CHEM 122 - F08

LAB: CORRECTIONS, SUGGESTIONS, PRELAB & REPORT QUESTIONS

Last Modified: Tuesday, 23-Sep-2008 16:59:04 EDT

Always do INDIVIDUAL WORK (i.e. work ALONE) both in the lab and on your reports. The only time you can work together in the lab is if your TA gives you permission. You can discuss the lab with each other before and after lab but when it comes time to answer any questions it must be your own work.
ALWAYS WEAR SAFETY GLASSES - failure to do so will result in:
- First offense - loss of 1/10 points (10%) for that day's exp.
- Second offense - loss of additional 1/10 points (10%) for that day's exp.
- Third offense - loss of all points (100% pts.) for that day's exp.
You MUST view the relevant video at the beginning of lab prior to doing a lab. If you are late for lab and miss the video you must view the video on the computers in the Learning Resource Center - 160 CE. A form will be printed in 100 CE which should be picked up and given to the TA in lab.
Reports are due at the beginning of lab one week after the work is completed. A penalty of 10% per day, max 50% per week, is assessed for late labs. They should be graded and returned the following week.
Changes or corrections generally will be minimal and announced before the lab. Make sure your lab manual is the 2008 printing.
Exp. 11 (Calorimetry and Hess's Law):
- Prelab Problems: 3 on p. 5
- Report Questions: 1, 2 on p. 15
- Corrections, changes or hints to the lab manual:
  - There is a videotape for Exp 11. Prior to lab you should read over the experiment and prepare a notebook if you have not done so already.
  - You will obtain a digital thermometer dispaly unit and probe from Room 231 or 331. An OSU picture ID or driver's license must be traded for them. See Appendix E.
  - Do NOT use your mouth for pipetting. Instead, use a blue pipetting bulb from the locker. Refer to Appendix C in the Laboratory Manual for a discussion of pipetting technique. The bulb should NOT be forced onto the pipette. It "sits" on top.
  - Each tea cup (styrofoam cup) should have a lid. Punch a hole in it for the thermister probe. Keep the tea cup calorimeter away from boiling water. Place the cup in a 400-mL beaker to stabilize it.
  - Copper cylinders must be kept in boiling water at least 10 minutes.
    Measure the temperature of the boiling water with the mercury thermometer (so heat is not transferred to the probe). Do NOT assume it is boiling at 100^oC.
  - Do NOT use a negative value for the heat capacity of the tea cup. If the correctly calculated value turns out negative (due to experimental error) then assume the heat capacity of the tea cup is zero
  - See Appendix D for a discussion of graphing. All three graphs look essentially the same.
    You can use Excel for manipulating your data, doing calculations and graphing. If you don't know how to use Excel take a look at my example for exp 13. You can always come to me for help.
    If you do use Excel for doing calculations you must still show the required sample calculations. If you are supposed to obtain a slope from a graph use the slope from Excel but explain how you would get a slope (and that you used the slope Excel determined). For this exp. you can plot the points (scatter graph) and then draw your required lines in by hand. Make sure you have Excel draw grid lines on the graph so you can get more accurate results for your time and temperatures from the graph. You could have Excel plot your straight lines but this would be a little trickier. Again, if you want help come see me.
  - If the temperature does not rise at least 5 degrees, check the probe. The easiest way to do this is to place it in boiling water. Defective display units or probes should be given to Room 231 or 331 when you get another one.
  - Return copper cylinders and cups to TA.
  - Return thermometers to storerooms.
  - All solutions can be poured down the drains.
- Waste disposal: Reacted and excess sample may be disposed down the drain while running water. Throw out empty plastic vials in the trash cans.
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Exp. 12 (The Ideal Gas Law: Determining Molecular Weight):
- Prelab Problems: 1, 2b, 5(computer pregenerated prelab) on pp 20-21
- Report Questions: 1, 3, 5 on p. 27
- Corrections, changes or hints to the lab manual:
  - TAKE ALL BROKEN MODIFIED FLASKS TO ROOM 231/331 CE. The modified flasks must be returned at the end of the session. Do NOT place them in your lab drawers.
