The chemical formula of sodium sulfate is Na2SO4. The chemical formula of barium chloride is BaCl2. BaCl2 + Na2SO4-----------BaSO4 + 2 NaCl BaSO4 is a white, practically insoluble in water precipitate; this property is important in gravimetric analysis.
It is because Barium and Strontium are both elements of same group and have same outer shell configuration of ns2 where n is the principal quantum number or shell number. Furthermore, they both are bonded to same element. Hence, the formula of Barium and Strontium Chloride are similar.
Nothing - barium chloride is soluble. You can however precipitate either the barium (e.g. with sodium sulphate, giving barium sulpate, or the chloride, e.g. with silver nitrate giving silver chloride precipitate.