Ln k sp h rt t s rt ln k sp h r s r 1 t y m x b

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ln K sp = - H RT T S RT + ln K sp = - H R S R + 1 T y = m x + b
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Experimental Procedure (groups of 2 students) Note: to complete this procedure, it will be necessary to carefully divide the tasks between partners. Take a moment before beginning to make a plan between the two of you to do this. Using a 5 mL pipet, add exactly 5 mL of distilled water to each of ten test tubes. Mark the level with a grease pencil or other marker and pour the water out. Each group is assigned a temperature at which to do the titrations. Mark two of the tubes with your assigned temperature. Your data will be pooled with data from your classmates to arrive at the temperature dependence from room temperature to about 45°C. Using a hot plate, prepare a mixture of 30-32 g of solid sodium borate decahydrate (i.e. borax) and 150 mL of distilled water in a 250 mL beaker. Begin heating the mixture, but do not allow the temperature of the mixture to exceed 50°C. If all of the borax dissolves, add more so that an excess of solid borax exists in the beaker at 45°C. If there is solid borax in the mixture, you can assume the water is as saturated with dissolved borax as it can get, and this is the condition you want. Once the mixture has exceeded, however slightly, the 45°C mark, remove the beaker to the bench top and replace it with a beaker containing 150 mL of distilled water (this will be used later, but it is best to begin heating it now). This water should also not be allowed to exceed 50°C. Taking Samples of the Borax Solution Continue to stir the borax/water mixture occasionally until it has cooled to the assigned temperature. Then as quickly as possible, and trying to avoid the transfer of any solid borax, carefully pour out 5 mL of the solution into the two test tubes marked with your temperature. Be sure to note the actual solution temperature before and after this transfer, and use the average if the readings differ by more than 1°C. It will be not be necessary to cool the sample in an ice bath to speed up sampling. Doing so risks introducing errors due to supercooling, and should be avoided. Experience has shown that a 5-degree temperature drop can occur in about 10 - 15 minutes, and your patience will be rewarded with excellent samples from which to obtain data.
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Titration of the Borax Samples The second water bath should have reached at least 45°C. If it has, try your best to keep it at this temperature. Place the samples in the hot water bath until any re-precipitated borax has dissolved. Carefully pour the dissolved borax solution into a 125 mL Erlenmeyer flask containing 50-75 mL of distilled water and 10 drops of bromocresol green indicator. More water may be added to the flask, and the test tubes can be rinsed with water from the hot water bath, to ensure that all the borax has been transferred and is in solution before beginning the titration. The borax samples should have a blue color before they are titrated with acid.
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