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Unformatted text preview: Experiment #2: Decomposition of KClO 3 Objective: Determine the value of the gas constant, R, by measuring the decomposition of potassium chlorate, KClO 3 , using a liquid displacement method. Background: Most gases obey the ideal-gas equation, PV = nRT , quite well under ordinary conditions, that is, at room temperature and atmospheric pressure. Here P is pressure, V is volume, n is moles of gas and T is absolute temperature. The gas constant, R, relates these quantities and is now known to have a value of 0.08206 L.atm/mol.K. Small deviations from this law are observed, however, because real-gas molecules are finite in size and exhibit mutual attractive forces. The van der Waals equation, E 2 na E P + 2 (V nb ) = nRT eq. 1 V E where a and b are constants characteristic of a given gas, takes into account these two causes for deviation and is applicable over a much wider range of temperatures and pressures than the ideal- gas equation. The term nb in the expression (V -nb) is a correction for the finite volume of the molecules; the correction to the pressure by the term n 2 a/V 2 takes into account the inter-molecular attractions. In this experiment you will test the validity of the ideal gas law by determining the gas constant by the thermal decomposition of potassium chlorate, KClO 3 . The evolution of O 2 from the sample can be measured gravimetrically (mass difference) and by measuring the volume of evolved gas by displacement of water. A manganese(IV) oxide, MnO 2 , catalyst will be used to speed up the decomposition reaction. The overall reaction is given by: 2KClO 3 (s) 2KCl(s) + 3O 2 (g) eq. 2 Analysis of the results will be performed using both the ideal-gas law and the van der Waals equation. The experiment will be performed in triplicate to establish the precision of the measured value. You will work with mixtures of potassium chlorate and potassium chloride, KCl. From eq. 2 you can see that KCl(s) is a product of the reaction and, consequently, will not react. If a sample of the KClO 3 /KCl mixture is accurately weighed before and after the oxygen has been driven off, the mass of the evolved oxygen can be obtained by difference. From this moles of O 2 can be determined. The oxygen can be collected by displacing water from a bottle or closed flask, and the volume of gas can be determined from the volume of water displaced. The measurement of displaced water is performed in such a way that the initial and final temperature and pressure are identical. This way the ratio of moles of gas to volume of gas in the closed system must be constant. moles of gas to volume of gas in the closed system must be constant....
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- Fall '08