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CHEMISTRY, M8 EQ-Bank 23

and react to form an ionic compound according to the general equation

where and are numbers representing the ratio in which and combine.

Spectrophotometry was used to determine the stoichiometric ratio between and . To do this, eight 10 mL samples were prepared by reacting solutions of with solutions of in varying ratios. All and solutions had the same concentration. The absorbance of the samples is tabulated below.
 

  1. On the grid, construct a graph of absorbance against volume of solution from 0.00 mL to 6.00 mL, and draw TWO lines of best fit.  (3 marks)
     

     

  1. The reaction proceeds according to the general equation
  2.       .
  3. Find the values of and . Justify your answer with reference to the data given and the graph in part (a).   (3 marks)

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a.  
           

 

b.   Find values of and :

→ The graph shows that as the volume of compound decreases between 10mL and 7.5mL, there is an increase in ions. This indicates an excess of ions limiting the products.

→ The absorbance reaches a maximum when the 2.5 mL of compound is added to 7.5 mL of compound . Since the concentrations of the initial solutions are equal (given), equal volumes contain equal moles.

→ The ratio of volumes at peak absorbance = 2.5 : 7.5 = 1 : 3 (i.e. the correct stoichiometric ratio).

→ Hence and .

→ The curve then turns down sharply, indicating a reduction in .

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a.  
           

 

b.   Find values of and :

→ The graph shows that as the volume of compound decreases between 10mL and 7.5mL, there is an increase in ions. This indicates an excess of ions limiting the products.

→ The absorbance reaches a maximum when the 2.5 mL of compound is added to 7.5 mL of compound . Since the concentrations of the initial solutions are equal (given), equal volumes contain equal moles.

→ The ratio of volumes at peak absorbance = 2.5 : 7.5 = 1 : 3 (i.e. the correct stoichiometric ratio).

→ Hence and .

→ The curve then turns down sharply, indicating a reduction in .

CHEMISTRY, M8 EQ-Bank 12

A colorimeter was used to calculate the percentage of iron in a 0.200 gram tablet. The tablet was dissolved and oxidised, then reacted with thiosulfate according to the equation

The resulting solution was made up to 200 mL with distilled water. The absorbance of the final solution was measured to be 0.6105.

The calibration curve shows the absorbance of various concentrations of .
 

Calculate the percentage of iron in the tablet.   (3 marks)

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0.307%

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CHEMISTRY, M8 2018 HSC 20 MC

The Winkler method is used to determine the amount of dissolved oxygen in a water sample. The procedure involves the following sequence of reactions.

Step 1. 

Step 2. 

Step 3. 

When a 5.00 L sample of water was analysed using the Winkler method, a total of mol of thiosulfate was required in Step 3.

What concentration of oxygen was present in the original sample?

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Consider Step 3:

 

Consider Step 2:


 

Consider Step 1:


♦ Mean mark 45%.

CHEMISTRY, M8 2016 HSC 27

The volume of gas formed at 25°C and 100 kPa as hydrochloric acid was added to a pure sample of aluminium is shown in the graph.
 

Calculate the original mass of the aluminium sample used in the reaction.   (4 marks)

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0.109 grams

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→ The graph shows that all the aluminium has reacted when no more gas is being produced, at a volume of 0.150 L.


♦ Mean mark 48%.

CHEMISTRY, M8 2019 HSC 20 MC

The manganese content in a 12.0 gram sample of steel was determined by measuring the absorbance of permanganate using the following process.

The steel sample was dissolved in nitric acid and the   ions produced were oxidised to by periodate ions, , according to the following equation.

The resulting solution was made up to a volume of 1.00 L, then 20.0 mL of this solution was diluted to 100.0 mL. The absorbance at 525 nm of the resulting solution was 0.50.

A calibration curve for was constructed and is shown below.
 

What was the percentage by mass of manganese in the steel sample?

  1. 0.019%
  2. 0.096%
  3. 0.48%
  4. 1.0%
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From graph, 0.5 absorbance corresponds to 0.25 mg L ¯1.

 
   
 
   

 

 
   
   

 


♦♦ Mean mark 43%.