Asked by Ana

How many mmol ( millimoles) of HCl must be added to 130.0 mL of a 0.40M solution of methyl amine (pKb = 3.36) to give a buffer having a pH of 11.59?
Thank you for your help!
Answered by DrBob222
This looks almost like the KOH/acetic acid above. If you still have trouble show what you've done and where you're stuck as well as what you don't understand and I can help you through it.
Answered by Ana
the correct answer is 5.2 mmol; however, I get lost in the calculations.
It seems easy to solve: I have pH, pKb, and HH eq, but then I get stuck to derive x correctly
Answered by DrBob222
Did you convert pKb given to pKa?
Answered by DrBob222
mmols CH3NH2 - 0.40 x 130 mL = 52

..........CH3NH2 + HCl ==> CH3NH3^+
I..........52.......0........0
add.................x.............
C.........-x.......-x........x
E.........52-x......0........x

11.59 = 10.64 + log (52-x)/(x)
8.91 = (52-x)/x
8.91x = 52-x
9.91x = 52
x = 5.24 which rounds to 5.2
Answered by Ana
Thank you! I finally have solved it
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