Posted by **Lucy** on Sunday, November 4, 2007 at 3:47pm.

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?

- chemistry -
**DrBob222**, Sunday, November 4, 2007 at 3:51pm
PV = nRT

then n = g/molar mass. Post your work if you get stuck.

- chemistry -
**Lucy**, Sunday, November 4, 2007 at 5:13pm
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.

- chemistry -
**DrBob222**, Sunday, November 4, 2007 at 5:56pm
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 = 273-73 = 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.

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