[Beetle]

Hello all!

I need to find the dissociation constant of an unknown weak base which is 0.100M and has a pH=7.28

I don't know if my approach is right but i performed an imaginary titration with HCl and added 25mL of 0.1M HCl to 50mL of my unknown Base (Because that would be halfway to the equivalence point and therefore Kb=[OH-]??)

This meant that i would have 0.1x.025 moles of HCl reacting with the same amount of base leaving a solution with 0.0025 moles of base in 75mL of H2O

Then [OH-]=0.0025/0.075 Moles/Litre
                =0.0333mol/L
                =Kb

Is this right?
The Kb seams far too high to me but I can't see how to figure it out or to check my answer

[Borek]

Assume you know Kb value and start calculation of pH (or pOH) - and once you will have ready equation combining concentration, Kb and pH, simply solve for Kb - you know concentration and pH so you should have no problems with Kb calculation.

[Beetle]

Thank you that makes sense now.

I ended up with a pKb of 12.44 which is heaps better. Thank you for your help