Lab+1+Gas+Laws - It for lab PDF

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Chemistry 162: Experiment 1 THE IDEAL GAS LAW PRELAB ASSIGNMENT Before coming to lab:  Complete the following sections in your lab notebook for this lab exercise before attending lab: Title, Name, Date, Purpose, Procedure (in your own words). Be ready to have your instructor check this at the beginning of the period. Also, set up a Data Table(s) for the data you will be collecting (see the end of the introduction for an example table.) You will not be allowed to perform the lab if you are not dressed appropriately for the lab, have the lab procedure written in your own words, and come to lab on time.  Complete the Prelab questions found on the last page and hand it in as you come into the lab room on your way to your lab bench.

INTRODUCTION The objectives of this experiment are to determine the molar mass of an unknown liquid compound of relatively low boiling point and to practice the mathematics of the ideal gas law. To determine molar mass, we must be able to find both the mass in grams and the number of moles present in a particular sample. For liquid samples, making a weighing to measure mass does not usually present any difficulties. On the other hand, there may not be an obvious procedure for determining the number of moles. Measurement of extensive physical quantities, such as volume, will not lead to the number of moles present unless we have also obtained detailed knowledge about the molecular dimensions and how molecules pack together to find a number density. To use extensive chemical quantities, as in stoichiometry, we may need to find the identity or class of the unknown compound to choose what to react with it as a basis for measurement. Either of these methods could involve considerable challenges. If a sample can be vaporized, the ideal gas law allows us to calculate the number of moles if we can measure the pressure, volume, and temperature of the sample. Thus it is usually much easier to find the molar mass of a sample as a gas or vapor than as a liquid or solid. We will take advantage of this observation and analyze a liquid unknown by converting it to vapor. The procedure will have you place portions of a sample in a round flask and vaporize the liquid by immersion in a hot water bath. As the sample become gaseous, it requires much more volume. From approximately 5 mL, it will expand to fill the flask, driving out the air initially present as well as excess sample. At the point when only gaseous sample remains in the flask, we will determine the temperature, pressure, volume, and mass to gain sufficient data to calculate the molar mass. Moles of sample will be found by solving the ideal gas law PV = nRT for its symbol, n. Then the ratio of grams of mass to moles can be evaluated to give molar mass. Referring to details of the measurements, temperature will be found by placing a thermometer in the hot water bath surrounding the sample flask and assuming that this water and the sample have reached uniform temperature. The pressure of the gas will be taken as equal to the lab barometric pressure since there will be a pinhole in the sample cap to allow pressures in the flask and the surrounding lab air to equalize. The volume of the flask will be found in a separate operation by finding the mass of distilled water that fills the flask and using water density to calculate the volume that corresponds to the measured mass. To find the mass of a sample, we ordinarily weigh it in a container and then subtract the empty container mass. If we weigh the gaseous sample while it occupies the entire flask, then subtract the mass of the “empty” flask (actually filled with air), the result will not equal the sample mass but rather the difference Lab 1 – Gas Laws - p1

between the sample mass and the mass of an equal volume of air. To avoid this, we could create a vacuum in the flask before weighing it. However, there is an easier way to reduce the error. Instead of weighing the sample as a gas, we will allow it to cool down in the flask and condense back into a liquid. As a liquid, it will occupy much less volume, so air will come back into the flask and be weighed with the sample. Thus the air mass with the sample and in the “empty” flask will nearly cancel the error only due to the very small amount of air displaced by the sample as liquid.

MATERIALS AND SAFETY The vapors of the unknown liquid samples are moderately toxic and may be irritating if inhaled. The laboratory ventilation is adequate to prevent high concentrations from building up in the lab air space. However, you should avoid excess contact with the sample as vapor and as liquid. After the vaporizing operations, the amount of liquid residue is quite small and we will collect the bulk of it to evaporate in the lab hoods.

PROCEDURE Choose one of the samples to analyze. The samples will be stationed in reservoir bottles at the equipment cart or in a hood with dedicated graduated cylinders. Be sure to record the unknown number. The samples themselves are colorless but may have small amounts of dye added for identification. Obtain a thermometer and round-bottomed flask from the cart and cut two small squares of aluminum foil, 4-5 cm per side, to use as covers for the flask. Use a pin to poke a hole in the center of one of these foil squares. For reference in the rest of the procedure, the foil without a hole will be called the dry cap. It is to be used for all weighings. The foil with a hole will be called the wet cap. It will be used for heating operations in the water bath. Record the barometric pressure in the lab and record the density of water at the lab temperature from the lab chart (on the side of a central hood). Check that the flask is dry. Weigh it with the dry cap using an electronic balance and record the mass. To prevent the flask from rolling on the balance pan, use the cork ring to support the flask. Set up a ring stand, iron ring, and insulated wire mesh pad to support a 600 mL beaker above a Bunsen burner as shown by your instructor. You will perform two trials of vaporizing the sample liquid. If your sample was issued in a test tube, pour about half (without measuring) into the round flask. Otherwise, go to the sample reservoir and use the small graduated cylinder to place about 5 mL of the unknown liquid in the flask. Do not take the cylinder or reservoir bottle away from their designated area. With the sample in the flask, use the wet cap to cover it. Use a utility clamp attached to the neck of the flask and the ring stand to set the flask in the beaker. Be sure the flask does not rest on the bottom of the beaker. Add enough tap water to the beaker to cover most of the flask but not to so high a level as to risk boiling over upon heating. Light the burner and bring the water to a gentle boil. The liquid sample will slowly evaporate. Watch the level of the sample in the flask to observe when the evaporation is complete. Note that a dye will form a residue that does not evaporate. Continue the heating for a few minutes after you determine that no sample liquid remains. Then use the thermometer to measure the temperature of the boiling water in the beaker and record the value. Do not leave the thermometer standing in the beaker. Hold it in place while you take the measurement then promptly remove it to a safe location. Turn off the burner gas. When it is not too hot to handle, detach the utility clamp from the ring stand so that you can lift the clamp and remove the attached flask from the water bath. Take the clamp and flask to a sink and run cold tap water over the outside of the flask to cool it down. Be careful not to run water over the cap. When the flask is cool and you see a volume of liquid appear from condensation, detach the clamp and thoroughly dry the outside of the flask with paper towels. Then make a rapid switch from Lab 1 – Gas Laws - p2