  - Pressure units of atm must be used in the IGL. The volume should be in liters.
  - Evenly distribute tygon tubing on glass tip and flask. Remove tubing as you would a champagne cork (gradually - a little on each side). A buret clamp should be used to hold the flask in the hot water. You should carry the flask in their hand.
  - The modified flasks are to be IMMERSED IN, not floated on, the hot water contained in a trough. Keep the trough FULL! The vapors from the unknowns are flammable so flames are not to be used near the troughs. Each trough is heated with an electric hot plate.
  - Both the mass of the EMPTY flask and that of the flask containing the condensed liquid should be determined with the same analytical balance. However, if the mass is greater than 110 g, use the PE 600. Use a PE 600 balance to determine the mass of the flask when filled with water.
    The mass of the empty flask must be obtained when the flask is dry. Teh flask with condensed liquid must be cool. (Cold water may be uesed.)
  - Check that the barometric pressure is reasonable (above 735 mm Hg).
  - Use disposable pipets (from the window) attached to a water aspirator for evaporating acetone in the flask (Fig. 12.3), and for removing water (Fig. 12.4).
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Exp. 13 (Vapor Pressure and Enthalpy of Vaporization):
- Prelab Problems: 1, 4, 9(computer generated prelab) on p. 32
- Report Questions: 1, 4, 5 on p. 39
- Corrections, changes or hints to the lab manual:
  - Any Pressure Unit may be used on the computer prelab. However, to compare an answer with that of the computer, mm Hg (or torr) should be used.
    The same is true for the report sheet itself. However, the unit of mm Hg is shown and recommended. It will make graphing easier and more understandable.
  - You will be making plots of ln P vs. 1/T (in kelvin) and "drawing" a best fit line to your data points. The slope of this line gives -delH_v/R, from which you can calculate delH_v. Each graph you do should take up the whole page so the data points can be spread out. Proper labels of the graph, axes and data points should be included.
    I would suggest you use Excel or some other spreadsheet with graphing capabilities. This makes the data manipulation and graphing much easier, especially when you need to make corrections. Also, these programs will plot the "best-fit" line, give you the equation for this line, the slope and the correlation coefficient, R² (which indicates the quality of the fit of the line to the data points, with a value of 1 if every point lies on the line). You still need to show sample calculations of how the get the slope even though you will be using the slope from the generated graph for further calculations.
    If you do not know how to use a spreadsheet program get help from a friend. However, they can only help you learn the program itself. You still need to do the work for your data yourself and turn in your results. You can look at an example of how to use Excel for Exp. 13 by clicking the following link.
    - Exp 13 - Using MS Excel
  - Knowns will be assigned in groups of three students. You share this data. You report only one set of data for each of the three knowns in the set and report who you got the data from. Otherwise, zero points will be given for this data.
    The boiling points of the knowns can be found in the CRC Handbook of Chemistry and Physics in the SEL Library or online. The pressure corresponding to this boiling temp is 1 atm (760 mm Hg).
  - Use NEW serum stoppers for each determination (in box of syringes). Do not fold back the stopper too tightly. This could cause a leak. Additional stoppers are in Rooms 231/331.
  - Do not screw the buret clamp around the syringe barrel so tightly that the plunger stickes. Be sure the syringe is IMMERSED in the water bath - use 600mL beaker.
  - Do NOT use ALL your unkown sample for the boiling point determination. Approx. 1 mL is sufficient.
  - If your unknown crystallizes in a cool lab room it may be reliquified by placing the bottle in a beaker of warm tap water for 5-10 minutes.
  - Under NO circumstances are you to cooperate in the preparation of the report. ONLY DATA for the KNOWNS are shared and the report is to be INDIVIDUALLY prepared.
