A student observes that a 126 ml, dry flack with a foil exp weight 50.162 g. The

Question

A student observes that a 126 ml, dry flack with a foil exp weight 50.162 g. The student then adds 5 ml, of an unknown volatile liquid and heats the flask in a boiling water bath at 99.2 degree C until all liquid disappears. The flask, foil and unknown after heating that is dry and cool weight 50, 610 g. The barometric pressure in the laboratory is 743.3 mm Hg; the laboratory temperature average 22.3 degree C; the vapor pressure of the unknown liquid is 160 mmHg at 20 degree C, and the density of dry air at STP is 2929 g/L. What are experimental conditions of T and P that will he used to calculate the mass of air in the flask? Calculate the density of air at the experimental conditions identified in part a. Calculate the mass of air in the flask before the liquid was added. Calculate the partial pressure of air in the flask after the liquid was heated then cooled. Calculate the mass of air in the flask after the liquid was heated and then cooled. Calculate the mass of air lost. Calculate the mass of liquid, vapor and air in the flask before you correct for air lost. Calculate the mass of liquid, vapor and air in the flask after you correct for air lost. Calculate the molecular weight of the unknown liquid.

Explanation / Answer

a)

experimental conditions of T and P

T = 22.3°C

P = 743.3 mm Hg

b)

D of air = P*MW/(RT)

MW = 29 g/mol

R = 62.4 mm Hg L /molK

T = 22.3+273

Pgas = Ptotal - Pvapor = 743.3-160 = 583.3 mm Hg

D = 583.3* 29 / (62.4 *295.3) = 0.9179 g/L

c)

mass of air in flast...

V = 126 mL

so..

V = 0.126 L

mass = D*V = 0.9179*0.126 = 0.11565 g of air

d)

partial pressure of air after heating and cooling

Pgas = Ptotal - Pgas = 743.3-160 = 583.3 mm Hg

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