Limiting Reactant

Experiment (3) Limiting Reactant Objectives: To determine the limiting reactant in a salt mixture. To determine the percent composition of a salt mixture. Techniques: To effectively complete the experiment's objectives, review the following techniques: Technique 2d, 3 Transferring a liquid. Technique 5 Decanting a liquid from a solid. Technique 6 Flushing a precipitate from a beaker. Technique 7 Folding filter paper for a filter funnel. Technique 8 Filtering procedures. Technique 19b Weighing on a balance, ± 0.001 g. Introduction: Two factors that limit the yield of products in a chemical reaction are (1) the amounts of staring materials (reactants) and (2) the percent yield of the reaction. Many experimental conditions-for example, temperature and pressure-can be adjusted to increase the reaction's yield but because the chemicals react according to fixed mole ratio (stoichiometrically), only a limited amount of product forms from given amounts of starting materials. The reactant restricting the amount produced is called the limiting reactant in the chemical system. To better understand the concept of the limiting reactant, let's look at the reaction studied in this experiment. The equation's molecular form is: Since the reactant salts are soluble, soluble, the ionic equation is: is insoluble, and NaCl Presenting only the reacting ions in the reaction (removing spectator ions), we have: Two moles of phosphate ion from 2 moles of Na3PO4. 12H2O (F.W. = 380.2), or 760.4g reacts with 3 moles of barium ion from 3 moles of BaCl2. 2H2O (F.W. = 244.2), or 732.6 g, producing 1 mole of Ba3(PO4)2 precipitate (F.W. = 602.0), or 602.0g, if the reaction proceeds to completion. Suppose however only 700.0 g of Na3PO4.12H2O and 700.0 g of BaCl2.2H2O dissolve in water and are mixed. How many grams Ba3(PO4)2 are then produced? Since the equation reads in mole, not grams, the moles of each reactant are first determined: Equation 3.3: indicates that reacts with present, is enough . With present for all of the Again, using Equation 3.3 and the 3.2 mol ratio of to to react? . reacts with Since only with the of the , is the limiting reactant, reactant. Another words, all of the excess of added to the system reacts reacts, but not all the remains unreacted. the excess . An Once the limiting reactant is known, the theoretical yield of product is calculated from the balanced equation. Since is the limiting reactant, we determine the moles and grams of that are that are expected to precipitate: Produces . Produces . Produces Thus . form if the reaction proceeds to completion. In this experiment and unknown salt mixture of is added to water where they form insoluble . And 2HO . Weighing the initial salt mixture and the product's final weight, we determine the limiting reactant and the salt mixture's composition. Experimental Procedure: Two trials are recommended for this experiment. To hasten the analyses, weight duplicate unknown salt mixtures and simultaneously follow the procedure for each. Label the beakers accordingly for Trial 1 and 2 to avoid intermixing samples and solution. A- Precipitation of Ba3(PO4)2: 1- 2- Weigh on weighing paper (+0.001 g, Technique 19b) about 1.0g of an unknown Na3PO4. 12H2O-BaCl2.2H2O salt mixture (see your instructor). Transfer it to a 400-mL beaker and add 200mL (±0.2mL) of distilled (or de-ionized) water. Stir the mixture with a stirring rod for about 1 minute and then allow the precipitate to settle. Cover the beaker with a watch glass and maintain the solution hot (80-90oC) on a steam bath or with a low flame for 30 minutes (Figure 3.1). Allow the precipitate to settle. 3- Decant (Technique 5) two 50-mL (±0.2 mL) volumes of the supernatant liquid into separate 100mL beakers, labeled beaker I and Beaker II. Save for part B. Figure (3.1): Warming the precipitate. 4- 5- 6- Figure (3.2): Transfer of Ba3(PO4)2 precipitate to filter The precipitate may be gravity-filtered (Techniques 7 and 8a) or vacuum-filtered (Figure 3.2) using fine porosity filter paper, such as What man No.42. See your instructor before continuing. Weigh (+0.001g) a piece of filter paper and seal it into the filter funnel with a small amount of distilled water. Discard the water in the receiving flask. Have your instructor approve your filtering apparatus. While the solution is hot, quantitatively transfer the precipitate (Technique 6) to the filter funnel. Remove any precipitate on the beaker's walls with a rubber policeman and, with hot water, wash the precipitate on to the filter (Technique 6). Rinse the precipitate with two 5.mL portions of hot water. First air-dry the precipitate on filter paper, and then in a 100oC constant-temperature drying oven overnight or until the next laboratory period. Weigh (+0.001g) the precipitate and filter paper. B- Determination of Limiting Reactant: From the following two tests (Figure 3.3) we determine the limiting reactant in the original salt mixture. Some cloudiness may appear in both tests but one shows more definite precipitate formation. 1- Test for excess . Add 2 drops of 0.5M (0.5m BaCl2) to the 50 mL of supernatant liquid in Beaker!. If a precipitate forms, then excess and is the limiting reactant. Figure (3.3) is an Data and Results: Trail 1 Trail 2 A- Precipitation of Ba3(PO4)2: Unknown number …………..………… 1- Mass of salt mixture (g) ………… ………… 2- Mass of filter paper (g) ………… ………… 3- Mass of filter paper and Ba3(PO4)2 ………… ………… 4- Mass of Ba3(PO4)2 (g) ………… ………… 1- Limiting reactant in salt mixture ………… ………… 2- Excess reactant in salt mixture ………… ………… 1- Moles Ba3(PO4)2 Precipitated: M.Wt. = 602.2 g/mol ………… ………… 2- Moles BaCl2.2H2O reacted ………… ………… 3- Mass BaCl2.2H2O reacted (g): M.Wt. = 244.2 g/mol ………… ………… 4- Moles Na3PO4.12H2O reacted (g): M.Wt. = 380.2 g/mol ………… ………… 5- Mass Na3PO4.12H2O reacted (g): M.Wt. = 380.2 g/mol ………… ………… 6- Mass of salt mixture A.1 (g) ………… ………… 7- Mass of excess reactant, B.2 (g)#6-#3or#5 ………… ………… 8- Percent BaCl2.2H2O in mixture ………… ………… 9- Percent Na3PO4.12H2O in mixture ………… ………… B- Determination of Limiting Reactant: Calculation: * Show your calculations for one trial on a separate sheet.