Question

Calculate the molarity and the molality of C₃H₈O₂ for a solution of 20.0 g of C₃H₈O₂ (molar mass 76.09 g/mol) in 80.0 g of water (molar mass 18.02 g/mol). The density of the solution is 1.016 g/mL.

Answer 1

Number of moles of C₃H₈O₂ = Mass/Molar mass

= 20.0/76.09 mol

= 0.26 mol

Total mass of the solution = Mass of C₃H₈O₂ + Mass of water

= 20 + 80 g

= 100 g

Volume of the solution = Mass/Density

= 100 / 1.016 mL

= 98.42 mL

Volume of the solution in Liter = 98.42 / 1000

= 0.0984

Molarity of a solution = Number of moles of solute / Volume of solution in Liter

Molarity of given solution = 0.26 / 0.0984

= 2.64 M

Molality of a solution = Number of moles of solute / Mass of solvent in Kilogram

   = Number of moles of C₃H₈O₂ / Mass of water in Kilogram

= 0.26 / 0.08

= 3.25 m