When calcium chloride and ammonium phosphate are mixed, an insoluble precipitate of calcium phosphate forms and falls out of solution. After careful analysis of the purified precipitate, 8.16 * 10^25 atoms of calcium are found in the solid. How many other atoms are in the purified precipitate? (Note: the precipitate is only calcium phosphate)

Answer 1

The ppt is Ca3(PO4)2.
You have 8.16E25 atom Ca ions.
That's 8.16E25 x (1 mol/6.02E23) = ? mols Ca and divided by 3 = mols Ca3(PO4)2.
From there you know you have twice that mols P atoms (mols P x 6.02E23 = atoms P) and 8x that number of mols O atoms (atoms O = mols O x 6.02E23).