Question

When a small amount of 12 M HNO3(aq) is added to a buffer solution made by mixing CH3NH2(aq) and CH3NH3Cl(aq) , the pH of the buffer solution changes from 10.64 to 10.62. Which of the following equations represents the reaction that accounts for the fact that the pH does not change significantly when the HNO3(aq) is added?
a. CH3NH2(aq) + H+(aq) â CH3NH3+(aq)

b. CH3NH3+(aq) + H+(aq) â CH3NH42+(aq)

c. NO3- (aq) + H+(aq) â HNO3(aq)

d. OH- (aq) + H+(aq) â H2O(l)

Answer 1

Answer:

a. CH3NH2(aq) + H⁺ → CH3NH3⁺

Explanation:

The mixture of a weak base as CH3NH2 with its conjugate acid CH3NH3Cl produce a buffer. As the weak acid is in equilibrium with water, the mixture of the weak base and its conjugate base produce that the acid or base released react avoiding the change in pH.

For example, when a strong acid as HNO3 reacts, the weak base will react producing the conjugate base, that is:

CH3NH2(aq) + H⁺ → CH3NH3⁺

Right answer is:

a. CH3NH2(aq) + H⁺ → CH3NH3⁺