How to find pka value

Hello,
If I guessed right you are talking about the Chemistry notion of the pKa
Let the equilibrium constant be Ka =1.8x10^(-5). Then by definition pKa=-log(Ka) , log in base 10
pKa = - log(1.8x 10^(-5))
First set [MODE] to [COMP] by pressing [MODE] 1.
[LOG] [(] 1.8 [x] [SHIFT] [10 to the x] [(-)] 5 [)] . [(-)]
The [(-)] key is the change sign key : a small - between parentheses. The last [(-)] sign in the expression takes the negative of the whole calculation to obtain the correct value of the pKa.
The value you should find for the above exemple is 4.74

Hope it helps.