Question

In a titration of a monoprotic weak acid with sodium hydroxide, the ph at the half-equivalence point is 4. 20. What is the experimental ka value of this acid?.

Answer 1

Ka value of monoprotic weak acid:

The experimental ka value of this acid is 6.3 × $10^{-5}$  

Half-equivalence point:  

• The pH of the solution will be equal to the pKa of the weak acid at the half-equivalence point.
• At the equivalence point, the strong base OH- will neutralize the weak acid in the case of a weak monoprotic acid, which I'll denote as HA.

   

• HA + OH⁻  →  A⁻ + H₂O
• The moles of OH are equivalent to the moles of the acid since the acid is monoprotic:

At half equivalence point, pH = pKₐ

The pKa is determined by the weak acid's Ka acid dissociation constant.

pKₐ = -log(Kₐ)

Kₐ = $10^{-pKa}$

At the half equivalence point,

Kₐ = $10^{-pH}$

Given,

pH = 4.20. By putting the value;

Kₐ = $10^{-4.20}$

     = 6.3 × $10^{-5}$  

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