Comparing the values of x shows that forms with the lowest concentration of sulfate ions and is therefore the first precipitate to form. This method gives a quantitative comparison of the relative amounts of sulfate ions needed to form a precipitate. A qualitative comparison can be made from observation of the solubility constants. Barium sulfate has the smallest solubility constant. Thus a very small amount of sulfate causes a precipitate to form. The larger solubility constants of the other compounds imply that a higher concentration of sulfate ions is needed to form a precipitate. The first precipitate will be the compound that precipitates at the lowest concentration of sulfate ions. In this case, that compound is barium sulfate.
