A magnesium sulfate heptahydrate solution,which is 18.00% by weight in the anhydrous compound,has a density at 20 °C of 1.20 g/mL.What is the molality of the anhydrous compound in the solution? (Atomic masses in amu: H = 1.0,O = 16.0,S = 32.1,Mg = 24.3)
A)5.54 M
B)1.79 M
C)1.82 M
D)1.25 M
E)1.49 M

Question

Quizplus · Accepted Answer

To find the molality, first calculate the mass of the solution using its density (1.20 g/mL). For 1000 mL, the mass is 1200 g. The mass of the anhydrous MgSO4 in the solution is 18% of 1200 g, which is 216 g. The molar mass of MgSO4 is 120.4 g/mol. The moles of MgSO4 are 216 g / 120.4 g/mol = 1.79 mol. The mass of the solvent (water) is 1200 g - 216 g = 984 g = 0.984 kg. Molality is moles of solute per kg of solvent: 1.79 mol / 0.984 kg = 1.82 m.

A Magnesium Sulfate Heptahydrate Solution,which Is 18