Suggest a reason why it is desirable to use an excess of barium ion in the precipitation.

Hello your question is poorly written hence i will give you a more general answer

answer :  The reason why it is desirable to use an excess of barium ion in the precipitation is to form the compound needed,

Explanation:

Natural Precipitation occurs when condensed atmospheric water vapor falls due to the gravitational pull exerted on it by the clouds. precipitation can be in various forms like ; rain , ice pellets and snow . e.t.c

Hence the reason why it is desirable to use an excess of barium ion in the precipitation is to form the compound needed,

Also in order to form the precipitation, we will also add excess of barium ion which will help form the required precipitate and also  filter out the mass of Sulphate.

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).

the answer would be a