All experiments this semester: Weigh and record to  ± 0.0001 g.

Part A:  You will not be required to clean the crucibles at the end of the experiment.  Return them to the sideboard after emptying the ash in the solids waste beaker.

Part B:  You will not have to prepare the beakers as mentioned in the first paragraph below the zinc reaction caution on page 392.

Both Parts:

You may perform all three runs for the zinc iodide experiment in parallel

Calculate the standard deviation and the %RSD for all three runs.

Report the average and standard deviation properly and the %RSD in your conclusion.  Also include a statement as to the reasonableness of your result.  Identify the metal ribbon.  Do the chemical and physical properties of the metal ribbon match those of your assigned identity?

All liquid wastes will be collected in the waste bottle in the back hood.

The text mentions several things that are very important to the success of the experiment.  This section will not include points made in the text in future Notes.  It will be up to you to ask yourself why the text mentions these points and supply your own answer.  This is an excellent way to prepare and write the discussion section of your report.

Metal Oxide

• Preparation of the crucibles to achieve a constant weight by heating until the bottoms are red hot. (look inside the crucible.  This does not take very long.)
• There is very little that will not vaporize when a crucible is heated red hot.  This includes contaminants in addition to water and fingerprint oils.
• The analytical balances you will use read to four decimal places.  Record all four decimal places.  The greater the number of significant figures you can use in your calculations, the greater will be your accuracy.
• Not touching the crucibles with your fingers
• (one very good reason is a hot crucible doesn't look hot.  What is the other reason mentioned in the text?)
• Storing the cooling crucibles in the dessicator.
• It would be a shame to reabsorb all that water vapor you spent so much time desorbing.
• Slow heating prevents burning of the metal.
• You will also notice an appreciable amount of smoke in addition to the very hot, very bright, very energetic flame.  This smoke is particulate metal oxide being carried off by the thermal plume of the flame.  If you can see it the analytical balances you are using can measure it.
• Using a very low flame to drive off the ammonia and water from the oxide
• Spattering is a good way to lose your product and a good way to get burned.  Also too strong a thermal plume will levitate the metal oxide from the crucible once dehydrated.
• Using moist litmus paper
• The moisture will help trap the vapor and make the reaction easier to see.
• Cooling the crucibles the same length of time each time.,
• Limiting the variable parameters to one is vital to the success of every experiment.  It increases your reproducibility and therefore increases your precision.  Environmental variables that are always present include pressure, temperature, and in some cases light to name just a few.  This experiment's results are independent of pressure, but not temperature.  The temperature of the crucible will have an affect on its mass as measured by the balance.  Can you think of reasons why this is so?  Another category of variables are procedural.  Cleanliness of glassware and handling and preparation of reagents are important.  We will discuss quantitative transfers later.
• Record all Data
• A few students each year discover just how difficult it is to write a report without data.  The simple solution to the question as to what is important to record is to record everything.  Times, temperatures, amounts, observations about the conditions and appearances of reactions, deviations from the written procedure, units, and significant figures are very important.  Having too much data will always insure you have all the information you need.

 
Metal Iodide
 
• Accurately weigh each item and record the masses
• Set beaker slightly off center on the stir plate to insure agitation of the zinc metal
• Do not boil the mixture
• Do not lose any unreacted zinc powder during the washes
• Wash the metal with acetone

You should be able to describe why each of these points are important.

Things the text didn't say but are a good idea

Space your crucibles close enough together during the first part of the experiment such that they can be heated almost simultaneously.  Heat the crucibles far enough apart after the metal ribbon coils are added such that you can easily and quickly place the cover on one without knocking over the other two.  This is a big problem when the crucible you knock over contains ash you worked hard to create.

Sand the metal ribbon before coiling and weighing it.  You need to remove the metal oxide covering the surface.  Can you tell me why this is critical to the success of the experiment?  Why will the presence of a metal oxide on the surface of the same pure metal affect the results of your experiment and the answers to your calculations.

Coil the metal ribbon in a fashion that it will fit completely inside the crucible.  If the coil extends above the top of the crucible it is likely that the ash will fall outside the crucible.  Some overlap may be necessary.

A constant weight is achieved when the final decimal place for two consecutive measurements differ by only ± 4.

Swirl or shake the beaker containing the liquid or washes and the unreacted zinc powder to collect the zinc in one small pile.  Slowly rotate the tilted beaker one quarter turn and remove the liquid using a Pasteur pipette.  Watch for particles of zinc powder that are so light that the surface tension of the liquid is sufficient to make them "float".

