Is there a relation between Keq and ph?

Question

Hello,

I have an exam next week and the question above came up in previous years.

I know if (ph) gets higher (Kb) gets higher too, but for (Ka) it gets lower.

A teacher told that the higher the keq the lower the ph, but I'm not convinced.

It's better if you answer in detail.

Sorry for my bad English.

Thank you in advance. ^^

Answer 1

Ka, the acid dissociation constant, is in fact, an equilibrium constant.  So, let's look at that first.

Let HA be a weak acid.  It dissociates as follows:  HA <==> H⁺ + A^-

Ka = [H⁺][A^-]/[HA], so the HIGHER the value of Ka the greater the degree of dissociation and the higher will be the [H⁺], so the pH will be LOWER (more acidic).  So, the teacher was correct in making that statement.

 

Don't confuse the terms "higher" and "lower" Ka with the exponent.  For example a Ka of 1x10^-5 is higher or greater than a Ka of 1x10^-8.  So, an acid with a Ka = 10^-5 will have a lower pH than an acid with a Ka = 10^-8, at the same concentration.

 

Does this help you in understanding the relationship between Keq and pH?