A saturated calcium-lactate solution contains 1.2 mol Ca(lac)₂ in 9 L. Its pH is 8.4 and the salt is 90% dissociated. Find lactic-acid pK_a; log2=0.3,log3=0.48.

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Formal salt concentration=1.2/9=0.1333 M. With 90% dissociation [lactate]=2×0.9×0.1333=0.24 M. pH8.4 gives [OH⁻]≈2.5×10⁻⁶ M. K_b≈[OH⁻]²/0.24≈2.6×10⁻¹¹; K_a=K_w/K_b≈3.8×10⁻⁴ and pK_a≈3.42. This uses the small-hydrolysis approximation.

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