First you must recognize that this is a .
Next you must write the for the products.
You know that ##NO_3## has an ionic charge of -1 and ##S## has a charge of -2. So the ionic charge on ##Bi## is +3 while on ##Ba## it is -2.
After the cations have changed partners the formulas of the products are ##Bi_2S_3## and ##Ba(NO_3)_2##.
Next you use the solubility rules to determine if there are any precipitates.
In this case the important rules are:
So
##Ba(NO_3)_2## is soluble.
##BaS## is soluble because ##Ba## is in Group 2.
##B_2S_3## is insoluble because ##Bi## is in group 15.
The unbalanced molecular equation is
##Bi(NO_3)_3(aq) + BaS (aq) Bi_2S_3(s) + Ba(NO_3)_2(aq)##
The balanced molecular equation is
##2Bi(NO_3)_3(aq) + 3BaS(aq) Bi_2S_3(s) + 3Ba(NO_3)_2(aq)##
The total ionic equation is
##2Bi^(3+)(aq) + 6NO_3^(-)(aq) + 3Ba^(2+)(aq) + 3S^(2-)(aq) Bi_2S_3(s) + 3Ba^(2+)(aq)+ 6NO_3^(-)(aq)##
To get the net ionic equation we cancel the spectator ions.
##2Bi^(3+)(aq) + color(red)(cancel(color(black)(6NO_3^(-)(aq)))) + color(red)(cancel(color(black)(3Ba^(2+)(aq)))) + 3S^(2-)(aq) Bi_2S_3(s) + color(red)(cancel(color(black)(3Ba^(2+)(aq)))) + color(red)(cancel(color(black)(6NO_3^(-)(aq))))##
##2Bi^(3+)(aq) + 3S^(2-)(aq) Bi_2S_3(s)##
Here’s a good video on solubility rules and net ionic equations.