 

Introduction

Analytical balances are accurate and precise instruments to measure masses. They require a draft-free location on a solid bench that is free of vibrations. Some modern balances have built-in calibration masses to maintain accuracy. Older balances should be calibrated periodically with a standard mass.

Here are a few tips to follow to insure proper care whenever you are using an electronic balance of any kind:

• Do not bump or place objects on the bench after zeroing the balance.
• Mass powders on paper or dishes. Handle objects with tongs, tweezers, gloves, or paper to prevent fingerprints.
• Let hot objects cool before massing.
• Mass hygroscopic materials rapidly since they will absorb water during massing.
• When making repetitive massings always use the same procedure.
• DO NOT go off and leave spilled chemicals on or around the balance! Report any spill to the instructor so that she may clean it up in a proper manner, re-calibrating if necessary.  Some will stain, some will corrode.  The greatest problem is you are weighing chemicals you do not transfer to your experiment.
• Try to use the same balance each time.  This will decrease the inaccuracy and imprecision of your experiment by reducing propogational errors.

 

Picture of the Mettler  AJ100 analytical balance:

Operating Instructions for the Mettler Balance: Before turning on, be sure that the doors are closed and the pan is not touching the draft ring. Turn on by pushing down on the gray bar and letting up quickly. Push again if the digital readout does not show 0.0000 g. When adding the chemical to be massed open both side doors. Hold the sample vial over the weigh paper, weigh boat, or glassware with one hand and transfer the chemical with a clean spatula in the other hand.  Close both doors and do not jar the lab bench.  Allow display to stablize; (g) will appear after the nummerical display. Read (record) result, remove object from pan, leave doors closed and re-zero if necessary.

Measures to decrease time at the balance and a few cautions:

Tare or zero the balance.  This will save you the time of subtracting your container.  Be careful if you choose to tare, or zero, your balance.  Although this is an effective technique and will limit the number of calculations you may have to perform, you may tare values you do need for calculations.  Always tare each time in the same manner.

If sufficient glassware is available and storage of the chemicals is not important, other than to avoid contamination, do all of your weighing for all runs at the same time.

Do not spend a great deal of effort trying to transfer exactly the amount of chemical called for in the text.  If you calculate the moles of each chemical and compare them to the stochiometric molar ratios from balanced equations for the reaction you will notice that one of the chemicals is inevitably in large excess.  Any amount within ± 10% is good enough.

Carry all smaller containers in a larger container that will still prevent the small containers from tipping.  A 200 mL beaker is an excellent way to transport the crucibles used in this experiment.

• Do not heat glassware to dryness.
• Do not heat sand baths above 3.5 to 4 on the heat control dial.
• The temperature of a substance will not increase until the phase change is complete.  Water will not reach temperatures above 100 °C until all of the water has changed from the liquid phase to the gas phase.  This is below the failure point of the pyrex container.  However,  the next phase change from the gas phase is the disassociation of the water molecule.  The pyrex container will be destroyed before this temperature is reached.  This is also true for the sand bath.  Before the sand will change from the solid state to the liquid state the failure temperature of the pyrex will be reached and you will end up buying a very unusable sand bath.
• Control the bunsen burner flame with the gas control knob and air intake vents only.  Do not use the gas supply valve on your bench to meter gas.  The gas supply valve should be fully on or off.  If you have difficulty lighting the burner turn off the gas supply valve and wait a few moments.  Get the attention of your TA.
• The hottest part of a flame is the blue inner tip.

• If the experiment calls for 1 mL instead of 1.000 mL estimate the volume using marks on beakers or a single full squeeze of the bulb on a pasteur pipette instead of graduated cylinders.  Beaker marks are very effective for volumes of 5 mL or larger..
• Perform other tasks while waiting for a task to complete that does not require your constant attention.  Sanding, coiling, and possibly weighing ribbons can be accomplished while the crucibles are being heated and cooled to constant weight.  If the ribbons are ready just as you are achieving constant weight the ribbons may be added immediately and save you a trip to the balance room.  Assembling apparatus needed for later steps in the experiment such as vacuum filtration setups or preparing hot baths or ice baths and chilled washes in advance of need.
• Keep a large beaker under your hood to collect liquid waste and glassware washes generated during the experiment in.  Then you can make one trip to the waste hood.

Reactions:  (These may not be provided in future Notes and you will have to derive them yourself)

M is any metal

xM_(s) + air  M_aO_b(s)  + M_c(OH)_{d (s)} + M_eN_f(s)
M_c(OH)_{d (s)}  + H₂O_(l)  M_aO_b(s) + H₂O_(g)
 M_eN_f(s) + H₂O_(l)  M_aO_b(s) + NH_3(g)
Zn_(s) + I_2(s) ZnI_2(a)