Question

To determine the molar mass of a volatile liquid, we used the Dumas method with a test tube, tin foil cover, and hot water bath to convert the liquid to a gas and then measure its pressure, temperature, amount, and volume.

a) Why must you leave the test tube in the hot water for several minutes before taking a reading and removing it? What is happening during this time?

b) How was the experiment designed to ensure the pressure of gas in the test tube was the same as the atmospheric pressure? Explain.

Answer 1

In Dumas method ,a volatile liquid is placed in a glass bulb of known volume and the bulb is placed in a boiling water bath until the volatile liquid vaporises completely.Once it is vaporised ,the excess of volatilized gas pushes out of the bulb through the opening on it, leaving behind only substantial volume of gas that fills the bulb.During this time,the pressure of the gas inside the bulb equals the atmospheric pressure (as liquid boils at atmospheric pressure), so barometric pressure is measured.This barometric pressure,P, gives the value of the gas inside the bulb,and also temperature,T, is noted.Later,the volume of the bulb,V, is determined by filling it with water,.And also the mass of the vapour left inside the bulb is weighed by sealing the bulb immediately after the excess gas leaves the bulb.

Ideal gas law,PV=nRT=(w/M)RT is used to calculate the molecular weight of the volatile liquid.

w=mass of gas inside the sealed bulb

M=molecular weight

R=universal gas constant

a)The test tube is left in hot water for several minutes so that all of the liquid volatilizes and completely.

b)Once the liquid is vaporised ,the excess of volatilized gas pushes out of the bulb through the opening on it, leaving behind only substantial volume of gas that fills the bulb.During this time,the pressure of the gas inside the bulb equals the atmospheric pressure (as liquid boils at atmospheric pressure), so barometric pressure is measured.This barometric pressure,P, gives the value of the gas inside the bulb