A solution containing 306.5 g of Mg(NO3)2 per liter has a density of 1.114 g/mL. What is the molarity of the solution ?

Answer 1

Molarity (M) is defined as moles of solute, Mg(NO3)2, per liter of solution.

From the mass of Mg(NO3)2 we can calculate the moles:

moles Mg(NO3)2 = 306.5 g x 1 mol / 148.3 g = 2.067 moles

The problem states that this is per liter of solution, so the molarity = 2.067 mol / 1 L = 2.067 M

Now, since the problem provides the density, perhaps you meant to ask what is the molality of the solution. If that's the case, molality is defined as moles of solute per kg of solvent. We already know the moles of solute, so we now need to find the kg of water. Using the density we can find the mass of the solution and then the mass of water.

1 L solution x 1000 ml / L x 1.114 g / ml =1114 g of solution

1114 g - 306.5 g = 807.5 g H2O = 0.8075 kg H2O

molality = moles/kg = 2.067 mols / 0.8075 kg

molality = 2.560 m