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Exp. 15 (Variation of Solubility with Temperature and Solvent):
- Prelab Problems: 2, 5, 6, 7 on p. 44
- Report Questions: 1, 3, 5 on p. 50
- Corrections, changes or hints to the lab manual:
  - Do NOT attempt to insert a thermometer into a rubber sopper!
  - The experiment is A, B (one weight in water for each part) and C (one weight in 70:30 water/dioxane).
  - Do NOT stir too vigorously. You don't want to ram the stirrer through the bottom of the tube.
  - Oxalic acid may dissolve the thermometer markings. These may be renewed by scraping the thermometer with graphite from an ordinary pencil.
  - In your reports, answer the question "How did the addition of dioxane affect the solubility of your solute?"
  - Look at Appendix D for a discussion of graphing techniques.
- Waste Disposal:
  Dichromate/water and dichromate/water/dioxane go in the inorganic salts waste container.
  Oxalic acid/water and oxalic acid/water/dioxane go in the red solvent can.
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Exp. 16 (Freezing Point Depression):
- Prelab Problems: 4, 5 (computer generated prelab) on pp. 52-53
- Report Questions: 1(a,c,d), 2 on p. 58
- Corrections, changes or hints to the lab manual:
  - Pick up a digital thermometer and freezing point apparatus from Room 231/331 CE by leaving OSU ID or driver's license. Instructions are in Appendix E.
  - Use 600 mL beakers from your drawers. Take no more than 25 mL cyclohexane to fill the buret.
  - Use a DRY test tube for the cyclohexane. The f.p. of your cyclohexane solution of the unknown could fall below 0^oC. Thus any water could freeze to ice and obscure the f.p. of the cyclohexane solution.
  - Cyclohexane is flammable and toxic.
- Waste Disposal: Collect cyclohexane solutions in a beaker to be disposed in the red solvent can. Dissolve solids in cyclohexane and dispose in the red solvent can. Throw empty vials into a trash can.
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Exp. 17 (Determining a Rate Law and Rate Constant):
- Prelab Problems: 1(c,d) 2, 4, 5 on p. 64
- Report Questions: 1(b,c), 3, 4 on p. 74
- Corrections, changes or hints to the lab manual:
  - Organization
    - You will work in teams of 2. Each member is responsible for two of the four solutions. Each team will need 5 cuvettes. Each team will have one spectrophotometer.
    - Each solution takes about 75 minutes for the reaction to run to completetion. Thus, all four solutions must react simultaneously. This will require precise planning, timing and coordination. A reading is to be taken about every 5 minutes for each solution. Thus, a reading is taken about every minute. If you miss one just go on to the next one.
      Time zero is when the solutions are mixed. Thus by the time you take your first reading the time is actually not 0 but however much time it took you to put the cuvette in the spectrophotometer and take the reading after mixing. You should really take this into account.
    - Suggested organization of 3-hour session.
      - First hour - organization and preparation.
      - Second hour - taking absorbance readings (heat solutions - Beer's Law data)
      - Third hour - continue with absorbance readings clean up and doing rough graph of Beer's Law.
    - If the final absorbance value is attained in less than 50 minutes, then a mistake in mixing has been made and you should repeat that run.
  - Complete a rough Beer's Law graph prior to leaving the laboratory. You should redo a better one for the report.
  - Each student is to report the data for all four solutions. While you share the data, the lab reports (including calc. and graphs) are to be done INDEPENDENTLY.
  - You can use the MicroChem program 56 in 160 CE for data manipulation. However, I suggest you use a spreadsheet such as Excel. It makes doing any corrections very easy and you can have Excel generate the graphs for you. Look at the handouts related to this exp. and the web page for a tutorial. You need to have the slope of Beer's Law graph, initial Cr conc., and absorbance values for either method.
  - Handout for Exp. 17
  - Exp 17 - Using MS Excel
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Exp. 18 (Variation of Reaction Rate with Temperature):
- Prelab Problems: 1b, 2, 3, 5 (computer pre-lab) on pp. 77
- Report Questions: 2, 3, 4b, 5b, 7 on p. 82
- Corrections, changes or hints to the lab manual:
  - Do your own work. Boil your own water.
