Question

A gaseous hydrocarbon (an organic compound consisting of C & H) weighing 0.232g occupies a volume of 102mL at 23 degrees Celcius and 749mmHg.
Calculate the molar mass and identify the compound. The compound is:
a)C2H4
b)C3H6
c)C4H8
d)C5H10
e)C6H12
How is this problem worked out to get "c"...C4H8??? Do I use PV=nRT?...if so...how is it used for this problem?

PV=nRT

P = (749/760) atm.
V 0.102 L
solve for n. I get 0.004139
R = 0.08205 L*atm/mol*K
T = 273 + 23

# mols = g/molar mass.
you have mols and you have g. Solve for molar mass.
I get 56.
If it is a hydrocarbon, then 1C and 2H units is 14. 56/14 = 4 units of CH2
(CH2)4 = C4H8 OR you can just go down the list and calculate the molar mass of each of the answers.
a is 28, b is 42, 3 is 56, d is 70 and d is 84. Easy to pick out.

great! thank you!

so basically all that I had to do was 13/0.232? PV=nRT really had nothing to do with the problem then?

You used the ideal gas law PV=nRT to solve for the number of moles.

Mass= number of moles * molar mass

then you solved this for molar mass.