pH help??

[nysoprano]

This is the question i need help in:
A 0.200 M solution of a weak base in water has a pH=10.40. Calculate the value of kb for this base.

I know the answer but i need help on how to get that answer. Thanks

[Pyrrhus]

If its a weak base then it doesn't diassociates completely in OH ions, so you need to use the equilibrium solution.

[Sirus]

This may help: see post #5 here (http://www.physicsforums.com/showthread.php?t=51566).