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CHEMISTRY, M8 2013 VCE 2

The strength of the eggshell of birds is determined by the calcium carbonate, , content of the eggshell.

The percentage of calcium carbonate in the eggshell can be determined by gravimetric analysis.

0.412 g of clean, dry eggshell was completely dissolved in a minimum volume of dilute hydrochloric acid.

An excess of a basic solution of ammonium oxalate, , was then added to form crystals of calcium oxalate monohydrate, .

The suspension was filtered and the crystals were then dried to constant mass.

0.523 g of was collected.

  1. Write a balanced equation for the formation of the calcium oxalate monohydrate precipitate.   (1 mark)

  2. Determine the percentage, by mass, of calcium carbonate in the eggshell.   (3 marks)

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a.    

b.    %

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a.    
 

♦♦♦ Mean mark (a) 30%.

b.    

CHEMISTRY, M8 2016 VCE 2*

A common iron ore, fool’s gold, contains the mineral iron pyrite, .

Typically, the percentage by mass of  in a sample of fool’s gold is between 90% and 95%. The actual percentage in a sample can be determined by gravimetric analysis.

The sulfur in is converted to sulfate ions, as seen below:

This is then mixed with an excess of barium chloride, , to form barium sulfate, , according to the equation

When the reaction has gone to completion, the  precipitate is collected in a filter paper and carefully washed. The filter paper and its contents are then transferred to a crucible. The crucible and its contents are heated until constant mass is achieved.

The data for an analysis of a mineral sample is as follows.
 
     
  1. Calculate the percentage by mass of in this mineral sample.  (4 marks)

  2. State one assumption that was made in completing the calculations for this analysis.  (1 mark)

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a. 

b.    Answers could have included one of the following:

→ All the sulfur is converted to .

→ Precipitate was pure.

→ No was lost in washing the sample.

→ was the only precipitate.

→ The sample was fully dehydrated.

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a.   


 

Molar ratio 


 

b.    Answers could have included one of the following:

→ All the sulfur is converted to .

→ Precipitate was pure.

→ No was lost in washing the sample.

→ was the only precipitate.

→ The sample was fully dehydrated.

♦ Mean mark (b) 39%.

CHEMISTRY, M8 2018 HSC 16 MC

An investigation was carried out to determine the calcium ion concentration of a 2.0 L sample of tap water. Excess was added to the sample. The precipitate was filtered, dried and weighed. The mass of the dried precipitate was 400 mg.

What was the concentration of calcium ions in the sample of tap water?

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♦ Mean mark 41%.

CHEMISTRY, M8 2015 HSC 29

The procedure of a first-hand investigation conducted in a school laboratory to determine the percentage of sulfate in a lawn fertiliser is shown.
 

  1. Suggest modifications that could be made to the procedure to improve the results of this investigation. Justify your suggestions.   (4 marks)

  2. Calculate the percentage of sulfate in the original fertiliser sample.   (3 marks)

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a.   Modification 1:

→  The should be added slowly. This would help more of the sulfate ions come into contact with barium and form a precipitate.

Modification 2:

→ Barium sulfate is a very fine precipitate. By heating the mixture gently for some time, the precipitate is able to flocculate into larger particles, which would be trapped by the filter.

Modification 3:

→ The pores of most filter paper are still too large to effectively capture the precipitate. The experiment should use a small pore filter, like a sintered glass crucible.

Modification 4:

→ Multiple samples of the fertiliser should be analysed and the average value calculated to achieve more valid results. This would mitigate inaccuracies caused by unevenly distributed sulfate in the mixture.

b.   45.9%

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a.   Modification 1:

→  The should be added slowly. This would help more of the sulfate ions come into contact with barium and form a precipitate.

Modification 2:

→ Barium sulfate is a very fine precipitate. By heating the mixture gently for some time, the precipitate is able to flocculate into larger particles, which would be trapped by the filter.

Modification 3:

→ The pores of most filter paper are still too large to effectively capture the precipitate. The experiment should use a small pore filter, like a sintered glass crucible.

Modification 4:

→ Multiple samples of the fertiliser should be analysed and the average value calculated to achieve more valid results. This would mitigate inaccuracies caused by unevenly distributed sulfate in the mixture.
 


♦ Mean mark (a) 49%.

b.   


Mean mark (b) 54%.

CHEMISTRY, M8 2015 HSC 18-19 MC

Question 18

How could the reliability of the analysis of the pond water be improved?

  1. Analyse more samples from the same pond
  2. Use 50 mL of distilled water as a control sample
  3. Analyse samples from different ponds on the site
  4. Remove other contaminants from the sample before the analysis

 
Question 19

What was the concentration of lead ions in the sample?

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Question 18


 

Question 19


♦ Mean mark (Q19) 52%.

CHEMISTRY, M8 2022 HSC 28

The iron content of an impure sample (4.32 g) was determined by the process shown in the flow chart.
 

  1. Identify the brown precipitate formed at the end of step 3.   (1 mark)
  1. Calculate the percentage of iron in the original impure sample if 4.21 g of iron() oxide was collected. Assume that all the iron was converted to iron() oxide.   (4 marks)
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a. 

b.  
   
   

 

 
 

 
Therefore, the percentage of iron in the original impure sample is 68.2%.

CHEMISTRY, M8 2021 HSC 28

A 5.30 g sample of an alkali metal hydroxide was dissolved in water. After mixing with excess , the precipitate was collected, dried, measured and found to have a mass of 4.61 g.

Identify the alkali metal hydroxide. Support your answer with calculations and a balanced equation.   (4 marks)

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Since alkali metals are in group 1, they have a +1 charge.

 
   
   

 


 

 
   
   

 
This is consistent with the molar mass of potassium () from the periodic table.

Therefore, the unknown alkali metal hydroxide is (potassium hydroxide).

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Since alkali metals are in group 1, they have a +1 charge.

 
   
   

 


 

 
   
   

 
This is consistent with the molar mass of potassium () from the periodic table.

Therefore, the unknown alkali metal hydroxide is (potassium hydroxide).


♦ Mean mark 46%.

CHEMISTRY, M8 2021 HSC 17 MC

A sample was contaminated with sodium phosphate. The sample was dissolved in water and added to an excess of acidified to produce a precipitate of .

If 24.21 g of dry was obtained, what was the mass of sodium phosphate in the original sample?

  1. 1.225 g
  2. 1.521 g
  3. 1.818 g
  4. 2.115 g
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Each precipitate molecule has one molecule of

Each molecule has one molecule of

→  

→

 
   
   

 


♦♦ Mean mark 36%.