Ka and Kb

[yoyohomieg5432]

I had a question where they give you a 1M solution of bicarbonate, and they want you to find the pH. They give you the Ka of the conjugate acid, carbonic acid.

I solved this problem by finding Kb and then writing a Kb expression to solve for [OH] and then find pH from that.

It seems like back in my gen chem days there was a way to solve this directly by writing a pKa equation instead. I haven't been able to figure it out though. Am I right or wrong about this?

---

Members don't see this ad.

 

[sdm33]

Pka = - log (Ka)
Use Henderson-Hasselbalch equation to calculate the base/acid ratio
Ph = PKa + Log ( Conj base / Weak Acid )

 

[yoyohomieg5432]

Pka = - log (Ka)
Use Henderson-Hasselbalch equation to calculate the base/acid ratio
Ph = PKa + Log ( Conj base / Weak Acid )

how would that work? only the concentraction of bicarbonate is given. if i tried to use henderson there'd be 2 unknowns, ph and the acid concentration

 

[polyatomic]

If you only know the concentration of bicarbonate...you can use Berkeley's special pH equation. It's a spin off of the Henderson-Hasselbach:

pH = 0.5*(pKa) - 0.5*(log [HA])

NOTE: this is only used for weak acids, such as bicarbonate, and would obviously never be used for strong acids.

 

[BerkReviewTeach]

I had a question where they give you a 1M solution of bicarbonate, and they want you to find the pH. They give you the Ka of the conjugate acid, carbonic acid.

I solved this problem by finding Kb and then writing a Kb expression to solve for [OH] and then find pH from that.

It seems like back in my gen chem days there was a way to solve this directly by writing a pKa equation instead. I haven't been able to figure it out though. Am I right or wrong about this?

This is a simple question hidden in a difficult (tricky) package. How would you get the pH for a 1.0 M zwitterion? The same way you would if it were 0.10 M, 2 M, and so on. The pH of any zwitterion is the pI, and it's found by averaging the two pKa values that flank that structure. For glycine, it's an average of pKa1 and pKa2 while for histidine it's an average of pKa2 and pKa3.

In the case of H2CO3, the pH of a solution of bicarbonate can be found by averaging pKa1 and pKa2, which are 6.4 and 10.8 respectively. No matter what the concentration may be, the pH at the first equivalence point is 8.6.