Question

In: Chemistry

1. Which one of the following is true? A. London forces do not exist between molecules...

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

Solutions

Expert Solution

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

queen_honey_blossom answered 2 years ago
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