Laboratory 1



Stoichiometry and Limiting Reagents

Prepared by

Edward L. Brown, Lee University and

Kathleen Reschke, West Virginia Wesleyan College

To develop an understanding of the relationships between a reaction’s stoichiometry, the limiting reagent, and the theoretical yield.

|Apparatus |

|Pyrex test tube (2) |Glass stirring rod |

|Hot plate |Glass Funnel |

|Test tube clamp |25 mL Erlenmeyer flask |

|Chemicals |

|Na3PO4·12H2O (solid) |BaCl2 (solid) |

|0.10 M Na3PO4·12H2O |0.10 M BaCl2 | |

|Distilled Water | | |

The concept of limiting reagent is as important to the study of chemical reactions as it is to the production of automobiles. Suppose you were the manager of an automobile plant that had previously run out of tires and steering wheels. You told your workers to keep making cars, but without tires (5 per car including the spare tire) and steering wheels. After several days, your plant has 50 cars ready to ship except they are missing tires and steering wheels. Two trucks simultaneously pull into the unloading dock – one has 200 tires and the other has 75 steering wheels. A quick calculation involving limiting reagents tells you that you can complete the assembly of 40 cars and ship them later that day.

To arrive at the answer, most people would calculate the maximum number of cars that can be made from each reagent: the unfinished cars, the tires and the steering wheels.

The “theoretical yield” of finished cars is the minimum number of finished cars that can be produced from the available reactants (unfinished cars, tires, steering wheels). The only way more finished cars can be produced at this plant is for more reactants to be added.

Calculating the theoretical yield in a chemical reaction is accomplished in the same manner: each reactant is converted into either the number of products formed or the mass of products formed. In the case with the cars, each reactant was converted into the number of finished cars, but we could have easily determined the mass of finished cars if we had been give the conversion factor, 1 finished car = 2500 lbs. In chemistry, we generally use this additional conversion factor and determine the mass of product formed since it is very difficult to count the number of molecules formed. The atomic masses displayed in the periodic table allow us to calculate the grams of product contained in 1 mole.

Mole is a word that represents a number. Exactly like the words below.

Dozen . . . . . . . 12

Gross . . . . . . .144

Ream . . . . . . .500

Grand . . . . . 1000

Mole . . . . . . . 602,214,179,000,000,000,000,000

or

6.022 × 1023

or

6.022E23

To calculate theoretical yields in chemistry, one need only perform the following calculation for each reagent – the reactant that gives the fewest grams of Product (theoretical yield) is the Limiting Reactant.

Suppose that one of the 40 finished cars was totaled while it was being loaded onto the tractor trailer. Since the theoretical yield was 40, but the actual yield was 39, then the percent yield is 97.5%.

A related calculation found in limiting reactant problems is to determine the mass of a reactant remaining in excess at the end of the reaction. In the example above, there would be 10 unfinished cars and 35 steering wheels left over (in excess) after making as many cars as possible. There are two ways this number can be calculated:

from the theoretical yield

from the limiting reactant

Let’s apply what we have learned at the automobile plant to a typical chemical reaction:

CaCl2 + Na2CO3 CaCO3 + 2 NaCl

Suppose at the beginning of the reaction we add 0.750 g CaCl2 to 0.750 g Na2CO3.

1. Which reactant is the limiting reactant?

2. What is the theoretical yield of Calcium carbonate?

3. How much of the non-limiting reactant remains in excess after the reaction has gone to completion?

Limiting Reactant

The limiting reactant is CaCl2 (it places a limit on the amount of CaCO3 that can be formed).

Theoretical Yield

The smallest amount of product (CaCO3) that can be formed is 0.676 g. Also, it is the amount of product that can be formed from the limiting reactant.

Mass of Na2CO3 in Excess

from the theoretical yield

from the limiting reactant

Approximately 0.0337 g Na2CO3 is present in excess at the completion of the reaction.

