Explain why a 0.1 M NaHCO3 solution is basic, and estimate its pH.

Explain why a 0.1 M NaHCO3 solution is basic, and estimate its pH.

• A. Around 8.3
• B. Around 7.0
• C. Around 6.0
• D. Around 9.0

Answer: (A)

Bicarbonate ion is amphiprotic: it can both accept a proton to form carbonic acid and donate a proton to form carbonate. In water, this dual behavior sets the pH toward the average of the two dissociation constants of carbonic acid. The relevant pKa values are about 6.35 for H2CO3 ⇌ H+ + HCO3− and about 10.33 for HCO3− ⇌ H+ + CO3^2−. Taking the average gives pH ≈ (6.35 + 10.33)/2 ≈ 8.3. So a 0.1 M NaHCO3 solution is basic, with pH around 8.3. The concentration only slightly nudges this value; the typical pH for such a solution sits between 8 and 9, closer to 8.3.

Bicarbonate ion is amphiprotic: it can both accept a proton to form carbonic acid and donate a proton to form carbonate. In water, this dual behavior sets the pH toward the average of the two dissociation constants of carbonic acid. The relevant pKa values are about 6.35 for H2CO3 ⇌ H+ + HCO3− and about 10.33 for HCO3− ⇌ H+ + CO3^2−. Taking the average gives pH ≈ (6.35 + 10.33)/2 ≈ 8.3. So a 0.1 M NaHCO3 solution is basic, with pH around 8.3. The concentration only slightly nudges this value; the typical pH for such a solution sits between 8 and 9, closer to 8.3.