Question

a solution is known to contain Be2+ and Ra2+. which pair of ions listed below could be used, in the order given, to selectively precipitate the two cations from the solution?

Answer 1

Step 1 - Eliminating soluble compounds

Since we want to precipitate Be(2+) and Ra(2+), both of which belong to the II A family in the periodic table (Be Mg Ca Sr Ba Ra), we can eliminate some possibilities:

a) NO3(-) and CH3COO(-): pratically all nitrates and acetates are very soluble in water. Therefore, this item can't be the correct answer.

b) Br(-): bromides are usually very soluble as well. Of course it depends on the cation, but it is soluble with Be(2+). Therefore, this item can't be the correct answer as well.

c) SO3(2-): sulfites are usually soluble in water. We should expect RaSO3 to be soluble as well, since most sulfites with cations of the II A family are soluble

Step 2 - Finding the insoluble salts

Sulfides (S2-) are usually very insoluble in water, as well as carbonates CO3(2-)). Therefore, both BeS and RaCO3 would precipitate in solution and could be added to a solution in order to precipitate these cations.

The answer thus is the first item, S(2-) and CO3(2-).