Determine substance from molar heat of vaporization

[rasen58]

The molar heat of vaporization for a certain compound is 30.8 kJ/mol. If 0.770 kJ were required to vaporize 1.95 g of the compound, its molecular formula could be

a. C3H6

b. C4H10

c. C5H10

d. C6H6

e. C7H14

 

I tried using the formula q = Hvap x (mass/molar mass)

and plugged in the numbers

so .770 kJ = 30.8 kJ/mol x 1.95 g/molar mass

 

Solving for the molar mass, I got .0128.

What should I do after that?

Edited March 12, 2014 by rasen58

[hypervalent_iodine]

A molar mass of 0.0128 doesn't make sense. Hydrogen, the smallest element in the periodic table, has a MW of 2 g/mol (counting for the fact that it's diatomic in it's elemental state).

 

The formula you've used to calculate that looks correct, but you have gone wrong somewhere when actually trying to do that calculation. I get a different result to 0.0128, so perhaps try that part again and see what you get.

 

The questions has asked you to identify which of the possible compounds it refers to and you seem to have identified that you need to solve for molar mass. How did you know to do that? What is it about the molar mass you've calculated that will help you to distinguish between the compounds listed?

[rasen58]

Oh I see now.

I did the calculation wrong. The molar mass is actually 78, which is the same as D.

Thank you!