Which of the following salts when added to Pure H2O will cause a significant change in the PH of the solution?
b) LiNo3
c)NaF
d)All the above
e) NaCl
Answer:
The pH of pure water is 7.0 The concentration of H+ ions and OH- ions are equal and are
1.0 x 10^-7. They are in equilbrium according to the following equation:
H2O ===== H+ + OH-
Only a) and b) will cause a change in the pH of the pure water because each salt contains the negative ion that is found in a weak acid.
1) Br= will remove H+ ions from the water to form the weak acid HBr and the [OH-] will increase relative to the [H+] and the soluion will become basic. The pH will be greater than 7.0
2) F- will do what the Br- does to form the weak acid HF. The solution will become basic and the pH will be greater than 7.0
3) Both LiNO3 and NaCl have ions that form strong acids that are completely ionized in water so neither ion will be able to form an acid in the water and remove H+ ions from the water.
d all of the above.
e) NaCl
It is lithium nitrate. All other salts are salts of strong acid and strong base. Hence, their conjugates are all weak. For example, [K(H2O)+] is a weak acid and hence does not donate a H+ion to water molecule. So, it does not undergo cationic hydrolysis. Similarly, Br- ion does not undergo anionic hydrolysis for it is a weak conjugate base. But LiNO3 is a salt of a weak base (LiOH) and strong acid(HNO3). So it under goes cationic hydrolysis. OH- ions are formed in solution and pH changes.
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