Final answer:
The first precipitate to form when silver nitrate is added to a solution containing both Cl- and Br- ions is silver chloride (AgCl), due to its higher solubility product constant compared to silver bromide (AgBr).
Step-by-step explanation:
The formula for the precipitate that forms when silver nitrate solution is added to a solution containing both chloride and bromide ions at 0.050 M concentration depends on the solubility product constants (Ksp) of silver chloride (AgCl) and silver bromide (AgBr).
Since AgCl has a higher Ksp than AgBr, it will precipitate out of the solution first. Thus, the formula for the precipitate that begins to form when silver nitrate is added to the solution is AgCl, a white precipitate.