Review and Extend Stoichiometry

Question

What minimum mass of cobalt (II) nitrate (MM of Co(NO3)2 = 182.94 g/mol) must be added to 60.0 mL of a 0.0890 M sodium phosphate solution in order to completely precipitate all of the phosphate as solid cobalt (II) phosphate?

2Na3PO4(aq) + 3Co(NO3)2(aq) → Co3(PO4)2(s) + 6NaNO3(aq)

a. 2.93 g

b.0.976 g

c.1.46 g

d.0.651 g

e.0.325 g

Answer 1

2Na₃PO₄(aq) + 3Co(NO₃)₂(aq) ==> Co₃(PO₄)₂(s) + 6NaNO₃(aq)

moles Na₃PO₄ present = 0.060 L x 0.0890 mol/L = 0.00534 moles

moles Co(NO₃)₂ needed = 0.00534 mol Na₃PO₄ x 3 mol Co(NO₃)₂ / 2 mol Na₃PO₄ = 0.00801 mol Co(NO₃)₂

mass of Co(NO₃)₂ needed = 0.00801 moles x 182.94 g/mol = 1.47 g (to 3 significant figures)