The ionisation constant of an acid- Ka is the measure of strength of an acid- The Ka values of acetic acid- hypochlorous acid and formic acid are 1-74 - 10-5- 3-0 - 10-8 and 1-8 - 10-4 respectively- Which of the following orders of pH of 0-1mol dm-3 solutions of these acids is correct-

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Common Ion Effect

The ionisatio...

Question

The ionisation constant of an acid, $K_{a}$ is the measure of strength of an acid. The $K_{a}$ values of acetic acid, hypochlorous acid and formic acid are $1.74 \times 10^{- 5}, 3.0 \times 10^{- 8}$ and $1.8 \times 10^{- 4}$ respectively. Which of the following orders of pH of $0.1$ mol $d m^{- 3}$ solutions of these acids is correct?

Acetic acid > Hypochlorous acid > Formic acid

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Hypochlorous acid > Acetic acid > Formic acid

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Formic acid > Hypochlorous acid > Acetic acid

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Formic acid > Acetic acid > Hypochlorous acid

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Solution

The correct option is D Hypochlorous acid > Acetic acid > Formic acid
$K_{a}$ is a measure of the strength of the acid i.e., larger the value of $K_{a}$ , the stronger is the acid.
Thus, the correct order of acidic strength is
$H C O O H > C H_{3} C O O H > H C l O$
Stronger the acid, lesser will be the value of pH. Hence, the correct order of pH is $H C l O > C H_{3} C O O H > H C O O H$ .

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The ionisation constant of an acid, Ka is the measure of strength of an acid. The Ka values of acetic acid, hypochlorous acid and formic acid are 1.74×10−5,3.0×10−8 and 1.8×10−4 respectively. Which of the following orders of pH of 0.1mol dm−3 solutions of these acids is correct?