QUESTION 129 (5 marks)
The idealised conductometric curve shows a weak monoprotic acid titrated with NaOH. It has been corrected for dilution. A 30.0 mL acid sample has equivalence volume 15.0 mL when the NaOH concentration is 0.0800 mol L⁻¹.
(a) Explain why conductivity initially rises gradually. [2 marks]
(b) Explain why the gradient becomes steeper after equivalence. [1 marks]
(c) Calculate the acid concentration. [2 marks]
Practice marking scheme
Answer
Before equivalence, mostly molecular acid is converted into its sodium salt, increasing ion concentrations. After equivalence excess mobile OH⁻ accumulates. The acid concentration is 0.0400 mol L⁻¹.
Working
The weak acid initially supplies relatively few ions. Neutralisation converts HA into dissolved A⁻ while Na⁺ is added, increasing conductivity gradually. Beyond equivalence Na⁺ and excess, highly mobile OH⁻ accumulate, giving the steeper rise. mol; .
Marking criteria
- Identifies the low initial ionisation of the weak acid. [1 mark]
- Explains the gradual rise through formation of Na⁺ and A⁻ ions. [1 mark]
- Explains the steeper rise through excess highly mobile OH⁻ after equivalence. [1 mark]
- Calculates acid amount = 0.00120 mol. [1 mark]
- Calculates acid concentration = 0.0400 mol L⁻¹. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.