A 1.00 liter solution contains 0.40 M ammonia and 0.52 M ammonium iodide

Question

If 0.130 moles of barium hydroxide are added to this system, indicate whether the following statements are true or false.

(Assume that the volume does not change upon the addition of barium hydroxide.)

A. The number of moles of NH3 will remain the same.

B. The number of moles of NH4+ will decrease.

C. The equilibrium concentration of H3O+ will increase.

D. The pH will remain the same.

E. The ratio of [NH3] / [NH4+] will decrease.

 

 

Answer 1

The solution of NH₃ and NH₄I creates a buffer since you have a weak base (NH₃) and the conjugate acid (NH₄⁺).  

When OH^- (as in barium hydroxide) is added, it reacts with the acid (NH₄⁺) to reduce the amount of NH₄⁺ and increase the concentration of NH₃.  This occurs as follows:  NH₄⁺ + OH^- ===> NH₃ + H₂O  (since water is formed, this maintains the pH).  Thus ...

 

A. The number of moles of NH3 will remain the same. - FALSE

B. The number of moles of NH4+ will decrease. - TRUE

C. The equilibrium concentration of H3O+ will increase. - FALSE

D. The pH will remain the same. - TRUE (to a degree. It will change slightly).

E. The ratio of [NH3] / [NH4+] will decrease. - FALSE. It will increase as NH₃ goes up and NH₄⁺ goes down.