4NO3),

an active ingredient in nitrogen fertilizers.

 

The percentage purity of ammonium nitrate in a fertilizer can be determined by back titration.

 

In one experiment, the following steps were carried out:

 

Step 1: A 2.50 g sample of fertilizer was boiled with 20.0 mL of 1.25 M sodium hydroxide solution

until no more ammonia was released.

 

NH4+(aq)      +       OH-(aq)       à     NH3(g) +      H2O(l)

 

Step 2: The mixture was cooled and the remaining hydroxide ions were titrated with 0.150 M hydrochloric acid using bromocresol purple as an indicator. The titre value was 15.7 mL.

 

Nitric acid reacts with ammonia according to following balanced chemical equation:

 

HNO3(aq)             +      NH3(g)        à     NO3-(aq)              +      NH4+(aq)

 

(i) Bromocresol purple is purple in base, yellow in acid. State the colour change that would indicate

the end point of the titration.

 

________________________________________________________                (2 marks)

 

(ii) Calculate the number of moles of sodium hydroxide solution initially supplied.

 

 

 

 

 

 

 

                                                                                                         (2 marks)

 

(iii) Calculate the number of moles of hydrochloric acid solution delivered from the burette.

 

 

 

 

 

 

                                                                                                         (2 marks)

 

 

Hydrochloric acid reacts with sodium hydroxide according to following balanced chemical equation:

 

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

 

(iv) Calculate the number of moles of sodium hydroxide in excess after the reaction with the fertilizer. This is the amount that reacted with the hydrochloric acid solution.

                                                                                                              

                 

                                                                                                                                                            (2 marks)       

(v) Calculate the number of moles of ammonium ions that reacted with the sodium hydroxide solution.

 

 

 

 

 

 

 

 

 

 

 

 

                                                                                                                                                                             (4 marks)

(vi) Calculate the number of moles of ammonium nitrate in the fertilizer.

 

 

 

 

                                                                                                                                                         (1 mark)

(vii) Calculate the mass of ammonium nitrate in the fertilizer.

 

 

 

 

 

                                                                                                                                                         (2 marks)

(viii) Calculate the percentage, by mass, of ammonium nitrate in the fertilizer

 

 

 

 

 

 

                                                                                                                                                         (2 marks)

 

(ix) If in Step 1, the mixture was not allowed to react completely, state and explain the effect this

would have on the value obtained for the % purity.