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An unknown acid is titrated with a strong base. At the half-equivalence point (i.e., at the volume halfway to the equivalence point), the pH is 4.2. What is the pKa of the unknown acid?

Respuesta :

Answer : The  pKa of the unknown acid is, 4.2

Explanation : Given,

pH = 4.2

As we know that, at half-equivalence point the moles of salt formed is equal to the moles of acid remaining.

Using Henderson Hesselbach equation :

[tex]pH=pK_a+\log \frac{[Salt]}{[Acid]}[/tex]

Now put all the given values in this expression, we get:

[tex]4.2=pK_a+\log (\frac{1}{1})[/tex]

[tex]4.2=pK_a+\log 1[/tex]

[tex]4.2=pK_a+0[/tex]

[tex]pK_a=4.2[/tex]

Therefore, the pKa of the unknown acid is, 4.2

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