When you put soluble ions in solution together, they are all bumping around, and when the right ones find each other, they form the precipitate- In this case, lead iodide is insoluble (see exceptions of soluble compounds), and when they find each other, they form a solid.
______ PbBr 2 (s)+ ______ NaNO 3 (aq)
FeCl3 (aq) + 3NaOH(aq) = Fe(OH)3 (s) + 3NaCl (aq)
solute takes up space
.427 M NaCl .335 M KCl
60. g KCl
you want .15 mols in a 0.50 mols/L soln, so you need 0.30 L