20 mL of a 0.1 mol L⁻¹ solution of an acid is titrated against a 0.1 mol L⁻¹ solution of sodium hydroxide - HSC - SSCE Chemistry - Question 17 - 2024 - Paper 1
Question 17
20 mL of a 0.1 mol L⁻¹ solution of an acid is titrated against a 0.1 mol L⁻¹ solution of sodium hydroxide. A graph of pH against the volume of sodium hydroxide added... show full transcript
Worked Solution & Example Answer:20 mL of a 0.1 mol L⁻¹ solution of an acid is titrated against a 0.1 mol L⁻¹ solution of sodium hydroxide - HSC - SSCE Chemistry - Question 17 - 2024 - Paper 1
Step 1
Identify the type of titration based on pH graph
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Answer
The graph shows a clear and significant increase in pH as sodium hydroxide is added. This typically indicates the titration of a weak acid with a strong base. Therefore, the acid used must be a weak acid.
Step 2
Determine the equivalence point
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Answer
The equivalence point can be inferred from the graph where the pH sharply increases, usually around the 20 mL mark in this case. This is important for identifying the acid as it reflects the nature of the acid's dissociation.
Step 3
Compare the pKₐ values of the options
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Answer
The pKₐ value at the equivalence point of a weak acid titrated with a strong base should be greater than 7. Based on the options:
Acid 1 has a pKₐ₁ of 4.76, indicating it’s a weak acid;
Acid 2 is a strong acid;
Acid 3 has a very low pKₐ₁ of 1.91, indicating it’s a strong acid as well;
Acid 4 has a pKₐ₁ of 4.11, which is also a weak acid.
Since we observe a clear equivalence point and pH transition, Acid 3 (1.91 and 6.30) is a strong acid and likely not correct. Acid 1 and 4 could potentially be the correct options but Acid 4 has the suitable second dissociation for neutralization with a strong base.
Step 4
Conclude the identification of the acid used
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Answer
Based on this analysis, the most fitting choice for a weak acid used in this titration is Acid 4, with pKₐ values suggesting it's a weak acid that shows the expected behavior in the titration with sodium hydroxide.
