Protocol: You will be doing six determinations using different concentrations of

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Protocol: You will be doing six determinations using different concentrations of potassium nitrate in order to examine the thermodynamics of KNO3 dissolving in water. Each time you will record the temperature when you first see crystals begin to form in the solution. CAUTION: Potassium nitrate is both an oxidizing agent and an irritant. 1. Get your gloves, goggles and lab coats on. Place a 600 mL beaker filled ¾ full with water on a hotplate and secure it with a ring attached to a ring stand. Turn on the hotplate to begin heating the water. Retrieve a clean test tube fitted with a rubber two-hole stopper and a stirrer. If the test tube isn't clean, you know what to do. Attach a clamp to the ring stand to hold your test tube. 2. Mass 15 grams of potassium nitrate (KNO3) on a tared piece of weigh paper. Record the exact mass on your Data Sheet and carefully transfer it to your clean test tube. 3. Add 12 mLs of distilled water to your test tube with the KNO3 in it. Put your test tube in the clamp and heat it in your hot water bath with stirring until all of the KNO3 dissolves 4. Determine the volume in your test tube by filling an identical test tube with tap water until the volumes in both test tubes are identical. Use a graduated cylinder to measure the volume in the test tube filled with water and record it on your Data Sheet. 5. Remove the test tube from the hot water bath. In one hole of the stopper, place the metal stirrer then put the stopper on the test tube. Now add the thermometer to the second hole. Allow the test tube to cool while slowly stirring. Watch carefully and record the temperature when you first see KNO3 crystals begin to appear. Look for snowflake-like white crystals. At this temperature the solid is assumed to be in equilibrium with the solution. 6. Add 4 mLs of distilled water to your KNO3 test tube. Again, warm the solution while stirring until all of the crystals have dissolved. Determine the solution volume as you did previously (step 4) and cord the volume on the Data Sheet. 7. Cool the solution and record the temperature when crystals first appear. 8. Repeat steps 6 and 7 until you have data for six different concentrations of potassium nitrate. 9. Discard your KNO3 solution in the Hazardous Waste and thoroughly clean your test tubes, stopper and stirrer. Wash your hands thoroughly with soap and water before leaving lab

Explanation / Answer

4.

Srxn = So[K+(aq)] + So[NO3-(aq)] - So[KNO3(aq)]

Srxn = [(102.5 J/mol K) + (146.4 J/ mol K)] – 132.9 J/mol K = 116 J/mol K

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