Accumulation of lactic acid (HC_{3}H_{5}O_{3}), a monobasic acid in tissues leads to pain and a feeling of fatigue. In a 0.10M aqueous solution, lactic acid is 3.7% dissociated. The value of dissociation constant, K_{a} for this acid will be

  • Option 1)

    2.8\times10^{-4}

  • Option 2)

    1.4\times10^{-5}

  • Option 3)

    1.4\times10^{-4}

  • Option 4)

    3.7\times10^{-4}

 

Answers (1)

 

Ionization constant of weak acids -

Consider 

HX(aq)+H_{2}O(l)\rightleftharpoons H_{3}O^{+}(aq)+\bar{X}(aq)
 

K_{a}=dissociation\:or\:ionization\:constant

- wherein

K_{a}=\frac{C^{2}\:\alpha ^{2}}{C(1-\alpha )}

K_{a}=\frac{C\alpha ^{2}}{1-\alpha }
 

C=initial\:concentration\:of\:undissociated\:acid

\alpha =extent\:upto\:which\:HX\:is\:ionized\:into\:ions.

 

 Degree of dissociation \alpha =\frac{3.7}{100}=0.037

According to Oswald's formula

K_{a} =\alpha ^{2}c = (0.037)^{2} \times 0.10 = 1.4 \times 10^{-4}

 


Option 1)

2.8\times10^{-4}

incorrect

Option 2)

1.4\times10^{-5}

incorrect

Option 3)

1.4\times10^{-4}

correct

Option 4)

3.7\times10^{-4}

incorrect

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