Determine the molecular weight of a compound?

Question

Determine the molecular weight of a compound which, when vaporized, occupies a volume of 142 mL at 100 degrees C and a pressure of 768 mmHg. The condensed vapor weighs 0.532 g.

I need help.

I got this far:

P= 768 mmHg / 760 = 1.01

V= 142 mL = .142 L

n=

R= .08205 L*atm/ mole K

T= 100 + 273= 373 Kelvin

I think the formula I should use is:

MW = massRT/PV

I'm not too sure where to go from here??!

Answer 1

'n' in the formula is the number of moles.

PV = nRT rearranges to:

n = PV/RT

n = (1.01 x 0.142) / (0.08205 x 373) = 0.00469 moles

You know that your vapor weighs 0.532 g and that you have 0.00469 moles.

mw of gas = mass/ no. of moles = 0.532 / 0.00469 = 113.5g/mole