# CHEMISTRY!!!=0

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Determine pH of each solution:

b) 0.20 M CH3NH3I

I did....

CH3NH3^+ +H2O -> CH3NH2 + H3O^+
0.20 ---------------0--------------0
0.20-x--------------x------------x

so i got...

? =x^2/0.20-x

i looked up in the back of the book and found the Kb of CH3NH2 to be 4.4 X 10^-4.

do i put this number in for the question mark up above or do i have to do that Ka=Kw/Kb thing? How do i know if i use ka or kb?....

• CHEMISTRY!!!=0 -

You looked up Kb which is right.
You want Ka for CH3NH3^+ and they don't make tables for those.
You ALWAYS know that KaKb=Kw; therefore, if you have Kb and want Ka it's Ka = Kw/Kb. If you have Ka and want Kb it's Kb = Kw/Ka.

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