Ethanoic acid (CH₃COOH) is a weak acid in water - VCE - SSCE Chemistry - Question 7 - 2005 - Paper 1
Question 7
Ethanoic acid (CH₃COOH) is a weak acid in water.
a. Write an equation showing the ionisation of ethanoic acid in water.
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Worked Solution & Example Answer:Ethanoic acid (CH₃COOH) is a weak acid in water - VCE - SSCE Chemistry - Question 7 - 2005 - Paper 1
Step 1
a. Write an equation showing the ionisation of ethanoic acid in water.
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Answer
The ionisation of ethanoic acid (
CH₃COOH) in water can be represented by the following equilibrium equation:
CH3COOH(aq)⇌CH3COO−(aq)+H+(aq)
Step 2
b.i. Calculate the hydrogen ion concentration in a 0.100 M solution of ethanoic acid.
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Answer
To find the hydrogen ion concentration
[H+], we can use the formula for pH:
pH=−log[H+]
Given that the pH is 2.88, we can rearrange this to:
[H+]=10−pH=10−2.88≈1.32×10−3 M
Step 3
b.ii. Calculate the acidity constant of ethanoic acid at 25°C.
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Answer
The acidity constant (Ka) can be calculated using the formula:
Ka=[CH3COOH][CH3COO−][H+]
Substituting the values:
Initial concentration of CH₃COOH = 0.100 M
[H+]ext,calculatedfrompartb.i,≈1.32×10−3 M
At equilibrium:
[CH3COO−]=[H+]=1.32×10−3extM
[CH3COOH]extatequilibrium=0.100−1.32×10−3≈0.100extM (since 1.32 x 10^-3 is negligible)
Now substituting:
Ka=0.100(1.32×10−3)(1.32×10−3)≈1.74×10−5
Step 4
c.i. Comparing two 0.10 M solutions of methanoic and ethanoic acids, which solution would have the higher pH? Give a simple qualitative explanation for your answer.
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Answer
Methanoic acid (HCOOH) has a higher acidity constant compared to ethanoic acid (CH₃COOH), as stated in the question. Since methanoic acid dissociates more completely in solution due to its higher acidity, it produces a greater concentration of H+ ions. Consequently, the solution of ethanoic acid will have a higher pH than that of methanoic acid because a lower concentration of H+ ions corresponds to a higher pH.
Step 5
c.ii. Equal volumes of both solutions were titrated against a 0.10 M solution of NaOH. Which of the solutions, if either, would require the greater volume of the NaOH solution for complete neutralisation? Explain your conclusion.
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Answer
In the titration of a weak acid with NaOH, the weaker the acid, the more volume of titrant is usually required for neutralisation.
As ethanoic acid is weaker than methanoic acid, it will require a greater volume of 0.10 M NaOH to reach the equivalence point compared to the methanoic acid solution. This is because the lower dissociation of the weak acid means there are fewer H+ ions available to react with NaOH, requiring more NaOH to achieve neutralisation.
