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

The correct answer is a.

When a small amount of strong acid, such as HNO3, is added to a buffer solution, the acid reacts with the weak base component of the buffer to form its conjugate acid, which has a negligible effect on the pH of the buffer. In this case, the CH3NH2(aq) is the weak base component of the buffer, and it reacts with H+ from the added HNO3 to form CH3NH3+(aq):

CH3NH2(aq) + H+(aq) → CH3NH3+(aq)

This reaction is represented by answer choice a, and it accounts for the fact that the pH of the buffer does not change significantly when the HNO3 is added. Answer choices b, c, and d do not represent the correct reaction that occurs in the buffer solution when HNO3 is added.