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ChemLab113-0039 - EXPERIMENT 3 Analysis of the Soda Ash...

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Unformatted text preview: EXPERIMENT 3 Analysis of the Soda Ash Unknown Introduction You will finish the Block 1 experiments this period by measuring the mass percent Na2C03 in your soda ash sample through titration with the knowu molarity HCl solution that you measured in Experiment 2. The average molarity you obtained is used for that purpose. When you come to the laboratory this period you will find your soda ash unlmown in a vial on your lab bench. Empty the pure Na2C03 from your weighing bottle into the appropriate disposal container. Rinse the weighing bottle well with distilled water, dry it thoroughly with a Kimwipe, and finish the drying with ten minutes of heating in an oven. Then half fill the weighing bottle with the soda ash unknowu and keep both the weighing bottle and the vial of remaining soda ash in your desicooler when you are not using them. Directions for Analyzing the Soda Ash Unknown The soda ash is titrated in very nearly the same way as the pure sodium carbonate you titrated last period. Begin by preparing your buret and by using the red solution to adjust the pH meter to read 4.00. As in Experiment 2, make that adjustment only at the beginning of the period. Before the precise titrations can be carried out it is necessary to perform a rough titration to ascertain the ideal range of soda ash sample masses to use. That range varies considerably from one unknowri to another. For the rough titration weigh out 0.4—0.5 g of soda ash by difference from your weighing bottle into the 150 mL beaker with stirrer that you use for titrations. Record the mass in your notebook. Dissolve the soda ash sample in 50 mL of distilled water measured out with a graduated cylinder and note the pH reading on the meter. If it is within the range 10.60-11.20, you can proceed with the titration. If it is outside that range, have your teaching assistant replace the red solution, and then repeat the calibration before going on to the titration. Begin the titration exactly as you did in Experiment 2. However, continue the rapid dropwise addition until the pH reaches 4.00 or slightly below, instead of stopping at 4.70 as in Experiment 2. Take a final buret reading at that point and record it. Then go to the computer and follow the directions on the screen. The computer will use your soda ash mass and initial and final buret readings to specify the ideal mass range for your unknown, and it Will give you a printout showing that range. Once the ideal mass range is known use it in three titrations carried out exactly as in Experiment 2. You will also work with the computer exactly as you did in that experiment. However, in this case the computer will calculate a Na2C03 mass percent for each titration instead of an HCl molarity. In addition it will calculate a different measure of the scatter, parts per thousand, from that calculated in Experiment 2. When the computer indicates whether your results are acceptable or unacceptable, hand in your data and printout in the former case 01' 55 ...
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