Chem 1000 Exp#3-2020 PDF

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1 EXPERIMENT 3 Chemical Analysis by Gas Evolution Read before laboratory: Textbook, Chapter 5, sections 5.2-5.7 Student Laboratory Handbook

Principles Chemical analysis is the determination of the composition of a material. In this lab, the amount of active ingredient in two common antacids will be determined. The method of choice in this lab is that of gas evolution. As seen in numerous television ads, dissolution of one of these antacid tablets produces bubbling in the test solution. You will collect the gas evolved by this bubbling action, from which the amount of active ingredient in the original tablet can be determined. Gas evolution is a very convenient method for chemical analysis. Gases are easily collected from the reaction mixture, since the gas bubbles out of the solution. Additionally, the volume of gas is easily related to the number of moles of gas by the ideal gas law. A convenient method of gas collection is with the use of a U-tube containing a liquid such as water. There are two important points to remember when working with this type of apparatus. First, the liquid over which the gas is being collected will most likely have some vapour pressure caused by the evaporation of the liquid itself. Therefore, in order to know the pressure of just the gas you are intending to collect, you must subtract the vapour pressure of the liquid from the total pressure. For this experiment, the vapour pressure of water at various temperatures is well known and can be found in the experiment 3 folder on Blackboard. Secondly, in order to know that the pressure in the tube is the same as the atmospheric pressure, the tube must be balanced, i.e. there must be equal levels of water in the two arms of the tube. Before making each volume measurement, ensure that this is the case. The presence of calibration errors, small leaks, and the fact we are using the ideal gas law for a real gas, the apparatus will not measure the “correct” amount of gas. Thus, the apparatus must be calibrated before being used to measure an unknown sample. This is done by measuring a pure substance, one that evolves a known amount of gas. The ratio between the known amount that should have been measured and the actual amount measured on the particular apparatus can be considered to be a percent yield. That is the actual volume of gas captured divided by the theoretical volume that would have been captured presuming no loses or other errors. If we assume that this percent yield stems only from reproducible or systematic errors, we can use the percent yield calculated with a standard sample to correct the volume of gas collected from an unknown sample.

Antacids The acid contained in the stomach is mainly hydrochloric acid, HCl. In aqueous solution, this acid dissociates into H+ (actually H3O+) and Cl-. The H+ ion is the active acid in solution. Antacids were developed to counteract the effect of abnormal acid secretion in the stomach, by raising the pH through a chemical reaction that uses up the H+ ions. Most of them are based on a carbonate, CO32or bicarbonate, HCO3-,salt. These salts are basic, and react with H+ as follows:

2 H+ (aq) + CO3 2-(aq) → CO2 (g) + H2O (l)

(1)

2 +

H (aq) +

HCO3-

(aq) → CO2 (g) + H2O (l)

(2)

An antacid tablet is not 100% active ingredient, however. They all contain binders to hold the materials together in a tablet form, some contain an analgesic (such as aspirin) to relieve pain, many contain sugar to make the medicine more palatable and other fillers. Gas evolution analysis can be used to measure the amount of active ingredient in an antacid tablet by measuring the amount of CO2 evolved when the tablet is reacted with acid (simulating the reaction of the antacid with your stomach juices). It is important to note that none of the other ingredients in these particular tablets will react with HCl solution to produce a gas. Thus, all the gas that is evolved is CO2, and all the CO2 comes from the active ingredient: carbonate or bicarbonate.

Experimental Procedure There are three sections to this lab. In Part A, you will measure the amount of gas evolved from a Tums antacid sample. In Part B, you will determine the percentage yield of your apparatus using a known standard; using this information, you can then determine the amount of active ingredient in the Tums antacid you used in Part A. In Part C, you will determine the amount of active ingredient in a second type of antacid, to compare with that of Tums. You perform the calibration experiment second because it’s a much more vigorous reaction, which shouldn’t be attempted without an easier run to test the apparatus set-up.

Part A: Analysis of TumsR Assemble the apparatus as instructed. The Mohr burette is mounted vertically on a burette clamp. From the bottom, a long hose is attached that leads to a levelling reservoir. This reservoir should be clamped near the top of the stand. The top of the burette is connected to a test tube, in which the reaction will take place. Attach your thermometer to the retort stand using a small clamp so that you have a measure of the temperature in the lab near your apparatus. This temperature may vary throughout the lab period, so be sure to take a reading during each part of the experiment. You will also need to record the air pressure during each part of the experiment using the barometer. Weigh a dry large test tube in a 125 mL Erlenmeyer flask to the nearest 0.0001 g (the flask is used to ensure the test tube stays upright while weighing). Weigh approximately 0.400 g of the powdered TumsR onto a piece of weighing paper. Then carefully roll it up (with the powder in the middle) and while holding the test tube on an angle, insert the paper with the sample into the test tube such that the entire paper is in the tube except about 1 cm at the top. Hold the test tube up-right and allow the sample to fall into the test tube and remove the weighing paper. This will ensure that the antacid will not stick to the sides of the test tube, but will all sit at the bottom. Weigh the tube, with the antacid, and Erlenmeyer flask again (to the nearest 0.0001g) and the subtraction of these two masses will give you an accurate mass of antacid in the tube. Add approximately 2.0 mL of 3.0 M hydrochloric acid to the small test tube (about ½ full) using the dropper provided. Dry the outside of the test tube thoroughly, as any liquid on the outside of the tube will adversely impact the experiment. Place the small tube containing the acid into the large tube containing the antacid, ensuring none of the acid spills out of the small tube. Clamp this tube to the other side of the burette clamp.

