Achieve Question 10 -- pKa and Ka

[Nidhi Thati 3I]

I was wondering how to approach Achieve question #10 on the week 2 homework. It says,

The conjugate acid BH+ has a pKa=8.55.
For this weak base, the predominant species present at pH 6.07 is: charged, neutral, unknown.

Do we need to compare the Kb value we get using the pH to the pKa of the conjugate acid? How do we interpret that?

[Lidia_1I]

you would just be comparing the pH and pKa. if the pH value is above the pKa value, then the predominant species is charged. if the pH is less than the pKa, the predominant species is neutral

[Justine Lim 3L]

For this question you wouldn't need to do any calculations! Just compare the pKa and pH values and remember that if pH > pKa: the weak base is neutral & if pH < pKa: the weak base is charged. Since for the question the pH is greater than pKa, it would be neutral.

[Justin Selby 2G]

Does anyone know if we will need to know what the predominant series means? I don't remember him talking about this in lecture.

[SanthoshNandakumar3L]

Hi, I understand the idea of pH vs pKa, but what is the rule for pOH and pKb. Can someone please write out the relationship of this scenario involving weak bases. Thank you!

[Megan Ho 2B]

For question 10, all you would have to do is compare the given values of pKa and pH. If pH=pKa, equal amounts of the protonated species and the deprotonated species will be present. If pH>pKa, the deprotonated species (B) will be more predominant, and if pH<pKa, the protonated species (BH+) will be more predominant. This is because the pKa is for the conjugate acid BH+ (the protonated species).

Since pH<pKa, this means that the protonated BH+ will be more predominant and thus the predominant species present would be charged. Hope this helps!