Question

At a certain temperature the vapor pressure of pure heptane (C,H.) is measured to be 375, torr. Suppose a solution is prepared by mixing 92.7 x of heptane and 61.6 g of acetyl bromide (CH,COBE). Calculate the partial pressure of heptane vapor above this solution. Round your answer to 3 significant digits Note for advanced students: you may assume the solution is ideal.

Answer 1

Molar mass of C7H16,

MM = 7*MM(C) + 16*MM(H)

= 7*12.01 + 16*1.008

= 100.198 g/mol

Molar mass of CH3COBr,

MM = 2*MM(C) + 3*MM(H) + 1*MM(O) + 1*MM(Br)

= 2*12.01 + 3*1.008 + 1*16.0 + 1*79.9

= 122.944 g/mol

n(C7H16) = mass/molar mass

= 92.7/100.198

= 0.9252

n(CH3COBr) = mass/molar mass

= 61.6/122.944

= 0.501

n(C7H16),n1 = 0.9252 mol

n(CH3COBr),n2 = 0.501 mol

Total number of mol = n1+n2

= 0.9252 + 0.501

= 1.426 mol

use:

Mole fraction of solvent C7H16,

X(C7H16) = n1/total mol

= 0.9252/1.426

= 0.6487

According to Raoult’s law:

P = Po*X(solvent)

p = 375.0*0.6487

p = 243 torr

Answer: 243 torr