Question

What is the molality and molarity of solution prepared by dissolving 35.0 mL of benzene (density = 0.877 g/mL) in 175 mL of hexane (density = 0.660 g/mL)? Assume volumes are additive.

Answer 1

volume of benzene = 35 mL

mass of benzene = volume x density

                             = 35 x 0.877

                             = 30.695 g

moles of benzene = mass / molar mass = 30.695 / 78.11 = 0.393

mass of hexane = 175 x 0.660 = 115.5 g = 0.1155 kg

volume of hexane = 175 mL = 0.175 L

molality = moles of solute / mass of solvent in kg

             = 0.393 / 0.1155

             = 3.4 m

molality = 3.4 m

molarity = moles / volume of solution

              = 0.393 / (0.035 + 0.175)

              = 1.87 M

Molarity = 1.87 M