Question

Methanol has a heat of vaporization of 38.278 kJ/mol and a density with is 80% that of water. Find it's cohesive energy density.

Answer 1

Cohesive energy density means the amount of energy needed to completely remove unit volume of any molecule from their neighbour molecules.

Cohesive energy E_coh =

And cohesive energy density e_coh = E_coh/V_m

_here

_{where T is boiling point}.

Boiling point of methanol 64.7⁰c

₌ 38.2 - 8.314(64.7+273)/1000 =38.2-2.8= 35.4 KJ

Molar volume of methanol = molar mass/density

Molar mass of methanol = 12+ 4+ 16= 32 g/ mol

Density of methanol is 80% of water. Density if water is 1g/dm³. Then density of methanol = 0.8g/dm³, molar volume= 32/0.8= 40dm³ = 0.04 m³

Hence E_coh = 35.41000/0.04= 8.85 10⁵ J/m³