A0.286-g sample of gas occupies 125 ml at 60. cm of hg and 25° c. what is the molar mass of the gas?

Using the Equation: PV=nRT
Where P is the pressure 60 cmHg or 600 mmHg or 600/760= 0.789 atm
V is the volume 125 ml or 0.125 L, n is the number of moles, R is a constant 0.082057, and T is temperature 25 °C or 298 K; 
Therefore:
0.789 × 0.125 = n × 0.082057 × 298
 n = 0.0987/24.45 
    = 0.004036 mol
0.004036 mole has a mass of  0.286 g
Hence; 1 mole has a mass of 0.286/0.004036 
  = 70.8 g /mol
Therefore the molar mass of the gas is 71 g/mol (2 sfg)

     

The teacher is mostly talking about the distillation of a mixture