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Question

Indicator pH range
Methyl violet 0.3-1.8
Methyl orange 2.8-3.8
Congo red 2.8-4.8
Methyl red 3.8-6.1
Bromothylmol blue 6.0-7.9
Phenol Red 6.8-8.6
The table above shows the pH ranges of some common indicators. Given that Ka is 7.3×1010 for boric acid (H3BO3), Choose an indicator that can be used for the titration of 0.10MKH2BO3 with 0.10 M HCl.

A
Methyl orange
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B
Congo Red
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C
Methyl Red
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D
Phenol Red
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Solution

The correct option is C Methyl Red
KH2BO3+HClH3BO3+KCl.

At equivalent point the concentration of H3BO3produced=0.050M H3BO3 then ionizes thus :

H3BO3+H2OH3BO3+H3O+
0.05 M x x

ka=[H3O+][H3BO3][H3BO3]=x20.05

i.e.7.3×1010=x2/0.05

x=6.04×106

pH=log(6.04×106)=5.22

This pH is in the range of methyl red indicator which will be suitable for the titration.

Hence the correct option is C.

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