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A titration was conducted by adding NaOH from a teflon-coated burette to HCl in a conical flask - HSC - SSCE Chemistry - Question 10 - 2005 - Paper 1
Question 10
A titration was conducted by adding NaOH from a teflon-coated burette to HCl in a conical flask. The pH in the flask was recorded during the titration and Curve A wa... show full transcript
Worked Solution & Example Answer:A titration was conducted by adding NaOH from a teflon-coated burette to HCl in a conical flask - HSC - SSCE Chemistry - Question 10 - 2005 - Paper 1
Step 1
What is the appropriate indicator for Curve B using the table?
Answer
To determine the appropriate indicator for Curve B, we need to analyze the pH change indicated by the titration curve.
Since the original titration (Curve A) involved HCl and NaOH and used bromothymol blue, we can infer that Curve B likely corresponds to a different acid.
From the table provided:
- Methyl orange works in the pH range of 3.1 - 4.4, which is typical for strong acid to weak base titrations.
- Methyl red operates in a range of 4.4 - 6.2, useful for similar types of reactions but slightly broader.
- Bromothymol blue is indicated for strong acid-strong base reactions.
- Cresolphthalein has a transition range of 8.1 - 9.7, suitable for weak acid-strong base titrations.
- Alizarin yellow is pertinent for weak acid-strong base titrations within the 10.1 - 12.0 range.
Given that Curve A indicated a strong acid-strong base reaction and Curve B suggests another acid, it is reasonable to conclude that it represents a weak acid reacting with NaOH. Therefore, the indicator that fits the sharp change in pH for weak acid-strong base titrations is Cresolphthalein (C), as it indicates the endpoint in the higher pH range.
Thus, the appropriate indicator for Curve B is Cresolphthalein (C).