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CHEMISTRY, M6 2024 HSC 29

150 mL of a 0.20 mol L sodium hydroxide solution is added to 100 mL of a 0.10 mol L sulfuric acid solution.

Calculate the pH of the resulting solution, assuming that the volume of the resulting solution is 250 mL and that its temperature is 25°C.   (4 marks)

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.

.

CHEMISTRY, M6 2023 HSC 23

The pH of two solutions, and , were measured before and after 10 drops of concentrated was added to each.
 


 

Explain the pH changes that occurred in solutions and .  (3 marks)

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→ The diagram shows that the pH of solution changes significantly with the introduction of the base , whereas the pH of solution only shows a small change in pH. This indicates that solution contains a buffer while solution does not.

→ When was added to solution , the addition of ions caused the increase in pH

→ In contrast, the ions react with the buffer solution in solution . This has the effect of minimising the change in and therefore pH.

Show Worked Solution

→ The diagram shows that the pH of solution changes significantly with the introduction of the base , whereas the pH of solution only shows a small change in pH. This indicates that solution contains a buffer while solution does not.

→ When was added to solution , the addition of ions caused the increase in pH

→ In contrast, the ions react with the buffer solution in solution . This has the effect of minimising the change in and therefore pH.

CHEMISTRY, M6 EQ-Bank 23

Propanoic acid dissociation in water can is represented in the following equation:

Explain how the pH of the propanoic acid solution would change if it was diluted.   (3 marks)

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→ Propanoic acid only partially ionises in solution and is defined as a weak acid.

→ Any dilution of the acid will result in a decrease of the concentration of all species (including the hydronium ion).

→ According to Le Chatelier, the decreasing concentrations of dissolved species will cause the equilibrium to shift to the right.

→ While this effect causes an increase in ionisation, it is not sufficient to counter the decrease in hydronium ion concentration caused by the original dilution.

→ Since the net effect causes the hydronium ion concentration to decrease, the solution will become less acidic and the pH will increase.

Show Worked Solution

→ Propanoic acid only partially ionises in solution and is defined as a weak acid.

→ Any dilution of the acid will result in a decrease of the concentration of all species (including the hydronium ion).

→ According to Le Chatelier, the decreasing concentrations of dissolved species will cause the equilibrium to shift to the right.

→ While this effect causes an increase in ionisation, it is not sufficient to counter the decrease in hydronium ion concentration caused by the original dilution.

→ Since the net effect causes the hydronium ion concentration to decrease, the solution will become less acidic and the pH will increase.

CHEMISTRY, M6 EQ-Bank 25

The graph shows changes in pH for the titrations of equal volumes of solutions of two monoprotic acids, Acid 1 and Acid 2.
 

Explain the differences between Acid 1 and Acid 2 in terms of their relative strengths and concentrations.   (3 marks)

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→ Acid 1 is a strong acid. Its initial pH = 1 and its equivalence point is at pH = 7.

→ Acid 2 is a weaker acid. Its initial pH ~ 2 and its equivalence point is >7.

→ Acid 2 has a higher concentration than Acid 1 as it doesn’t take that much more to neutralise it.

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→ Acid 1 is a strong acid. Its initial pH = 1 and its equivalence point is at pH = 7.

→ Acid 2 is a weaker acid. Its initial pH ~ 2 and its equivalence point is >7.

→ Acid 2 has a higher concentration than Acid 1 as it doesn’t take that much more to neutralise it.

CHEMISTRY, M6 2021 HSC 15 MC

What is the pH of the resultant solution after 20.0 mL of 0.20 mol is mixed with 20.0 mL of 0.50 mol ?

  1. 11.8
  2. 13.2
  3. 13.5
  4. 14.0
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Show Worked Solution

The reaction when  is mixed with is:

  ⟶

 
   
   

 

 
   
   

 
→ is the limiting reagent and is the excess reagent.

 
   
   

 

Therefore,

 
   
   

 


♦ Mean mark 54%.