Question

1. Which one of the following is true?

A. London forces do not exist between molecules of dimethylamine, (CH3)2NH

B. increasing the pressure above a liquid increases its boiling point

C. the surface of hexane, C6H14, in a glass tube will be a u-shaped meniscus

D. a water drop will spread out and “wet” a surface of polyethylene

E. amorphous solids have a well defined melting point

2. A scientist measured the vapor pressure of a substance at various temperatures and then plotted ln(Pvapor) on the y-axis versus 1/T on the x-axis and ended up with a straight line with a slope of -39.9 and a y-intercept of 0.200. What is the heat of vaporization for this substance?

A. 332 J/mol

B. 8.314 J/mol

C. 39.9 J/mol

D. None of these

E. 4.80 J/mol

Answer 1

1. Which one of the following is true?

A. London forces do not exist between molecules of dimethylamine, (CH3)2NH FLASE, they do, due to electron distribution

B. increasing the pressure above a liquid increases its boiling point . TRUE , increase in pressure will require more energy to vaporize the liquid molecules, thereofre higher BP

C. the surface of hexane, C6H14, in a glass tube will be a u-shaped meniscus. FALSE, will be convex, since no attractions between hexane due to adhesion

D. a water drop will spread out and “wet” a surface of polyethylene. False, since it is impermeable

E. amorphous solids have a well defined melting point. False, they change according to structure, similar to glass

Q2.

ln(P) vs 1/T

Recall that in equilibrium; especially in vapor-liquid equilibriums, we can use Clasius Clapyeron combination equation in order to relate two points in the same equilibrium line.

The equation is given as:

ln(P2/P1) = -dHvap/R*(1/T2-1/T1)

Where

P2,P1 = vapor pressure at point 1 and 2

dH = Enthalpy of vaporization, typically reported in kJ/mol, but we need to use J/mol

R = 8.314 J/mol K

T1,T2 = Saturation temperature at point 1 and 2

Therefore, we need at least 4 variables in order to solve this.

Substitute all known data:

ln(P2/P1) = -dHvap/R*(1/T2-1/T1)

if slope = -39.9

-dHVap/R = -39.9

Hvap = 8.314*39.9 = 331.7286 J/mo, choose A