Question 6.11: Determine the osmolarity of 5.0 × 10^–3 M Na3PO4.

Step 1. Na_3PO_4 is an ionic compound and produces an electrolytic solution:
Na_3PO_4 \xrightarrow[]{H_2O} 3Na^+ + PO_4  ^{3-}

Step 2. 1 mol of Na_3PO_4 yields four product ions; consequently
5.0 \times 10^{–3}  \frac{\cancel{mol  Na_3PO_4}}{L} \times \frac{4  mol  particles}{1  \cancel{mol  Na_3PO_4}} = 2.0 \times 10^{-2} \frac{mol  particles}{L}

Step 3. Using our expression for osmolarity,
2.0 \times 10^{-2} \frac{mol  particles}{L} = 2.0 \times 10^{-2}  osmol