A student is doing the lab studying Dumas Method of determination of the Molar M

Question

A student is doing the lab studying Dumas Method of determination of the Molar Mass. The results are summarized as follows: Mass of the flask, boiling stones, foil crop, and unknown liquid after cooling = 130.50 g Mass of the flask, boiling stones, and foil cop = 129.65 g Water bath temperature = 100 degree C Barometric pressure = 1.03 atm Volume of the flask = 300 mL a) Calculate the density of the unknown liquid. b) Calculate the Molar Mass of the unknown liquid c) The unknown volatile liquid is Cyclohexane. Calculate the percent error in your determination. e) Explain briefly the principles of the Dumas Method. Based on the results of your calculations does this method work appropriate for determination of the Molar Mass.

Explanation / Answer

Given

Volume = 300 ml = 0.3 L

Pressure = 1.03 atm

Temperature = 100 C = 373 K

R = 0.0821 L.atm/mol.K

ideal gas law

No. of moles n = PV / RT = 1.03 atm * 0..3 L / 0.0821 L.atm/mol.K * 373 K = 0.01009 moles

Mass of flask with substance = 130.5 g

Mass of flask without substance = 129.65 g

Mass of substance = 130.5 g - 129.65 g = 0.85 g

Molar mass = Mass of substance / No. of moles = 0.85 /0.01009 = 84.24 g/mol Answer (b)

Given molar mass of Cyclohexane = 84.2 g/mol

if liquid present is cyclohexane then

% error in molar mass = (84.24-84.2 / 84.2) * 100 = 0.0475 % Answer(C)

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