How many moles of barium are present in a 40,000,000 atom sample?

Different isotopes indicate that an element will have different numbers of

Using a value of ksp = 1.8 x 10-2 for the reaction pbcl2 pb+2(aq) + 2cl -(aq). if the value of ksp was determined to be only 1.2 x 10-2: too much solid has dissolved. additional precipitate is forming. the solution is unsaturated. the ions are now combining to reduce their concentrations.

What is the empirical formula for c4h8o2