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ChemLab113-0095 - required mass of magnesium represents...

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Unformatted text preview: required mass of magnesium represents only a few turnings.) Record the mass of the sample to four places past the decimal. Be careful to avoid the errors pointed out earlier in connection with the weighing out of small samples, i.e., making sure that g shows and closing the doors on the balance. When you return to the laboratory, record the temperature on the digital thermometer as the value at time zero and immediately add the magnesium to the calorimeter. Approximately one minute later and at about one-minute intervals alter that record the temperature on the thermometer. You should observe that the temperature increases rapidly, reaches a maximum within fifteen minutes, and then decreases slowly. (In some cases the temperature may remain constant at the maximum for several minutes.) Continue recording temperatures until you have five successive decreasing readings after the maximum temperature is reached or until the temperature remains at the maximum for fiVe successive readings. (You will probably not have to record temperatures for any longer than 20 minutes after adding the magnesium.) Empty, rinse and dry the calorimeter. Go to your computer and enter the data for the first measurement. Then carry out two more measurements with new samples of magnesium and HCl solution. After the data for all three measurements have been entered into the computer you will get a printout indicating whether or not this part of the experiment has been completed successfully. If successfully completed continue to Part [L If not completed successfully, see instructor. Carry out any additional measurements that may be required. After this first part of the experiment has been successfully completed, dispose of any unused magnesium in the appropriate disposal container and transfer all your magnesium oxide to the weighing bottle. If on any trial, pieces of unreacted Mg remain in calorimeter, the trial must be repeated. Directions for Measuring the Heat of Reaction of Magnesium Oxide with HCl Measurements in the second part of the experiment are carried out exactly as in the first part, except for the use of magnesium oxide instead of magnesium metal. In weighing out the MgO from your weighing bottle use a weighing boat in the balance room. The appropriate range of MgO sample masses is between 0.30 g and 0.35 g. If you weigh out more than that, you can return excess material to the weighing bottle. When adding the MgO to the calorimeter, make the addition quickly so little heat is lost while the calorimeter is open. As a result of the quick addition you may leave a tiny amount of MgO in the boat, but it is a losing proposition leaving the calorimeter open for an extended time while you attempt to transfer that last small amount. Try to add the MgO directly to the HC] solution, rather than having some of it fall on the thermometer probe or the sides of the calorimeter. After the MgO is added, the temperature will reach a maximum within a minute or two, and then show a slow decrease or an extended maximum, as before. To make sure all the MgO in the calorimeter gets in contact with the HCl solution it is a good idea to swirl or shake the calorimeter vigorously for a few seconds between one minute and two minutes after the addition of the MgO to make sure that all the MgO is in solution. If there is unreacted MgO at the conclusion of the trial, the trial must be repeated. After the data for three measurements have been entered in the computer, you will receive a printout as before. If the heat of reaction is not acceptable, see the instructor. 131 ...
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