Combining Stoichiometry

Description

Using 12-wells strips, different amounts of equimolar solutions are mixed. The volume of solid produced is observed. The maximum volume of solid is produced when the ratio of the combining volumes equals the ratio of coefficients from the balanced chemical equation.

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The chemicals are toxic. The alternative chemicals are less toxic. Trisodium phosphate is sufficiently basic to corrode eye tissue.

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Do not ingest the chemicals. Wear eye protection.

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1. All solutions are 0.1 M solutions.
2. Add 4 drops of distilled water to each of 8 wells in a clean 12-well strip. Add 1 drop of potassium iodide solution to the first well, 2 drops to the second, and so forth. End by adding 8 drops of potassium iodide to the 8th well.
3. Add 8 drops of lead nitrate solution to the first well, 7 drops to the second, and so forth. End by adding 1 drop of lead nitrate to the 8th well.
4. Mix each well with a clean toothpick.
5. (You may replace potassium iodide with sodium oleate and lead nitrate with calcium nitrate in the above procedure.)
6. Add 4 drops of distilled water to each of 9 wells in a clean 12-well strip. Add 1 drop of calcium nitrate solution to the first well, 2 drops to the second, and so forth. End by adding 9 drops of calcium nitrate to the 9th well.
7. Add 9 drops of sodium oxalate solution to the first well, 8 drops to the second, and so forth. End by adding 1 drop of sodium oxalate to the 9th well.
8. Mix each well with a clean toothpick.
9. After time has permitted the solids to settle, compare the volumes of solid in the wells with one another. Examine the strips at eye level. Note the well in which the maximum amount of yellow lead iodide (white, gelatinous calcium oleate) is observed. Note the well in which the maximum amount of white calcium oxalate is observed.
10. Flick the 12-well strips into a central container.
11. Wash the 12-well strips with cotton swabs.
12. (If the formation of calcium phosphate is studied, follow the same procedure as for calcium oxalate. Warming the calcium phospate helps the precipitate settle.)

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Name ___________________________ Class ________

Teacher__________________________

DoChem 071 Combining Stoichiometry

Watch the movie and answer the questions.

Each drop of starting reagents contains the same number of molecules of reagent which will be called d.

1. Write equations for each well for the reaction of Pb²⁺ and I^-. The equation for well 1 is given.

   Well-1	1d I- + 8d Pb2+ --> 0.5d PbI2 + 7.5d Pb2+
   Well-2
   Well-3
   Well-4
   Well-5
   Well-6
   Well-7
   Well-8

   Use the picture after settling to answer these questions.

2. Which well has the most yellow precipitate?
   1. How many drops of Pb²⁺ were in that well?
   2. How many drops of I^- were in that well?
   3. What is the limiting reagent in well-2?
   4. How many drops of the other reagent reacted in well-2?
   5. How many drops of the other reagent were excess reagent in well-2?
   6. What is the limiting reagent in well-7?
   7. Write a net ionic equation for the reaction.
3. Which well has the most white precipitate?
   1. How many drops of Ca²⁺ were in that well?
   2. How many drops of C₂O₄^2- were in that well?
   3. What is the limiting reagent in well-2?
   4. How many drops of the other reagent reacted in well-2?
   5. How many drops of the other reagent were excess reagent in well-2?
   6. What is the limiting reagent in well-7?
   7. Write a net ionic equation for the reaction.

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Purpose

To illustrate the combining stoichiometries associated with the formation of several insoluble salts.

