The pH of a 1.4M solution of barbituric acid HC4H3N2O3 is measured to be 1.93. Calculate the acid dissociation constant Ka of barbituric acid. Round your answer to 2 significant digits.

9.8 × 10⁻⁵

Explanation:

Let's consider the acid dissociation of barbituric acid.

HC₄H₃N₂O₃(aq) ⇄ C₄H₃N₂O₃⁻(aq) + H⁺(aq)

The pH is 1.93. We can find the concentration of H⁺.

pH = -log [H⁺]

[H⁺] = antilog -pH = antilog -1.93 = 0.0117 M

The concentration of the acid (Ca) is 1.4 M. We can fidn the acid dissociation constant (Ka) using the following expression.

[H⁺] = √(Ca × Ka)

[H⁺]² = Ca × Ka

Ka = [H⁺]² / Ca

Ka = (0.0117)² / 1.4

Ka = 9.8 × 10⁻⁵

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