The reaction of interest in this lab begins by combining sodium phosphate dodecahydrate and barium chloride dehydrate. One of the products, barium phosphate, is a solid that will be recovered and weighed. The balanced chemical reaction is

2Na3PO4·12H2O + 3BaCl2·2H2O Ba3(PO4)2(s) + 6 NaCl(aq) + 30 H2O(l)

The limiting reactant will be determined experimentally by taking the filtrate liquid after the reaction has gone to completion and dividing it in two portions. Add a few drops of 0.10 M Na3PO4 to one of the portions and a few drops of 0.10 M BaCl2 to the other. If a precipitate forms when Na3PO4 is added, then the sodium phosphate was the limiting reagent. If a precipitate forms when BaCl2 is added, then barium chloride was the limiting reagent.

In this experiment, the BaCl2·2H2O and Na3PO4·12H2O will be provided in containers labeled Reactant 1 and Reactant 2. During the online submission of the lab report, you will obtain enough information to determine the identity of Reactant 1 and Reactant 2.

Safety:

Wear safety goggles and use caution in handling all materials. Barium compounds may be hazardous if inhaled or swallowed.

Your Lab Instructor may wish to demonstrate some aspects of the lab procedure. Watch and listen closely for new information.

Procedure

1. Fill a 400 mL beaker ¾ of the way full with deionized water. Place it on a hot plate and set the hot plate to 120 °C. You can cover with a watch glass to speed up the heating process.

2. While the water is heating, Reactant 1 and Reactant 2 are located at the balance. Tare a piece of weighing paper on the balance and obtain 0.25 g ± 0.05 g of either Reactant 1 or Reactant 2. Using a second piece of weighing paper, obtain enough of the other reactant so that the total mass of Reactant 1 and Reactant 2 is 1.00 g ± 0.1 g.

3. Record the mass of Reactant 1 [Data Sheet].

4. Record the mass of Reactant 2 [Data Sheet].

5. Obtain two clean, dry Pyrex test tubes from your drawer.

6. Carefully transfer Reactant 1 into a test tube and add ~ 4 mL distilled water.

7. Carefully transfer Reactant 2 into a second clean, dry test tube and add ~ 4 mL distilled water.

8. Remove the watch glass from the beaker. Gently heat both test tubes in the water bath to dissolve the solids. Use your glass stirring rod to occasionally stir each solid, but be sure to clean it off in between.

9. As soon as the solids dissolve in the test tube, remove the heat and place the test tube in the test tube rack. Note: Do NOT allow the mixture to get too hot, avoid boiling.

10. Obtain a piece of filter paper and label it with your name and lab section.

11. Measure its mass and record the mass [Data Sheet].

12. Fold the filter paper into quarters. Open one fold to form a cone, place it in a funnel and seal the filter (wait to seal the paper until you are ready to filter) into place with a small amount of distilled water. Place a 25 mL Erlenmeyer flask beneath the funnel.

13. Add the contents of the test tube containing Reactant 2 into the test tube containing Reactant 1. A precipitate will form immediately.

14. Add ~ 1 mL warm deionized water from the water bath to the empty test tube, swirl and add this liquid to the test tube containing the precipitate.

15. Stir the contents of this test tube with a glass stirring rod – use a distilled water bottle to squirt water on the stirring rod as you remove it from the solution.

16. Filter the mixture through the funnel. The most efficient approach to filtering is to pour almost all of the liquid into the funnel first, leaving the solid behind in the test tube. Transfer the solid to the filter paper using a rubber policeman.

17. Wash any remaining precipitate into the funnel with a 2 mL portion of warm deionized water from water bath.

18. Wash the precipitate with two 2 mL portions of warm deionized water from water bath.

19. Save the filtrate (the liquid) for step 25.

20. Label and mass a clean watch glass [Data Sheet].

21. Carefully remove the filter paper and precipitate, lay them on your watch glass. Place these in the oven to dry overnight.

22. You will need to return to mass your product + filter paper + watch glass the following day. [Data Sheet]. Go to step 25.

23. When you return to measure the mass of your product + filter paper + watch glass allow them to cool for 5 minutes and determine their mass [Data Sheet].

