(Easy 10 points) Help calculating molar mass

This is from a practice sheet for a test next week. He said there will be similar problems on it. I keep getting stuck and get none of the answers. Can someone show me what it is and the work so I can understand?


A sealed flask contains 5.6 L of a gas at 3.5 atm and 31.1°C. If the density of the gas is 11.4 g/L, what is its molar mass? (You can assume that the gas behaves ideally.)

a. 15 g/mol
b. 1.3 g/mol
c. 81 g/mol
d. 145 g/mol
e. 455 g/mol


Many thanks. 10 points to best answer!

The ideal gas law is PV = nRT
P = pressure
V = volume available to the gas
n = number of moles of gas
R = ideal gas constant
T = temperature (in Kelvin--it NEEDS to be in Kelvin!)

You know:
V = 5.6 L
P = 3.5 atm
T = 31.1 °C = 304.25 K

You know the density of the gas is 11.4 g/L

Molar mass = g/mol

So, we need the number of moles, and the mass of the gas.

To get the mass of the gas, we use the density and the volume of the gas!

11.4 g/L * 5.6 L = 63.84 g

Now, we need the number of moles (n). To get this, we solve for n in the ideal gas equation.

n = PV/(RT)
n = (3.5 atm) * (5.6 L) / ((0.082 L*atm*K^-1*mol^-1)*(304.25 K)
n = 0.79 mol

Then, we simply divide the mass of the gas by the number of moles of gas...
63.84 g / 0.79 mol = 80.81 g/mol

Hope this helps!

e. 453 g/mole
Find the original mass first. 11.49 g/l x 22.4 = 257g/mole = M2

P1V1 / T1M1 = P2V2/ T2V2
Solve for M1
P1=3.5 V1=5.6L T1=257 M1=??
P2=1 V2=22..4L T2=273 M2=257g
Try again, and good luck