pH shortcut?

[MedPR]

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What is the pH of 0.050M H3CCH2CO2H (pKa=4.89).

A. 1.30
B. 3.10
C. 4.89
D. 9.78

So A and D are automatically out since the acid is a weak acid. TBR says

The pH is less than the pKa, so choice C is eliminated

.

How do you know the pH is less than the pKa without doing any calculations?

 

[fluoropHore]

tbr gives a shortcut eq: pH= 1/2pKa - 1/2log[ha]

or just use Ka equlilbrium constant to find the x [h+] and then use that in the pH=-logHA eq.

 

[MedPR]

tbr gives a shortcut eq: pH= 1/2pKa - 1/2log[ha]

or just use Ka equlilbrium constant to find the x [h+] and then use that in the pH=-logHA eq.

How do you know the pH is less than the pKa without doing any) calculations?

So I guess you can just tell based on the shape of that equation that the pH is probably always going to be less..

 

[kasho11]

Because weak acids do not dissociate that well in solution, there will be significantly more undissociated acid than base, so with the HH equation pH = pKa + log(Conjugate base/Acid)

... by having significantly more undissociated acid will result in a fraction less than 1. Taking the log of a number less than 1 results in a negative number. So the pH will be less than the pKa.

A and D should be out immediately, and C should automatically be sketchy because it's the same as the pKa meaning there would have to be an exactly even amount of conjugate base and acid (log of 1 = 0).


But for a short answer, weak acids don't dissociate fully, therefore the pH should usually be less than the pKa.