What is the molecular weight of a gas if a 15.0g sample has a pressure of 836 mm Hg at 25.0 degrees C in a 2.00 L flask?

What is the molecular weight of a gas if a 15.0g sample has a pressure of 836 mm Hg at 25.0 degrees C in a 2.00 L flask? a)167 b)1.35 c)176 d)11.1 e)none of the above Please explain... I need to understand how to do this. Thanks.I know that I have to use PV=nRT, then solve for n grams/molar massAnd, I know that:P=1.1 atm V=2.00 L n=? R= I think .0821 and T= ? I have no idea how to change 15.0g to moles, since I do not know what the element is to look at the atomic mass--I know that one mole is avagadro's number, but I'm still really not sure what to do here. Please help...