  - While heating solutions, keep them in the hood or point tubes toward hood.
  - The sample calc. are required (as always) and are worth 22 pts (ln t, 1/T, slope of line, E_a).
- Waste Disposal: All solutions containing phenol and tribromophenol must go into the metal salts waste container.
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Exp. 19 (Equilibrium and Le Chatelier's Principle):
- Prelab Problems: 1(a,b), 2c, 4, 6 on pp. 86-87
- Report Questions: 1, 2, 3 on p. 95
- Corrections, changes or hints to the lab manual:
  - Always wear SAFETY GLASSES. Using concentrated acid.
  - Part C: There will be two half filled disposable weighing dishes - one with CoCl₂^.6H₂, one with Co(NO₃)₂^.6H₂), at the front desk. These are for you to observe and use.
  - Part C: ABSOLUTE ethanol must be used.
  - Do NOT use 6 M HCl when concentrated HCl is required. Conc. HCl is in the hoods along the wall.
  - Exp 19 - Explanation of why addition of H₂O shifts Equilibrium for reaction (3)
- Waste Disposal: Sb(III), Co(II) in acid soln - neutralize with 6 M NaOH to pH about 7 first (persistance of white ppt.), and put into the Inorganic Waste Container.
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Exp. 20 (Analysis of a Solution of Two Acids):
- Prelab Problems: 4, 5 (computer pre-lab) on p. 101
- Report Questions: 1a, 2, 4 on p. 106
- Corrections, changes or hints to the lab manual:
  - General Directions for Quantitative Analysis - Titrations:
    - Using the buret and pipet.
      - Cleaning the buret - the solution should drain completely - no "beading".
      - Filling of buret - no air bubles in the tip. (Wire to clean out tip available in 231/331 CE.)
      - Reading the buret - estimate to the nearest 0.01 mL. Buret reading cards are available from the cart near 231 or 331 CE.
      - Pipet fills to 5.00 or 10.00 mL when bottom of meniscus is on line. Drain pipet which will leave a small amount of liquid in tip.
      - When filling pipet, make sure sufficient solution remains - don't suck up air.
      - Pipet and buret techniques are discussed in the introduction to the laboratory manual.
    - TROUBLE SHOOTING TITRATION RESULTS: In calc. use the CORRECT 4 SIG. FIG. MOLARITY OF THE STANDARD SOLUTION.
  - Use 0.05xxx M NaOH. Thymol blue is preferred for second titration. Titrated sample should be same shade as reference solution.
  - Titrate 10mL of the unknown to determine the acid concentration and 5 mL of the unknown for the concentration of the salt.
    Do not forget that in the second titration the phosphoric acid in the original sample has been titrated twice, once to H₂PO₄^- and again to H₂PO₄^2-. Suppose that the unknown is y molar in phosphoric acid and z molar in sodium dihydrogen phosphate. The first titration determines y, since the end-point corresponds to completion of the reaction:
```
			H₃PO₄  +  NaOH  -->  NaH₂PO₄  +  H₂O
			
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    The second titration again starts with fresh unknown, so the first portion of NaOH added changes the H₃PO₄ to NaH₂PO₄. After this portion has been added, the solution is y + z molar in NaH₂PO₄. Further addition of base will neutralize not only the z molar NaH₂PO₄ originally present in the unknown but also the y molar salt formed from the y molar H₃PO₄ originally present, forming Na₂HPO₄ from both when the end-point is reached. The chemical reactions for the second titration are:
```
                        H₃PO₄ (y molar) +  2 NaOH  -->  Na₂HPO₄  +  2 H₂O          
			
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```
                        Na₂HPO₄ (z molar)  +  NaOH  -->  Na₂HPO₄  +  H₂O          
			
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		    Thus, for the second end point:  M_Base x V_Base = V_Sample x 2y  +  V_Sample x z
		    
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