Understanding pH and Buffer Solutions: Acid-Salt Ratio and pKa vs. pH and pKb

[future_vet]

This is not a homework question, so I cannot give exact values or anything, sorry.

If we have to make a buffer solution, and we know the pH and the pKa of the acid we are going to use, would it be true to say that if the pH < pKa, we will need more of the weak acid than salt? If pH > pKa, we would need more salt?

Same thing for the pKb and pOH (14 - pH), would we need more salt if the pKb was bigger? More base if the pKb was smaller?

If we have to prepare a buffer with a target pH, and pH = pKa, we need the same quantity of acid and salt respectively, correct? (so that pKa = pH - log1, since log1=0)
BUT if we have pKa = 14 - pKb, what would be the reasonning then? I have a bit of trouble visualizing all that.

Thank you,

~J.

 

[Borek]

If we have to make a buffer solution, and we know the pH and the pKa of the acid we are going to use, would it be true to say that if the pH < pKa, we will need more of the weak acid than salt? If pH > pKa, we would need more salt?

OK

Think in terms - add more acid to lower pH

 

[future_vet]

So my first part would be right?
If the pH is lower than the pKa we need more acid?

Thanks,

~J.

 

[Borek]

Whatever pH you have, the only way to try to lower it is to add more acid. In concentrated solutions, or in case of really weak acid it may not work, but that's the only way of trying.

 

[future_vet]

Thank you!