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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: vnaraya3 on February 16, 2015, 07:19:45 PM

Title: Why is the pH of a 1:1 carb/bicarb sol'n 9.4, when the pKa is 10.3?
Post by: vnaraya3 on February 16, 2015, 07:19:45 PM

This was in a lab experiment in class. All the groups had a pH of about 9.4-ish (using digital pH meter) when they made the 1:1 buffer of NaHCO₃ and Na₂CO₃, even though the pKa of HCO₃ is around 10.3. Why was it consistently lower than the theoretical pH?

Title: Re: Why is the pH of a 1:1 carb/bicarb sol'n 9.4, when the pKa is 10.3?
Post by: Arkcon on February 16, 2015, 07:34:53 PM

Its true that theoretically, the pH of an equimolar mix of conjugate acid and conjugate base equals pka (because 1/1=1 and log₁₀1=0).  In order to explain your experimental results, you'll have to figure out why.  How did you prepare the 1:1 buffer?  Are you sure there's nothing else that could be present to affect the pH?