The freezing point of ethanol CH3CH2OH is 11730 C at 1 atmos

The freezing point of ethanol CH3CH2OH is -117.30 °C at 1 atmosphere. Chlorophyll is a green pigment found in plants and is nonvolatile and a non-electrolyte that dissolves in ethanol. How many grams of chlorophyll, C55H72MgN4O5, 893.5 g/mol, must be dissolved in 15.0 grams of ethanol to reduce the freezing point by 1.10 °C? kf for ethanol is 1.99 °C/m.

Solution

depression in freezing point = Kf x molality

1.1 C = 1.99 (C/m) x molality

molality = 0.55276 m

molalitty = 0.55276 mol/kg

now

moles of chlorophyll = mass of solvent ethanol (kg) x molality

moles of chlorophyll = 0.015 kg x 0.55276 mol / kg

moles of chlorophyll = 0.00829 mol

now

mass of chlorophyll = moles of chlorophyll x molar mass

mass of chlorophyll = 0.00829 mol x 893.5 g/mol

mass of chlorophyll = 7.408 g

so

7.408 grams of chlorophyll must be dissolved