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Determining the Kb of a base
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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
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