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• 2 12-well strip
• 4 mL 0.1 M sodium oxalate (1.34 g Na₂C₂O₄/ 100 mL soln; stored in thin-stem transfer pipet)
• 4 mL 0.1 M calcium nitrate (2.18 g Ca(NO₃)₂/ 100 mL soln; stored in thin-stem transfer pipet)
• 4 mL 0.1 M lead nitrate (0.66 g Pb(NO₃)₂/ 20 mL soln; stored in thin-stem transfer pipet)
• 4 mL 0.1 M potassium iodide (1.66 g KI/ 100 mL soln; stored in thin-stem transfer pipet)
• distilled water
• cotton swabs
• toothpicks
• Alternatives:
  • 4 mL 0.1 M sodium phosphate (1.64 g Na₃PO₄/ 100 mL soln; stored in thin-stem transfer pipets)
  • 4 mL 0.1 M sodium oleate (3.04 g NaC₁₇H₃₃COO/ 100 mL soln; stored in thin-stem transfer pipets)
  • plastic pipet tips

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• Put out only the amount of lead nitrate solution that will be used. Make up the minimum amount; store any excess for later use.
• Problems with disposal may be circumvented by using 0.1 M calcium nitrate with: 0.1 M sodium oxalate (Na₂C₂O₄); 0.1 M trisodium phosphate (Na₃PO₄); and 0.1 M sodium oleate (NaC_¹7H₃₃COO).
• Calibration of the Beral pipet dispensers may be needed. Choose pipets that give similar rates of delivery (i.e., similar drops/mL). You may use a separate plastic pipet tip attached to a Beral pipet in order to avoid the calibration problem.

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Teacher preparation: 20 minutes

Class Time: 30 minutes (Mix in early part of the period and observe settled precipitates at the end of the period.)

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The chemicals are toxic. The alternative chemicals are less toxic. Trisodium phosphate is sufficiently basic to corrode eye tissue.

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• Flick the 12-well strips into one central collection beaker. Treat the solution with 1 M sulfuric acid. Wait one week. Filter. Discard the filtrate at the sink. Save the solid for disposal with heavy metal wastes.
• (The calcium compounds may be discarded at the sink.)

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• The combining stoichiometries of chemicals often present learning difficulties to beginning chemistry students. This experiment shows graphically one outcome of the idea of a limiting reagent.
  • In these reactions:
    • Ca(NO₃)₂(aq) + Na₂C₂O₄(aq) --> CaC₂O₄(s)+ 2NaNO₃(aq)
    • Pb(NO₃)₂(aq) + 2 KI(aq) --> PbI₂(s) + 2KNO₃(aq)
  • The first chemical equation has coefficients of 1 and 1. This implies a 1:1 ratio. The second chemical equation has coefficients of 1 and 2. This implies a 1:2 ratio.
• It is instructive for students to write the equations using the actual number of drops they use (5:5; 6:4; etc.), and then reduce these to the smallest whole numbers.

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1. Well-1	1d I- + 8d Pb2+ --> 0.5d PbI2 + 7.5d Pb2+
   Well-2	2d I- + 7d Pb2+ --> 1d PbI2 + 6d Pb2+
   Well-3	3d I- + 6d Pb2+ --> 1.5d PbI2 + 4.5d Pb2+
   Well-4	4d I- + 5d Pb2+ --> 2d PbI2 + 3d Pb2+
   Well-5	5d I- + 4d Pb2+ --> 2.5d PbI2 + 1.5d Pb2+
   Well-6	6d I- + 3d Pb2+ --> 3d PbI2 + 0d Pb2+
   Well-7	7d I- + 2d Pb2+ --> 2d PbI2 + 3d I-
   Well-8	8d I- + 1d Pb2+ --> 1d PbI2 + 6d I-

2. 6
   1. 3 drops Pb²⁺
   2. 6 drops I^-
   3. I^-
   4. 1 drop
   5. 6 drops
   6. Pb²⁺
   7. 2 I^- + Pb²⁺ --> PbI₂
3. 5
   1. 5 drops Ca²⁺
   2. 5 drops C₂O₄²⁺
   3. Ca²⁺
   4. 2 drops
   5. 6 drops
   6. C₂O₄^2-
   7. Ca²⁺ + C₂O₄^2- --> CaC₂O₄

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• precipitate
• limiting reagent
• solution stoichiometry
• chemical equations

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