chemistry
posted by Lucy on .
If 0.385 g of a gas occupies a volume of 200mL at minus 73 degrees C and 750 torr, what is the molecular mass of the gas?

PV = nRT
then n = g/molar mass. Post your work if you get stuck. 
So far I have: (0.385g)(8.31dm3 times kPa)(200K)(1000cm3)/(7.40 kPa)(mol times K)(200 cm3)(1Dm3) which =639870 g/1480 mol which =432.345 and that is not even close to one of the answers. I don't know where I am going wrong.

I think your units are messed up.
PV = nRT
It's easier to use atmospheres than kPa. R is 0.08205 L*atm/mol*K when atm is used for pressure, P. V must be in liters. I don't know what grams is doing in PV = nRT
P = 750 torr/760 = ?? atm
V = 200 mL = 0.200 L
n = solve for this
R = 0.08205
T in K = 273 + C = 27373 = 200 K
So
(750/760)atm x 0.200 L = n x 0.08205 L*atm/mol*K x 200 K.
Solve for n, THEN,
n = # mols = grams/molar mass.
You have mols and grams, solve for molar mass. I hope this helps.