Referring to the table below, pick an indicator for use in the titration of each

Question

Referring to the table below, pick an indicator for use in the titration of each base with a strong acid.

Part A

C5H5N(pKb= 8.76)

( 8.76)

Part B

LiOH

2,4-dinitrophenol or bromphenol blue

NH3(pKb= 4.75)

( 4.75)

erythrosine B crystal violet phenolphthalein o-cresolphthalein or phenolphthalein bromocresol green or bromphenol blue pH 0 1 2 3 45 6 789 10 11 12 Crystal Violet Thymol Blue Erythrosin B 2,4-Dinitrophenol Bromphenol Blue Bromocresol Green Methyl Red Eriochrome Black T Bromocresol Purple Alizarin Bromthymol Blue Phenol Red m-Nitrophenol o-Cresolphthalein Phenolphthalein Thymolphthalein Alizarin yellow R Trademark of CIBA GEIGY CORP

Explanation / Answer

As given the acid is strong acid

i) Here the base is weak base

as pKb = 8.76 so

Kb = 1.74 X 10-9

After equivalence point there will be salt of weak base and strong acid and a drop of strong acid

So the pH will fall near 2-4

Out of given choices the suitable indicator will be Erythrosine B

ii) LiOH is a strong base

At equivalence point the pH of solution will be nearly 7.

the best indicator for strong base titration is phenolphtalein

However we can also use o-cresolphthalein

iii) NH3

for titration of NH3 and strong acid we can use bromocresol green or methyl red

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