Chemical Stoichiometry and Gases

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Experiment 8

Chemical Stoichiometry and Gases

LABORATORY PROCEDURE:

1. Place flask A on the balance and tare the balance. Weigh approximately accurately 10 grams of unknown solution (record the number or letter) into the flask. Add about 30 to 40 mL of distilled water to the flask. If the balance will not weigh flask A, use a weigh boat to weigh the unknown and then transfer it quantitatively to flask A.

2. Make sure the test tube to be filled with HCl in step 3 is the correct size to lean inside flask A as shown above.

3. Fill the small test tube with HCl to within about 2 cm of the top (it should not spill when it leans in flask A). Wipe the outside of the small test tube and carefully lower the small test tube of HCl into flask A without spilling.

4. Place saturated salt solution in the Florence flask (B) to a level as shown above. Place the rubber stopper (tightly) in flask B. Observe the salt solution level. The solution should be morethan3 cm below the level of the short glass tube in the stopper.

5. Place the rubber stopper in flask A tightly. Make sure all connections of rubber tubing to glass tubing and rubber stoppers are tight.

6. Remove the glass discharge tube (C) from the beaker (D). Connect rubber bulb to tube C. Release the pinchclamp on tube C while slowly squeezing the bulb. (If the bulb is squeezed too rapidly, salt solution might carry over into flask A). Hold the bulb (compressed) and apply the pinch clamp. (The entire apparatus should now be pressurized). Place the end of tube C in beaker (D). Release the pinch clamp. Salt solution should begin to rush into the beaker. Be sure to keep the tube C submerged in the solution in the beaker.

7. Raise the beaker so that the level of salt solution in the beaker is equal to the level of solution in flask B. The pressure in the apparatus is now equal to the pressure outside of the apparatus (the atmospheric pressure or barometric pressure). Apply the pinch clamp.

8. With the system airtight, lift the end of tube C from beaker and empty the liquid in beaker back into the container of salt solution (or a separate beaker for salt solution at your station). Place the end of tube C in the empty beaker D.

9. Start the reaction as follows. Remove the pinch clamp at the same time that flask A is tilted to empty the test tube of HCl into the sample in flask A. Gently swirl flask A for the next several minutes to insure that the reaction of the HCl and NaHCO3 is complete. Salt solution should be displaced into beaker D. (Remember this will represent the volume of gas produced).

10. When the reaction is complete (no more salt solution flows into beaker D even if flask A is shaken), hold beaker D up beside flask B so that the levels of the solutions in them are at the same height and clamp the pinchclamp. The pressure in flasks A and B are now at the same pressure they were initially, that is, at atmospheric (barometric) pressure. Remove the end of tube C from beaker D and measure the volume of salt solution in beaker D (with a graduated cylinder). The volume should e measured to the nearest mL.

11. POUR THE CONTENTS OF FLASK A (less the test tube) INTO THE WASTE CONTAINER.

12. Pour the salt solution from the graduated cylinder back into flask B.

13. Make three runs using a different sample each time.

14. Calculate the percent by mass of NaHCO3for each run and an average of the three runs (assume ideal behavior).

When finished, return all the salt solution to the container from which it was obtained. Rinse the flasks to remove any salt and set them on the lab bench to dry.