The mass of an evacuated 255 mL flask is 143.187 g . The mass of the flask filled with 356 mbar of an unknown gas at 25 ∘C is 143.348 g . Calculate the molar mass of the unknown gas.

Question

The mass of an evacuated 255 mL flask is 143.187 g . The mass of the flask filled with 356 mbar of an unknown gas at 25 ∘C is 143.348 g . Calculate the molar mass of the unknown gas.

Answer 1

Mass of unknown gas = 143.348 g - 143.187 g = 0.161 g

Molar mass of unknown gas = g / moles

We know the grams, so we need to find the moles, which we can do by using the ideal gas law

PV = nRT

P = pressure in atm = 356 mbar x 1 bar/1000 mbar x 0.987 atm/bar = 0.351atm

V = volume in L =255 ml x 1 L / 1000 ml = 0.255 L

n = moles = ?

R = gas constant = 0.0821 Latm/Kmol

T = temperature in Kelvin = 25C + 273 = 298K

Solving for moles (n), we have...

n = PV/RT = (0.351)(0.255) / (0.0821)(298)

n = 0.00366 moles

Molar mass = g / mols

Molar mass = 0.161 g / 0.00366 mols

Molar mass = 43.96 g/mol = 44.0 g/mol (could be CO₂)