Chemistry ideal gas law

Question

A has sample with a mass of 0.250 g is collected at 150.0 °C and 720. mm Hg. The volume is 85.0 mL. What is the molar mass of the gas? Could you identify this gas simply using a periodic table? Why or why not?

Answer 1

A useful equation to remember in order to determine the molar mass of a gas using the ideal gas law equation is:

MW = gRT / PV

where MW = molar mass of the gas in units of g/mol

g = mass of the gas = 0.250 g

R = gas constant = 0.0821 L atm mol^-1 K^-1

T = Kelvin temperature = 150.0 + 273.15 = 423.15 K

P = pressure of the gas in atm = 720. mm Hg x (1 atm / 760 mm Hg) = 0.947 atm

V = volume of the gas in Liters = 85.0 mL x (1 L / 1000 mL) = 0.0850 L

Now plug in the numbers and solve for MW

MW = (0.250 g x 0.0821 L atm mol^-1 K^-1 x 423.15 K) / (0.947 atm x 0.0850 L) = 108 g/mol

The gas cannot be identified as the calculated molar mass does not match the molar mass of any elemental gas from the periodic table. As a result, more information is needed in order to deduce the identity of the unknown gas.