What is the concentration of carbonate ions (CO3^2−) in a 0.090 M carbonic acid (H2CO3) solution, assuming complete dissociation?

• A. 0.090 M
• B. 0.045 M
• C. 0.180 M
• D. 0.030 M

Answer: (B)

Verified step by step guidance

1. Understand that carbonic acid (H2CO3) is a diprotic acid, meaning it can donate two protons (H+). It dissociates in two steps: first into bicarbonate ions (HCO3^−) and then into carbonate ions (CO3^2−).
2. Write the dissociation equations for carbonic acid: 1. H2CO3 ⇌ H+ + HCO3^− 2. HCO3^− ⇌ H+ + CO3^2−
3. Assume complete dissociation of carbonic acid, which means all of the initial H2CO3 will convert to CO3^2− through the two-step dissociation process.
4. Since the initial concentration of H2CO3 is 0.090 M, and each molecule of H2CO3 ultimately produces one CO3^2− ion, the concentration of CO3^2− will be half of the initial concentration of H2CO3 due to the two-step dissociation.
5. Calculate the concentration of CO3^2− by dividing the initial concentration of H2CO3 by 2: [CO3^2−] = 0.090 M / 2