An unknown compound was analyzed twice in the lab, and the following information

Question

An unknown compound was analyzed twice in the lab, and the following information was collected:

Mass of empty 250 mL flask, stopper and cork ring: 159.535 g

The density of water at 21.0 C is: 0.998734 g/mL

Mass of 250 mL flask, stopper, cork ring and water (filled) at 21.0 C: 440.250

Trial # 1 Trial # 2

The temperature of boiling water: 99.8 C 99.6 C

Atmospheric pressure: 996 mbar 993 mbar

Mass of flask after heating: 160.256 g 160.284 g

Calculate the molar mass for each trial, the average molar mass, and the parts per thousand for the experiment.

Explanation / Answer

trial # 1)

Molar mass determination of volatile liquid:

To determine the molar mass (M)of a volatile liquid,the liquid is evaporated in a flask of measured volume(V) being submerged in boiling water,so that the temperature(T) of water equals that of gas in the flask.After the liquid condenses,the mass of the flask+liquid is measured and substracted from the mass of flask to calculate the mass of liquid(m).Barometric pressure (P)is also noted and all the values plugged in the ideal gas equation,PV=nRT=m/M RT

or,PV=(RTm/M)

given:

P=Atmospheric pressure=996mbar

T=21+273=294K

R=8.314*10^-2 Lbar K^-1mol^-1

mass ofwater=(Mass of 250 mL flask, stopper, cork ring and water (filled))-(Mass of empty 250 mL flask, stopper

and cork ring)=440.250-159.535=280.715g

So volume of water=mass/density=280.715g/(0.998734 g/mL)=281.071 ml

volumeof flask=281.071 ml(=vol of water used to estimate volume)

mass of flask +condensed liquid=160.256g

mass of condensed liquid=(mass of flask +condensed liquid)-(Mass of empty 250 mL flask, stopper
and cork ring)=160.256g-159.535g=0.721 g

m=0.721g

PV=(RTm/M)

M=RTm/PV=(8.314*10^-2 Lbar K^-1mol^-1)(294K)(0.721 g)/(996*10^-3 bar)(281.071 ml *1L/1000ml)=62.953 g/mol

M=62.953 g/mol=molar mass of liquid

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