A sample of a volatile liquid is vaporized completely in a 240 cm3 flask at 135°C and 1200 mm Hg...?

A sample of a volatile liquid is vaporized completely in a 240 cm3 flask at 135°C and 1200 mm Hg. The condensed vapor (liquid) weighs 0.837 grams. What is the molecular weight? The liquid contains 64.9% carbon, 13.5% hydrogen, and 21.7% oxygen. What is the molecular formula?

Answer:
Calculate number of moles with formula pV=nRT

n= pV/RT

p= 133.2*1200 Pa V= 240*10^-6 m^3
R =8.31 J/K T= 135+273=408= 0.01132mole
this corresponds to 0.837g
so molecular weight is 0.837/0.01132=74g
mass of carbon 74*0.649=48g so C4
mass of hydrogen=74*0.135=10 so H10
mass of oxygen 0.217*74 =16 so O

formula C4H10O (alcool butyric)

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