The Dumas method was used to determine the molecular weight of an unknown liquid. The vapor occupied a 525mL flask at a temp of 114 degrees celsius & 850mm Hg pressure...

Question

The empty flask weighed 135.111g and when filled with the condensed vapor weighed 137.923g. Calculate the molecular weight of the unknown liquid?

Answer 1

Mass of unknown = mass of flask filled with vapor - mass of empty flask = 137.923 g - 135.111 g = 2.812 g

To get molecular weight, we now need the number of moles present. To find this, we solve for n (moles) in the ideal gas law: PV = nRT

n = PV/RT

P = pressure = 850 mm Hg x 1 atm/760 mm = 1.12 atm

V = volume = 525 ml = 0.525 L

R = gas constant = 0.0821 Latm/Kmol

T = temp = 114 + 273 = 387K

n = (1.12)(0.525) / (0.0821)(387)

n = 0.0185 moles

molecular weight = 2.812 g / 0.0185 moles = 152 g/mol