How does diluting a 0.1 M solution of lactic acid, HC₃H₅O₃, change its pH and percentage ionisation?
A - VCE - SSCE Chemistry - Question 20 - 2016 - Paper 1
Question 20
How does diluting a 0.1 M solution of lactic acid, HC₃H₅O₃, change its pH and percentage ionisation?
A. increase decrease
B. increase increase
C. decrease increase
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Worked Solution & Example Answer:How does diluting a 0.1 M solution of lactic acid, HC₃H₅O₃, change its pH and percentage ionisation?
A - VCE - SSCE Chemistry - Question 20 - 2016 - Paper 1
Step 1
pH Response to Dilution
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Answer
When a weak acid like lactic acid is diluted, the concentration of hydrogen ions ([H+]) in the solution decreases. The pH of a solution is given by the formula:
pH=−extlog([H+])
Therefore, as the concentration decreases, the pH increases.
Step 2
Percentage Ionisation Response to Dilution
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Answer
The percentage ionisation of a weak acid increases when diluted because the weak acid is less concentrated, allowing more of it to ionise compared to the undiluted state. This is expressed as:
where [A−] is the concentration of the ionised form and [HA] is the concentration of the un-ionised form. As the denominator decreases due to dilution, the percentage of ionisation becomes larger.
Step 3
Final Answer Selection
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Answer
From the analysis, we conclude that diluting the lactic acid solution results in an increase in pH and an increase in percentage ionisation. Therefore, the correct option is B: increase, increase.
