Can someone please help me with this problem? a gas was found to have a density of 1.13 g L-1 at 22.0 C and a pressure of 755 torr. What is the molecular mass?

Find the amount of molecules based on temperature, pressure (and volume).

(Mister Boyle,Charles,Gay-Lussac and Avogadro will help you with that)

 

Then its easy to find the end result.

 

P.S. nice site here to convert the units (if needed):

http://www.chemie.fu-berlin.de/chemistry/general/units.html

since theres no correction constant I assume ideal gas law is good enough (e.g. van der walls is not needed):

 

M = dRT/P

m = molecular mass

d = density

R = gas constant (.08206 (l*atm)/(moles*kelvin)

t = temperature in K

 

Derived from:

 

P=nRT/V

n = m / M (mass divided by Molar mass)

P = mRT/VM and m/v = density

P = dRT/M

MP = dRT

M= dRT/P

Yep MW = d x R x T / P is correct. Since no temp was given i would use 273.15K <or> 295K