How would I go about doing G-I? A student observes that a 126 mLdry flask with a

Question

How would I go about doing G-I?

A student observes that a 126 mLdry flask with a foil cap weighs 50.162g. The student then adds 5mL of an unknown volatile liquid and heats the flask in a boilingwater bath at 99.2 C(degrees) until all liquid disappears. Theflask, foil, and unknown after heating that is dry and cool weigh50.610 g. The barometric pressure in the laboratory is 743.3 mm Hg;the laboratory temperature averages 22.3 C(degrees); the vaporpressure of the unknown liquid is 160 mmHg at 20 C(degrees), andthe density of dry air at STP is 1.2929 g/L.

g) Calculate the mass of liquid, vapor and air in theflask before you correct for air lost.

h) Calculated the mass of liquid, vapor and air in theflask after you correct for air lost.

i) Calculated the molecular weight of the unknownliquid.

Explanation / Answer

g) mass of liquid, vapor and air in the flask before lost air correction = final weight of flask - initial weight of flask

                                                                                                                            = 50.610 - 50.162 = 0.448g

h) mass of air lost = Volume displaced x density = 126/1000L x 1.2929g/L = 0.1629g

thus mass of liquid,vaporand air in the flask after correction = 0.448-0.1629 = 0.2851g

i) vapor pressure of unknown is 160 mm

molar mass of volatile liquids is calculated by measuring their vapor pressure using the relation

    PM = wRT/V where P = pressure of gas, M= molar mass, w= weight of substance, R= universal gas constant= 0.0821 Latm/mol.K and T = absolute temperature

subtituing P = 160/760 atm, w= 0.2851g R= 0.0821 T= 22.3+273 K, V = 0.126L

we get Molar mass = 260.57 g/mol

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