General chemistry

Question

A scientist wants to make a solution of tribasic sodium phosphate, Na3PO4, for a laboratory experiment. How many grams of Na3PO4 will be needed to produce 550. mL of a solution that has a concentration of Na+ ions of 1.20 M?

Answer 1

Molar mass Na₃PO₄ = 163.9 g / mole

Molarity = moles solute / Liter solution

550.0 ml x 1 L / 1000 ml = 0.5500 L

1.20 M Na⁺ ions = 1.20 mols Na⁺ / L

1.20 mols Na⁺ / L x 0.5500 L = 0.6600 mols Na⁺

But note that there are 3 mols Na⁺ for each 1 mol Na₃PO₄, therefore...

mols Na₃PO₄ needed = 0.6600 mols Na⁺ x 1 mol Na₃PO₄ / 3 mol Na⁺ = 0.2200 mols Na₃PO₄

Grams Na₃PO₄ = 0.2200 mols Na₃PO₄ x 163.9 g / mol = 36.1 g Na₃PO₄ (3 sig.figs.)