Why must all chemical equations be balanced?

If the equation is not balanced, you have extra of one substance. If you try to do an experiment, you could wind up with a totally different outcome. Its kind of like baking a cake. If the ingredients aren't the right amounts, you dont get an edible cake.

It conforms what ever surface its in.

Since there is the same amount of agn03 in both silver chloride and silver nitrate they produce the same amount of moles. therefore silver chloride produces the same amount of silver nitrate, so your answer is 15.0 moles of silver nitrate. i hope this !