3 Fill the reservoir with room temperature deionized water, provided in a labelled carboy, to just below the fill-line then attach the reservoir to the tubing at the bottom of the burette. Close the lid and squeeze the reservoir to fill the burette with water until the level reads approximately 6 mL. Remove and set aside the lid, as the system needs to remain open for the rest of this procedure. Inspect the tubing for air bubbles and force any you find out by pumping the tubing with your fingers. Clamp the reservoir securely and as high on the retort stand as is physically possible and ensure that it is no more than half full of deionized water. Connect the rubber stopper tightly to the test tube. Check to ensure your apparatus is leak free by carefully moving the reservoir to a lower level (say, to the 40 mL mark on the burette). If there is a leak, the level in the burette will drop and continue to drop. Even if there isn’t a leak, the level will fall somewhat, but as long as the water level in the burette stabilises, the system is leak-free. Once you are convinced your apparatus is sealed, hold the reservoir such that the level of water in the reservoir and the burette are the same and record the water level in the burette to 0.05 mL. Re-clamp the reservoir securely to the retort stand before beginning the reaction. Unclamp the test tube and carefully pour the acid out of the small test tube so that it comes into contact with the antacid. You do not want any acid getting into the hoses or the burette. Mix the reaction with mild shaking occasionally, until the reaction is complete and the test tube has cooled to room temperature, this will take at approximately 5 minutes. Be sure to record a barometric pressure and a temperature for this part of the experiment. After the tube has returned to room temperature, hold the reservoir to the point where the water level in the reservoir is the same as that in the burette and read the final water level in the burette. Reclamp the reservoir, then remove the stopper from the test tube to open the system and allow the water to “re-level” back into the burette to be re-used for subsequent reactions.

Part B: Calibration with Sodium Bicarbonate This part of the experiment follows the same setup and measurement procedure as in Part A, with the following modifications. In a clean, dry test tube weigh out 0.110 to 0.130 g of sodium bicarbonate (NaHCO3, you may know this substance as baking soda) to the nearest 0.0001 g. * Ensure that your spatula is clean before you use it for another sample. Repeat the liquid-levelling step (to ~6 mL on the burette), and connect the 2nd test tube to the system, once again ensuring the connected system is leak free. Start this reaction, and continue measurements as per Part A. Remember to have water levels at the same height when taking burette readings. Be sure to record a barometric pressure and a temperature for this part of the experiment.

Part C: Determination of Amount of Active Ingredient in Another Antacid We will again repeat this process once more with another type of antacid of your choosing from the options provided for you. Be sure to record the active ingredient and name of your specific brand of antacid on your report template. The required mass of the antacid you are to use is provided on a label printed on the bottle you choose. Once you have measured your antacid into a clean, dry test tube and recorded the mass, repeat the liquid levelling step (to ~6 mL on the burette), and connect the test tube to the system once again, ensuring the connected system is leak free. Start this reaction, and continue measurements as per Part A.

4 Be sure to record a barometric pressure and a temperature for this part of the experiment. Remember to have water levels at the same height when taking burette readings.

Waste & Clean-up: Apparatus: -Mohr burette, reservoir and tubing need to just be drained of the deionized water stoppers should be wiped with wet paper towel Test tubes: -content should be rinsed into the main sink with lots of tap water (take care with the small test tube when removing it from the large tube) -wash tubes with lab soap (at each main sink) and warm water followed by several rinses with deionized water. Return large test tubes to your TA. Clean any common work spaces such as balance benches and dispensing stations. Return your equipment to the appropriate place. Wipe down your work station with wet paper towel.

Experiment 3: Gas Evolution: Equipment List Caddies: 125 mL Erlenmeyer flask 100 mL Beaker spatula 3 - 12mm x 75mm Test tubes Reservoir (Wash Bottle & Lid) Connecting pieces including stoppers, glass and silicone tubing (1@1/4"id x 68-70cm and 1@ 3/16"id x 40 cm) Other Equipment (Not in Caddy): 3 - 18 x 150mm Test tubes → clean and dry 50.0 mL Mohr burette thermometer (alcohol -20oC to 150oC)

Report: Complete the provided template and submit into Blackboard as one pdf. All raw data should be reported to the appropriate significance

5 Note 1: If you use the ideal gas law with the barometer reading (PTotal), you will get the pressure of CO2 and H2O. Since you only want the pressure of the product, CO2, i.e. Pproduct you need to adjust the total pressure to account for the vapour pressure of H2O. To do this, you can find PH2O_vap values in the table in the Experiment 3 folder on the course site, at the temperature at which you carried out the experiment, and use the relationship: PTotal = Pproduct + PH2O_vap.

Note 2: To determine % yield, use the information gathered in Part B of the experiment. From the mass of NaHCO3, determine the number of moles of CO2 that should be produced (theoretical), and then determine the number of moles of CO2 gas measured in the experiment (actual). The % yield 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐶𝑂2

calculated as:

is

% 𝑦𝑖𝑒𝑙𝑑 = 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐶𝑂2

Note 3: To determine the amount of active ingredient, convert the volume of gas collected to the number of moles of that gas using the Ideal Gas Law. Be sure to divide this number by the % yield (in decimal form) to correct for the systematic errors in the experiment. Convert that number to moles of active ingredient, using the stoichiometry of the reaction (be sure to show the balanced chemical reaction). Finally, report the value, as a mass percentage, of active ingredient in the commercially available antacid sample. The active ingredient in TumsR is CaCO3.

Note 4: Be sure to indicate the NAME and active ingredient of your second antacid and pay attention to this information so you can be sure of the reaction stoichiometry!

Show complete calculations with units and express answers to appropriate significance....