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Working with pH and ion concentration

Methylamine, CH3NH2, is a weak base that reacts according to the equation CH2NH2 + H2O <--> CH3NH3^1+ + OH^1-. The value of the ionization constant, Kb, is 5,25 x 10^-4. Methylamine forms salts such as methylammonium nitrate, (CH3NH3^1+)(NO3^1-).
(b) Calculate the pH of a solution maded by adding 0.0100 mole of a solid methylammonium nitrate to 120.0 mL of a 0.225-molar solution of methylamine. Assume that no volume change occurs. (The answer i got was pH=11.15)
(c) How many moles of EITHER NaOH OR HCl (state clearly which you choose) should be added to the solution in (b) to produce a solution that has a pH of 11.00? Assume that no volume change occurs.
This is from AP Chem Exam 1993 ques #1

Solution Preview

a) Calculate the hydroxide ion concentration, [OH -], of a 0.225 M solution of methylamine.

5.24 x10-4 = (x)^2/0.225

x = [OH -] = 1.09 x10-2

(we assumed that x moles of ch3nh2 ionizes and it produces x moles of each + ...

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