Question

A solution is prepared by mixing 25.0g pentane with 75.0g hexane. The density of the solution is 0.65g/mL. Determine the molarity and the molality of the solution.

*Please be as thorough as possible without skipping any steps. Thank you!

Answer 1

Since pentane is in a lesser amount than hexane, so pentane is taken as solute and hexane as solvent.

Amount of solute (pentane)=25.0 g

Molar mass of pentane =5 x Molar mass of C+ 12 x Molar mass of H=5 x 12 g/mol + 12 x 1 g/mol=60 g/mol + 12 g/mol=72 g/mol

Amount of solvent (hexane)=75.0 g

Total mass of solution=Amount of solute+Amount of solvent=25.0 g+75.0 g=100.0 g=100.0 g/1000 g/kg=0.1 kg (1 Mg=1000 g)

Density of the solution=0.65 g/mL

We know that density=mass/volume

So volume=mass/density

So volume of the solution=100.0 g/0.65 g/mL=153.85 mL=153.85/1000 mL/L 0.154 L (1 L=1000 mL)

Molarity=number of moles of solute/Volume of solution (L)

Number of moles of solute=mass of solute/molar mass of solute=25.0 g/72 g/mol=0.35 mol

So molarity=0.35 mol/0.154 L=2.27 M

So the molarity of the solution=2.27 M

Molality=number of moles of solute/mass of solution (kg)

=0.35 mol/0.1 kg=3.5 m

So molality of the solution=3.5 m