[GBuckT]

Hi there,

I had a question recently that went something like given that NH₃ has a Kb value of 1.8*10^-5  how many moles must be dissolved to give a pH of 11.1?

I started out doing

pOH = 2.9, 10^-2.9 = 0.00125M = [OH-]

and i know how to go from here if im working with strong bases but im not quite sure how to factor in the Kb value to my calculations, im terrible when it comes to numbers so thought 0.00125 / 1.8*10^-5 which gave me 6.94444444 moles which is obviously not right. Eventually i gave in and looked it up that a 0.09M solution will give a pH of 11 then just plugged that into my n=cV which did give me the correct answer but i didnt learn anything from it which kinda defeats the purpose of doing it in the first place.

Can anyone see tell me what i'm doing wrong? thanks.

[Astrokel]

Any volume given? Start working out from K

b

expression and ICE table.

[GBuckT]

580mL sorry

[UG]

K_b = [NH₄⁺][OH^-]/[NH₃]

Chuck substitute the numbers in.