Carbonic acid ( ) is a diprotic acid with pK_a1 = 6.37 and pK_a2 = 10.25
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Carbonic acid (H2CO3) is a diprotic acid with pK_a1 = 6.37 and pK_a2 = 10.25. A closed aquatic system maintains three distinct regions with the following measured pH values:
Region I: pH = 5.4
Region II: pH = 7.4
Region III: pH = 9.8
Assume that only the species H₂CO₃, HCO₃⁻, and CO₃²⁻ are present in appreciable concentration.
(1) For each region, identify the carbonic-acid–derived species that constitutes more than 50 % of the total carbonate species present. Provide reasoning for each region.
(2) Write balanced net ionic equations showing the acid–base transformations that interconvert the predominant species between Region I and Region II, and between Region II and Region III.
(3) Predict the sign (positive, zero, or negative) of the net charge on carbonic-acid–derived species that predominates in each region, and relate your prediction to the molecular formula of each species.
(4) Explain how the values of pK_a1 and pK_a2 enable the carbonate system to buffer the pH of natural waters, particularly near pH 7.4.