How many grams of Ca (NO3) 2 are needed to make 25.0 g of a 15.0% Ca (NO3) 2 (aq) ?

3.75 g.

Explanation:

mass percent is the ratio of the mass of the solute to the mass of the solution multiplied by 100.

mass % = (mass of solute/mass of solution) x 100.

mass of calcium nitrite = ? g,

mass of the solution = 25.0 g.

∴ mass % = (mass of solute/mass of solution) x 100

∴ mass of solute (calcium nitrite) = (mass %) (mass of solution) / 100 = (15.0 %) (25.0 g) / 100 = 3.75 g.