The formula for a chemical substance that will form a precipitate with SnCl2 solution but not with HgCl2 is Na2S.
When SnCl2 solution is mixed with Na2S, a reaction occurs where the Sn2+ ions from SnCl2 react with the S2- ions from Na2S to form SnS, a yellow precipitate. This precipitate indicates that a chemical reaction has taken place between the Sn2+ ions and S2- ions.
On the other hand, when HgCl2 is mixed with Na2S, no precipitate is formed. This is because HgCl2 does not react with Na2S to form a solid compound. Instead, the Hg2+ ions remain in solution.
The formation of a precipitate with SnCl2 and not with HgCl2 is due to the difference in reactivity of the metal ions. Sn2+ ions have a higher reactivity towards S2- ions compared to Hg2+ ions. Therefore, the Sn2+ ions readily react with S2- ions to form a solid compound, while the Hg2+ ions do not.
In summary, the formula Na2S can be used to form a precipitate with SnCl2 solution but not with HgCl2. This demonstrates the different reactivity of Sn2+ and Hg2+ ions with S2- ions.