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NTA Abhyas 2020
Exam
NTA Abhyas
Year
2020
Subject
Chemistry
Chapter
Ionic Equilibrium

NTA Abhyas 2020 · Chemistry · Ionic Equilibrium

An acid base indicator has K _ a = 3 × 1 0 ^ - 5 . The acid form of...

An acid base indicator has Ka=3×10−5{K}_{a} = 3 \times 1{0}^{- 5}. The acid form of the indicator is red and the basic form is blue. By how much must the pH change in order to change the indicator from 75% red to 75% blue (\log 3 =0.4770)= 0.4770)

Options

  1. A0.95
  2. B2.3
  3. C0.75
  4. D5

Correct answer

A. 0.95

Step-by-step solution

pH change = pH of 75 % blue - pH of 75% red
=[pKa+loglog7525]−[pKa+loglog2575]<br/>= \left[pKa + loglog\dfrac{75}{25}\right] - \left[pKa + loglog\dfrac{25}{75}\right]<br />= \log 3 + \log 3<br/><br />pH = 0.954
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