Equilibrium Question 890
Question: Select the best indicator from the given table for titration of 20 mL of $ 0.02,MCH_3COOH $ with 0.02 M NaOH. Given $ pK_{a},(CH_3COOH)=4.74 $
Indicator pH Range (I) Bromothy mol blue 6.0-7.6 (II) Thymolphthalein 9.3-11.05 (III) Malachite green 11.4-13 (IV) M-Cresol purple 7.4-9.0
Options:
A) I
B) II
C) III
D) IV
Show Answer
Answer:
Correct Answer: D
Solution:
At equivalent point $ pH=\frac{1}{2}[pK_{w}+pK_{a}+\log ,C] $
$ =\frac{1}{2}[14+4.74-2]=8.37 $
where $ [Salt]=\frac{\text{milli - moles of acid}}{\text{Total volume}}= $
$ \frac{20\times 0.02}{20+20}=0.01 $
For best indicator pH at equivalent point should be between colour transition range of indicator.
