Question

A solution has a 1 : 4 mole ratio of pentane to hexane. The vapour pressure of the pure hydrocarbons at 20°C are 440 mmHg for pentane and 120 mmHg for hexane. The mole fraction of pentane in the vapour phase would be .

• A. 0.549
• B. 0.200
• C. 0.786
• D. 0.478

Answer 1

Answer: (D)

0.478

Answer 2

$P _ { T } = P _ { p } ^ { 0 } x _ { p } + P _ { h } ^ { 0 } x _ { h }$ = $440 \times \frac { 1 } { 5 } + 120 \times \frac { 4 } { 5 }$

= $88 + 96$ = $184$ ; $P _ { p } ^ { 0 } x _ { p } = y _ { p } P _ { T }$ ; $\frac { 88 } { 184 } = y _ { p }$

$y _ { p } = 0.478$