Weight of flask and cover 128.7g Weight of flask with vapor 129.4g Pressure of v

Question

Weight of flask and cover 128.7g

Weight of flask with vapor 129.4g

Pressure of vapor, mm Hg 760

Pressure of, atm 1

Mass of water in flask 250g

Temperature of water in flask (boiling water) =92c

Temperature of vapor in K= 92C+273=365K

Temperature of water in the flask= 25t

the volume of vapor in the flask 250 ML

Using the volume of the flask (in liters) the temperature of the boiling-water bath (in kelvins), and the barometric pressure (in atmospheres), calculate the number of moles of vapor of flask is capable of containing. R=0.0821 L atm/mol K

PV=nRT

Solving for n

n=PV/RT

using the weight of unknown vapor contained in the flask, and the number of moles of vapor present calculates the molar mass of the unknown liquid

M= g/n

P= PRESSURE IN ATMOSPHERE

MM Hg

1 atmosphere

760 mm Hg

V= volume in liter

T= temperature

K= c+273

MM Hg

1 atmosphere

760 mm Hg

Explanation / Answer

Using,

moles of gas = PV/RT

with,

P = 1 atm

V = 0.250 L

R = gas constant

T = 365 K

we get,

moles of gas = 1 x 0.250/0.0821 x 365

                     = 0.008343 mol

molar mass of unknown liquid = mass/moles

mass = 129.4 - 128.7

         = 0.7 g

so,

molar mass of unknown liquid = 0.7 g/0.008343 mol

                                                 = 83.90 g/mol

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