Molecular Weight By Dumas Method

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Molecular Weight by Dumas Method In your text (Chang, 10th Ed) : 5.4 The Ideal Gas Equation, especially the Density and Molar Mass of a Gaseous Substance subsections Purpose: The experimental determination of the molecular weight of a volatile liquid sample and the density of its vapor are used to illustrate the uses of the Ideal Gas Law. Background: The Ideal Gas Law is: PV = nRT (Eqn. 1) where P = pressure in atm, V = volume in L, T = temperature in K, and R = 0.082056 L atm/(mol K), the Gas Constant It describes, with good precision, the behavior of many real gases over a wide range of pressures and temperatures. Even when there is deviation from ideality, it is useful as a starting point in physical analysis Recalling that density is mass/volume, Eqn. 1 can be re-written to derive the equation for the density of a gas, assuming ideal behavior: n = P Now multiply both sides by MW: n(MW) = P (MW) V RT V RT n(MW) is # moles x grams/mol = grams = mass, m Therefore: m = (P)(MW) = d , density (mass/volume, in grams/ liter ) (Eqn. 2) V RT where m = mass , in grams and MW = molecular weight, in grams/mole Therefore, at fixed P & T, the measurement of the mass of a vapor in a known volume will give the density of the vapor under those conditions. Since the MW and R are constant for a given gas, density measured at arbitrary experimental T, P conditions can be scaled to any desired T,P (usually STP, 273 K, 1 atm) by the following ratio: d1 T1 = d2 T2 (Eqn. 3) P1 P2 If P and T are also measured, the molecular weight of the substance can be determined by another easy algebraic re-arrangement of Eqn. 2: MW = m R T (Eqn. 4) PV Overview of Procedure: In this experiment, a small amount of a low boiling

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