A student observes that a 126 mL dry flask with a foil cap weighs 50.162g. The s
ID: 906344 • Letter: A
Question
A student observes that a 126 mL dry flask with a foil cap weighs 50.162g. The student then adds 5 mL of an unknown volatile liquid and heats the flask in a boiling water bath at 99.2C(degrees) until all liquid disappears. The flask, foil, and unknown after heating that is dry and cool weigh 50.610 g. The barometric pressure in the laboratory is 743.3 mm Hg; the laboratory temperature averages 22.3 C(degrees); the vapor pressureof the unknown liquid is 160 mmHg at 20 C(degrees), and the densityof dry air at STP is 1.2929 g/L.
a) What are the experimental conditions of T and P that willbe used to calculated the mass of air in the flask?
b) Calculate the density of air at the experimental conditions identified in part a.
c) Calculate the mass of air in the flask before the liquidwas added.
d) Calculate the partial pressure of air in the flask after the liquid was heated and then cooled.
e) Calculate the mass of air in the flask after the liquid was heated and then cooled.
f) Calculate the mass of air lost.
g) Calculate the mass of liquid, vapor and air in the flask before you correct for air lost.
h) Calculated the mass of liquid, vapor and air in the flask after you correct for air lost.
i) Calculated the molecular weight of the unknown liquid.
I understand the answer for a is 743.3 and 22.3. I've also seen that this question has been answered on chegg before but I don't understand how b is done. More specifically:
Where do you obtain 28.97 g/ml???
Explanation / Answer
28.97 g/mol is the average molar mass of air.
Air consists of 79 % nitrogen and 21 % oxygen opproximately. percentage of remaining can be neglected.
Molar mass of nitrogen = 28.02 g/mol
molar mass of oxygen = 32.00 g/mol
fraction of nitrogen in air is 0.79 and fraction of oxygen is 0.21
Molar mass of air = 0.79 *28.02g/mol + 0.21*32.00g/mol
= 28.86 g/mol
Average molar mass of gas = 28.97 g/mol
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