dumas method for the determination of the molecular mass of a volatile liquid pl
ID: 833173 • Letter: D
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dumas method for the determination of the molecular mass of a volatile liquid
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Each liquid used in this experiment has an appreciable vapor pressure at room temperature. This means that at room temperature a significant portion of the sample is .iporized, so that when the condensed vapor is weighed, the flask contains sample plus air. The empty" flask contained only air. The amount of air present during the last weighing is less than the amount of air in the "empty" flask because some of the air has been displaced by the vapor. Would this cause the experimentally determined value of the gram molecular mass to be greater than, less than, or unchanged from the theoretical value? Explain your reasoning. A student uses the method of this experiment to determine the gram molecular mass of an unknown volatile liquid. Tell whether each of the following procedural errors would cause the experimentally determined gram molecular mass to be too large, too small, or unchanged relative to the theoretical value. Explain your reasoning in each case. The student uses tap water in the water bath, which leaves a mineral deposit on the outside of the sample flask. The student seals that flask as soon as he removes it from the water bath, so that air is not allowed to re-enter the flask before weighing. When a student performs this experiment, he notices that when he removes the flask with the sample from the boiling water bath, a liquid appears in the flask. What is the source of this liquid, and does its presence introduce an error into the experimental results? Explain.Explanation / Answer
The Dumas method of molecular weight determination was historically a procedure used to determine the molecular weight of an unknown substance. The Dumas method is appropriate to determine the molecular weights of volatile organic substances that are liquids at room temperature.
The procedure entailed placing a small quantity of the unknown substance into a tared vessel of known volume. The vessel is then heated in a boiling water bath; all the air within the flask would be expelled, replaced by the vapor of the unknown substance. When no remaining liquid can be observed, the vessel may be sealed (e.g. with a flame), dried, and weighed.By subtracting the tare of the vessel, the actual mass of the unknown vapor within the vessel can be calculated. Assuming the unknown compound obeyed the ideal gas equation, the number of moles of the unknown compound, n, can be determined by
PV = nRT
where the pressure, P, is the atmospheric pressure, V is the measured volume of the vessel, T is the absolute temperature of the boiling water bath, and R is the ideal gas constant.By dividing the mass in grams of the vapor within the vessel by the calculated number of moles, the molecular weight may be obtained.
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