A student does this experiment and gets the following data. The mass of the empt

Question

A student does this experiment and gets the following data. The mass of the empty flask with the dry foil cap was 55.041 g. The sample was heated in water at 99.5 degree C then allowed to condense. The flask and condensed vapor weighed 56.039 g. To determine the volume of the flask, it was filled with water giving a total mass of 270.9 g. The barometric pressure in the lab was 752 mm of Hg. Use this data to find the molar mass of the sample unknown. Show all steps and unit conversions involved. Use 0.9982 g/mL for water density.

Explanation / Answer

Mass of empty flask with dry oil cap = W1 = 55.041 g

Mass of flask with dry oil cap + Condensed sample vapour = W2 = 56.039 g

Mass of Condensed sample vapour, W3 = W2 -W1 = 56.039 - 55.041 = 0.998 g

Mass of water + Flask =W4 = 270.9 g

Mass of water = W5 = W4 - W1 = 270.9 - 55.041 = 215.859 g

Volume of flask = Volume of water = Mass of water / Density of water = 215.859 g / 0.9982 g/mL

= 216.248 mL

Now PV =nRT

P = 752 mm = 752 /760 = 0.989 atm

V = 216.248 mL = 0.216 L

R = 0.082 L atm mol-1 K -1

T = 99.5 deg C = 372.5 K

or, n = PV / RT = ( 0.989 atm X 0.216 L) / ( 0.082 L atm mol-1 K -1 / 372.5 K)

= 0.007 mol

Molar mass = Mass / No of mole = 0.998 gm / 0.007 mol = 142.57 g/mol

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