Question

A solution contains Naphthlene (C₁₀H₈) dissolved in Hexane (C₆H₁₄) at a concentration of 12.60% Naphthalene by mass. Calculate Vapor Pressure at 25 degrees Celcius of hexane above the soultion. The vapor pressure of pure hexane at 25 degrees celcius is 151 torr

Answer 1

per 100 g solution we have Napthalene mass = 12.6g

Napthalene moles = mass / ( molar mass of napthalene) = ( 12.6g) / ( 128 g/mol) = 0.09844 mol

hexane mass = 100 - 12.6 = 87.4 g

Hexane moles = ( 87.4g) / ( 86g/mol) = 1.016 mol

mol fraction of napthalene = ( moles of napthalene) / ( napthalene + hexane moles)

= ( 0.09844 / 0.09844+1.1016) = 0.0817

we have formula Po-Ps = Po x Xb where Xb is mol fraction of solute napthalene ,

Po = hexane vapor pressure = 151 torr , Ps = vapor pressure of solution

hence ( 151-Ps) = 151 x 0.0817

Ps = 139 torr