Stoichiometry Lab Report

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Job's Method of Continuous Variation for Determination of Stoichiometry Purpose • Determine the stoichiometry of a chemical reaction experimentally • Determine the chemical formula of a precipitate • Determine the oxidation state of an ion in solution Introduction How can the stoichiometry of a chemical reaction or the formula of a compound be determined experimentally? It can be done using Job’s Method of Continuous Variation, which keeps the total number of moles of reactants constant throughout a series of mixtures and reactants, but varies the mole fraction of each reactant from mixture to mixture. Certain specific measurements are then taken for each of the mixtures. Since the maximum change will occur when the mole fraction of the reactants is closest to the actual…show more content…
Substituting in the expression for R, (1-x)/x, we find that the two lines intersect when R = (1-x)/x = {1 - [1/(1+k)]}/[1/(1+k)] = k. Because the amount of product increases as k is approached from either direction, the point of intersection of the lines occurs at the maximum amount of product obtainable. We have therefore shown that maximum product is obtained when R = k. This is what we set out to demonstrate. Example: A and B are known to react to form D, but the stoichiometry is uncertain. A Job's Method study yields the following data. Plot quantity of product versus moles A to determine the stoichiometry. moles A moles B grams product 0.2 1.8 2.5 0.3 1.7 3.75 0.4 1.6 5.0 0.6 1.4 4.38 0.8 1.2 3.75 1.0 1.0 3.12 The plot is shown in Figure 1. The value of k is clearly 4, because at the maximum, moles B/moles A = 1.6/0.4 = 4. Pre-Lab Questions Volume of 0.1 M Potassium Iodide (mL) Volume of 0.1 M Lead Nitrate (mL) Mass of Precipitate Formed (g) 1.0 19.0 0.12 2.0 18.0 0.23 3.0 17.0 0.36 4.0 16.0 0.46 5.0 15.0 0.58 6.0 14.0 0.70 7.0 13.0 0.81 8.0 12.0 0.92 9.0 11.0 1.04 10.0 10.0 1.15 11.0 9.0 1.30 12.0 8.0 1.38 13.0 7.0
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