Question

What volume of ethanol (density = 0.7893 g/cm3) should be added to 450. ml of water in order to have a solution that freezes at â15.0°c? (for water, kf = 1.86 °c/m.)?

Answer 1

According to this formula :
ΔTf= i Kf m
i is van't Hoff factor= 1
Kf = 1.86 
m the molality we need to assume it 
m= x moles of C2H5OH / Kg of mass 
∴ 15 = 1 * 1.86 * ( x moles of C2H5OH/ 0.45 kg)
∴X = 3.629 moles 
mass = no.of moles x molar mass of C2H5OH
          = 3.629 X 46 = 167 g 
∴the volume = mass / denisty
                      = 167 / 0.7893 = 211.57≈ 212 mL