What is the molecular weight of the gas? HELP!?

A 7.21 gram sample of gas occupies 2.56 liters at standard conditions. What is the molecular weight of the gas?

Answer:
2.56 liters / 22.4 liters per mole at STP = 0.1142857 moles

7.21 grams / 0.1142857 moles = 63.087507 grams per mole

So the molecular weight = [ 63.09 amu ]
PV = nRT. You know how much volume a given mass of gas occupies at a given pressure and temperature. You then know how many moles you have, hence the molecular weight of the gas.
PV=nRT=mRT/M

M=mRT/PV

do the math.
1 mole of gas occupies 22.4L under STP.
7.21g/2.56L = 2.81g/L
since 1mol = 22.4L of gas
2.81g/L= 2.81g/(1/22.4)mol
=62.944g/mol

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