Question

# Consider a solution prepared by dissolving 0.35 mol of CH3NH3+Cl- (methylamine hydrochloride) in 1.00 L of...

Consider a solution prepared by dissolving 0.35 mol of CH3NH3+Cl- (methylamine hydrochloride) in 1.00 L of 1.1 M CH3NH2 (methylamine). If 10 mL of 0.10 M HCl is added to this buffer solution, the pH of the solution will ________ slightly because the HCl reacts with the _________ present in the solution to produce a _______.

A. increase;     CH3NH2          weak acid

B. increase;      CH3NH3+Cl-; weak base

C. decrease,     CH3NH2;         weak acid

D. decrease;     CH3NH3+Cl-; weak base

E. increase;      CH3NH2;         weak base

moles of CH3NH3+Cl- = 0.35

moles of CH3NH2 = 1 x 1.1 = 1.1

pOH = pKb + log [salt / base]

= 3.30 + log [0.35 / 1.1]

= 2.803

pH = 11.197

if moles of HCl added = 10 x 0.1 / 1000 = 0.001

pOH = pKb + log [salt + C / base - C]

= 3.30 + log [0.35 + 0.001 / 1.1 - 0.001]

= 2.804

pH = 11.196

so pH decrease

CH3NH2 + HCl   --------------> CH3NH3+

so

option C) is correct

C. decrease,     CH3NH2;         weak acid

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