Question

One acid, HA, has a pKa of 3.16 and another, HB, has a pKa of 4.14. Which is the stronger acid and why?

Answer 1

PKa is defined as the logarithm of the inverse of Ka, i.e


pKa = log ( 1 / Ka)


Ka is the dissociation constant of the acid. The larger Ka the stronger the acid.


On the other hand, from pKa = log (1 / Ka) the larger Ka the smaller 1 /Ka, and so the smaller log (1/Ka).


So, the relation between Ka and pKa is inverse, which means that an acid with greater value of pKa will have lower value Ka, and so it will be  weaker or the smaller the pKa the stronger the acid.  


Therefore, in our case HA has the lower pKa ant it will be the stronger acid.



Answer: HA is the stronger acid, because it has the lower pKa and pKa is inversely related to the strength of the acid.