24. Place the product + filter paper + watch glass in the oven for another 5 minutes, cool and reweigh. If the mass is within 0.01 g of the previous mass, record this mass [Data Sheet]. Otherwise repeat the heating, cooling and weighing a third time.

Determining the Limiting Reactant

25. Place 5 mL each (the filtrate from step 19) into two different test tubes.

26. To one of the test tubes containing the filtrate from Step 19, add 4 drops 0.10 M BaCl2. Record your observations [Data and Online Report Sheet].

27. To the other test tube containing the filtrate from Step 19, add 4 drops 0.10 M Na3PO4. Record your observations [Data and Online Report Sheet].

28. Based on these observations, identify the limiting reactant [Online Report Sheet].

Identity of Reactant 1 and Reactant 2

29. Reactant 1 is ________________ [Online Report Sheet].

30. Reactant 2 is ________________ [Online Report Sheet].

Theoretical Yield If Reactant 1 is BaCl2

31. Assume that Reactant 1 is BaCl2 and Reactant 2 is Na3PO4·12H2O. Calculate the mass of Ba3(PO4)2 that can be produced from the starting mass of BaCl2 [Online Report Sheet].

32. Calculate the mass of Ba3(PO4)2 that can be produced from the starting mass of Na3PO4·12H2O [Online Report Sheet].

Theoretical Yield If Reactant 1 is Na3PO4

33. Assume that Reactant 1 is Na3PO4·12H2O and Reactant 2 is BaCl2. Calculate the mass of Ba3(PO4)2 that can be produced from the starting mass of BaCl2 [Online Report Sheet].

34. Calculate the mass of Ba3(PO4)2 that can be produced from the starting mass of Na3PO4·12H2O [Online Report Sheet].

Theoretical and Percent Yield

35. Based on the theoretical yield calculations and identity of the limiting reagent, what is the theoretical yield of Ba3(PO4)2 [Online Report Sheet]?

36. What is the actual yield of Ba3(PO4)2 [Online Report Sheet]?

37. What is the percent yield of Ba3(PO4)2 [Online Report Sheet]?

Waste Disposal

• Place the filtrate (liquid) in the waste container labeled Aqueous Waste.

• Place the solid Barium phosphate in the waste container labeled Solid Ba3(PO4)2.

Lab Report: Once you have turned in your Instructor Data Sheet, lab attendance will be entered and lab attendees will be permitted to access the online data / calculation submission part of the lab report (click on Exp #4 – Stoichiometry and Limiting Reagents). The lab program will take you in order to each calculation. If there is an error, you will be given additional submissions (the number and penalty to be determined by your instructor) to correct your calculation.

Post-Lab Questions: The questions for this lab can be found at . Do Not Wait Until The Last Minute!!!! Computer Problems and Internet Unavailability Happen, But Deadlines Will Not Be Extended!!

Late Submission: Late submission of the lab data / calculations is permitted with the following penalties: - 10 points for submissions up to 1 day late, - 20 points for submissions up to 2 days late.

Experiment 4

Student Data Sheet

|Mass of Reactant 1 | |g |

|Mass of Reactant 2 | |g |

|Mass of filter paper | |g |

|Mass of watch glass | |g |

|Mass of Ba3(PO4)2 + filter paper + watch glass (initial weighing) | |g |

|Mass of Ba3(PO4)2 + filter paper + watch glass (final weighing) | |g |

| | | |

|Observations: 0.10 M BaCl2 + filtrate | | |

|Observations: 0.10 M Na3PO4 + filtrate | | |

| | | |

Experiment 4

Instructor Data Sheet

|Mass of Reactant 1 | |g |

|Mass of Reactant 2 | |g |

|Mass of filter paper | |g |

|Mass of watch glass | |g |

|Mass of Ba3(PO4)2 + filter paper + watch glass (initial weighing) | |g |

|Mass of Ba3(PO4)2 + filter paper + watch glass (final weighing) | |g |

| | | |

|Observations: 0.10 M BaCl2 + filtrate | | |

|Observations: 0.10 M Na3PO4 + filtrate | | |

| | | |

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EXPERIMENT

4

OBJECTIVE

APPARATUS

AND

CHEMICALS

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