J.R. S. answered 10/19/21
Ph.D. University Professor with 10+ years Tutoring Experience
Start with a correctly balanced equation:
3MgCl₂(aq) + 2Na₃PO₄(aq) → Mg₃(PO₄)₂(s) + 6NaCl(aq) ... balanced equation
Use dimensional analysis and stoichiometry of the balanced equation to find moles of MgCl2 needed:
molar mass Mg3(PO4)2 = 263 g / mol
545 g Mg3(PO4)2 x 1 mol / 263 g x 3 mol MgCl2 / 1 mol Mg3(PO4)2 = 6.22 mols MgCl2 needed