Answer:
a. Ag+ and Ca2+
The pair of cations related to the question are as below:
a. Ag+ and Ca2+
b. Hg2^2+ and Ag^+
c. Ba^2+ and Na^+
d. Ca^2+ and Ba^2+
e. Pb^2+ and Ag^+
Explanation:
a. Ag+ and Ca2+ correct
Ag+ will precipitate with dilute hydrobromic acid (HBr) but Ca+2 will not
Ag+ + Br- -----------------> AgBr (s)
Ca+2 + 2Br- ---------------> CaBr2 (aq)
B) Hg2^2+ and Ag^+ Incorrect
Both Hg2^2+ and Ag^+ will precipitate. One has to precipitate while the other does not, so this is incorrect.
C) Ba2 and Na+ both won't precipitate. Incorrect
One has to precipitate while the other does not, so this is incorrect.
D) Ca2+ and Ba 2+both won't precipitate. Incorrect
One has to precipitate while the other does not, so this is incorrect.
E ) Both Pb2 and Ag+ will precipitate. Incorrect
One has to precipitate while the other does not, so this is incorrect.