A flask with a mass of 147.21 g is filled with a sample of volatile liquid and i

Question

A flask with a mass of 147.21 g is filled with a sample of volatile liquid and is heated to 98 degree C. Upon cooling the new mass of the flask with the recondensed liquid is found to be 147.68 g. The volume of the flask is determined to be 135.6 mL and the atmospheric pressure is 743.5 torr. Calculate the molar mass of the volatile liquid. If there was an error in the pressure measurement in the Problem #1 above, such that the atmospheric pressure was in reality lower than that measured, how would this affect the value determined for the molar mass?

Explanation / Answer

Calculate number of moles of the volatile liquid present at 98oC using ideal gas equation .

PV = nRT

P = 743.5 torr = 0.978 atm

V = 135.6 mL = 0.1356 L

R = 0.082 L atm/K/mol

T = 98oC = 98oC = 371 K

number of moles (n) = PV/RT = 0.978 atm * 0.1356 L/0.082 L atm/K/mol * 371 K = 4.36*10^-3 moles

Mass of the liquid = 147.68gm -147.21g = 0.47 gm

Molar mass = Mass /moles = 0.47gm/4.36*10^-3 moles = 107.8gm/mol

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