Lab manual_3 - UOIT Chem. 1800U, W10; Exp. 3 - 7.9 3....

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UOIT Chem. 1800U, W10; Exp. 3 - 7.9 64 3. MEASUREMENT OF THE IDEAL GAS CONSTANT, ‘R’ Objectives 1) To apply Dalton’s Law of Partial Pressures to a mixture of gases (water vapour and hydrogen); 2) To measure the ideal gas constant and compare the measured value with the accepted value (8.314 L kPa mol -1 K -1 . Introduction The behaviour of gases as the pressure, volume, temperature and amount of gas (i.e., the number of moles of gas) are changed was one of the earliest things to be studied systematically in science. Over 300 years ago Boyle observed that (to a very good approximation) the volume of a gas was inversely proportional to the pressure of the gas (at a constant temperature): (3.1) V P 1 Charles expanded (no pun intended!) on this and reported that at constant pressure the volume of a gas was directly proportional to its temperature: (3.2) VT Charles’s results showed that when the V vs. T data was extrapolated to a volume of 0 L the corresponding temperature was ~ -273 0 C. This temperature was then defined as “absolute” zero and is the basis of the Kelvin scale for temperature. At roughly the same time Gay-Lussac showed (not surprisingly!) that pressure of a gas was proportional to temperature: (3.3) P T It is also fairly clear that the volume of a gas depends on how much of it (the number of moles) is present:
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UOIT Chem. 1800U, W10; Exp. 3 - 7.9 65 (3.4) Vn (This was first stated in a slightly different way by Avogadro). These four laws can be combined into a single statement: (3.5) V nT P (3.6) P T and so: (3.7) P Vb n T = where ‘b’ is a proportionality constant. The proportionality constant is more commonly called ‘R’, the gas constant and this equation is written in its more recognizable form as: (3.8) PV nRT = which is known as the ideal gas equation. It should be noted this is an “ideal” case. For real gases this equation is an approximation and better models are used for more advanced applications of physical chemistry. However, it is a very good approximation for many gases (including hydrogen) at moderate pressures. The failures of the ideal gas law will be examined in higher level physical chemistry courses. Dalton’s Law of Partial Pressures is also very important in the study of gases. This law states that each gas in a mixture exerts a pressure proportional to its concentration. The total pressure is the sum of the individual “partial” pressures: (3.9) PP P P tot =++ + 123 .... (3.10) =+++ nRT V V V ... (3.11) =+ + + ( ...) nnn RT V (3.12) = nR T V tot
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UOIT Chem. 1800U, W10; Exp. 3 - 7.9 66 In this experiment you will react an accurately known mass of magnesium (measured on the analytical balance) with an excess of hydrochloric acid according to: Mg(s) + 2HCl(aq) ± MgCl 2 (aq) + H 2 (g) (3.13) From the stoichiometry of the reaction the number of moles of hydrogen gas can be calculated (assuming all the magnesium reacts). The volume of gas will be measured using a “gas burette”.
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This note was uploaded on 11/11/2010 for the course CHEMISTRY CHEM1800 taught by Professor Krista during the Winter '10 term at UOIT.

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Lab manual_3 - UOIT Chem. 1800U, W10; Exp. 3 - 7.9 3....

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