Calculating Hydroxide ion concentration help please

[PufferPete]

I think I know what I'm doing but I'm confused.

 

Basically I've been given this;

 

K= [NH4+(aq)] X [OH-(aq)] / [NH3(aq)] X [H2O(l)]

 

I've been given the values;

 

K= 3.19x10^-7

 

NH3(aq)= 1.00x10^-2 mol dm^-3

 

H2O(l)= 55.4 mol dm^-3

 

So I need to find the concentration of OH-. I know I have to rearrange the equation BUT since I have already been given a value for K what do I do with the H2O value? I thought I had to move that over to the left. Do I add it to K or multiply it to K?! Or am I way off?!

 

Any handy pointers are greatly appreciated!

 

Cheers

 

Pete.

[spookyjeff]

You seem to be including something you aren't supposed to. Remember only aqueous species are included in equilibrium quotients.

[Echoes]

Rearrange it so the OH is on the left - then apply what you know about rearranging equations....