the wet cap to the dry cap to cover the flask. Weigh the flask with the condensed sample and record the mass. Remove the dry cap and set it aside. Without discarding the sample residue, add another 5 mL of your fresh sample (either the other half from a sample tube or another 5 mL addition from the reservoir). Cover the flask with the wet cap and return it to the water bath. Repeat the vaporizing procedure and obtain a second set of temperature and mass values. After completion of the second trial, pour the residue into the waste container dedicated to the class, then rinse the flask out with tap water. Fill it to the top with distilled water and cover it with the dry cap. You must weigh the filled flask; however, the mass will probably exceed the capacity of the electronic balances. In that case, use the two-pan balance and the set of counterweights to measure the mass to the nearest 0.1 g. Be careful to correctly total the mass of counterweights and slider and to subtract the mass of the cork ring (which can be found with the electronic balance). This completes the data acquisition. Empty the flask and return it to the cart, along with the sample test tube (if issued) and thermometer. Dispose of the foil caps in an ordinary waste container and return other hardware items to the cart or lab storage drawers.

DATA SUMMARY TABLE Before you come to lab, create a table, clearly written, into your lab notebook to collect the following data for both trials (1 and 2). If you would prefer to originally record your data alongside your procedure, you may. However, everyone must have this table at the end of their procedure so I can clearly find all your data. It will be your responsibility to record your data in this table correctly. Not doing so may result in a loss of points. Make sure you organize your table in this particular order:  Unknown sample identification number  Mass of flask and dry foil cap  Mass of flask with condensed vapor  Temperature of water bath  Mass of flask with distilled water  Lab barometric pressure  Distilled water density

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POST LAB QUESTIONS AND CALCULATIONS Do your postlab in your lab notebook after your procedure and data summary table. Turn in ALL the carbonless copies of your lab notebook pages for this lab. This includes the procedure, all tables, postlab calculations, etc. Show your work for all calculations. Your lab will typically de due a week after performing the lab, unless noted otherwise. Note: The Summary of Results table is separate from the answers to the questions, meaning is it often the case that you have the same value as both the answer to a question and as part of your results table. It is meant to be a quick summary of what you found. While it is often useful to just include the main point of the lab (in this case just the average molar mass, for example), but at times you will be asked to summarize the answers to some of your questions to help with grading. Calculations: 1. Gas pressure in atmospheres 2. Volume of flask in liters For each trial (1 and 2) calculate the following: (show your work and clearly present your results) 3. 4. 5. 6. 7.

Gas temperature in Kelvin Moles of sample vapor Molar mass of sample (in g/mol) Average molar mass Percent difference between trial values

SUMMARY OF RESULTS TABLE Summarize your experimental results (answers to 1-7 above) in an organized data table. Note that for questions 3-5 you will have a separate value for each trial, whereas for questions 1, 2, 6, and 7 there is only one value.

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Lab 1 Prelab Questions Name____________________________________

Gas Laws

Lab Section________________

Instructions: Complete the following questions and hand in at the start of your lab period or when instructed by your instructor. Show your work with units and correct significant figures for all questions that involve a calculation. Circle your numerical answers. THERE ARE 2 QUESTIONS (see next page for the rest) 1. A student does this same experiment and gets the following data. The mass of the empty flask with the dry foil cap was 55.041 g. The sample was heated in water at 99.5ºC then allowed to condense. The flask and condensed vapor weighed 56.039 g. To determine the volume of the flask, it was filled with water giving a total mass of 270.9 g. The barometric pressure in the lab was 752 mm of Hg. Use this data to find the molar mass of the sample unknown. Show all steps and unit conversions involved. Use 0.9982 g/mL for water density.

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2. Below (a through d, see next page) is a list of some errors that could occur during the experiment. For each error, consider the effect on the data that would be obtained and trace the effect through to the calculation of molar mass. Circle whether the calculation will yield a higher or lower value than the actual molar mass. Then explain your reasoning for each case in detail. (Note: assume the sample had a molar mass equal to the result in question 1.) a.

A student removed the sample from the heating bath before the sample liquid had all evaporated. HIGH

LOW

Explanation

b.

A student did not dry the outside of the flask after removing it from the water bath to weigh the flask with condensed vapor. HIGH

LOW

Explanation

c.

After the heating process, the sample had not yet condensed when the foil caps were switched. HIGH

LOW

Explanation

d.

During the heat operation, the flask was left in the hot water bath very long past the time when it had all evaporated. HIGH

LOW

Explanation

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