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Finding the Ratio of Moles of Reactants in a Chemical Reaction

(adapted from Laboratory Experiments for AP Chemistry, Vonderbrink and Chem Fax by Flinn Scientific)

Each lab group will perform one of the experiments below and share their results with a PowerPoint Presentation.

Experiment One: Reaction of Sodium Hypochlorite with Sodium Thiosulfate in Sodium Hydroxide Solution

Objective: To use the method of continuous variations to determine the mole ratio of two reactants.

Introduction: First, solutions of the reactants are prepared in which the concentrations are known. Second, the solutions are mixed a number of times using different ratios of reactants. Third, some property of the reaction that depends on the amount of product formed or on the amount of reactant that remains is measured. This property may be the color intensity of a reactant or product, the mass of a precipitate that forms, the volume of gas evolved or the change of temperature. The volume of the resulting solution is kept constant, as well as the total number of moles of reactants. The optimum ratio, which is the stoichiometric ratio of the equation, should consume the greatest amount of reactants, form the greatest amount of product and generate the most change in the property being measured. For this experiment the property being measured is temperature change.

Procedure:

1. Commercial bleach contains NaClO (Sodium Hypochlorite). Most are listed at 5.25% NaClO by mass. This is approximately 0.67 M. Carefully prepare 0.50 M NaClO solution by diluting 372 mL of bleach to a total solution volume of 500 mL. The bleach should be fresh. We will later determine the molarity of our bleach in a separate experiment. Stir to mix well.

2. Prepare sodium thiosulfate, 0.50M in sodium hydroxide, 0.20 M. Dissolve 62 g of Na2S2O3 . 5 H2O in approximately 200 mL of distilled water. Add 4 g of NaOH and stir until dissolved. Dilute to 500 mL.

3. Bring both solutions to the same temperature by placing both solutions in a room temperature water bath.

4. Measure the temperature of both solutions. Record. Use the same thermometer for both measurements. Be sure to clean and dry in between readings.

5. Pour 5.0 mL of the diluted bleach into a Styrofoam cup, and then add 45.0 mL of the sodium thiosulfate solution. Stir with a thermometer and record the highest temperature reached by the mixture. Pour the solution our, rinse the cup and thermometer, and repeat the process using a different ratio of the two substances, always keeping the total volume at 50.0 mL. Continue testing various ratios until you have at least three measurements on each side of the one that gave the maximum temperature.

6. Plot your data. Place the change in temperature on the y axis and label the x axis with ml NaClO and with mL of sodium thiosulfate. Draw two straight lines that best fit your data, and determine where they intersect. Be sure to include the points at the 0:50 mL and 50:0 mL ratios. Find the stoichiometric mole ratio of reactants from the point of intersection on your graph.

7. In your laboratory report include answers to the following questions.

a. Explain how this method allows you to find the mole ratio of reactants.

b. Why must you keep a constant volume of reactants?

c. Is it necessary that the concentrations of the two solutions be the same?

d. What is meant by the term limiting reagent?

e. Does the measurement of temperature of the measurement of volume limit the precision of your data? Explain.

f. Which reactant is the limiting reagent along the upward sloping line of your graph? Which is the limiting reagent along the downward sloping line?

g. Why is it more accurate to use the point of intersection of the two lines to find the mole ratio rather than the ratio associated with the greatest temperature change?

h. If the two solutions used are not at the same initial temperature, a correction must be made to find the correct change in temperature. How should this be done?

Experiment Two: Reaction of Sodium Hypochlorite with Potassium Iodide in Sodium Hydroxide Solution

Objective: To use the method of continuous variations to determine the mole ratio of two reactants.

Introduction: First, solutions of the reactants are prepared in which the concentrations are known. Second, the solutions are mixed a number of times using different ratios of reactants. Third, some property of the reaction that depends on the amount of product formed or on the amount of reactant that remains is measured. This property may be the color intensity of a reactant or product, the mass of a precipitate that forms, the volume of gas evolved or the change of temperature. The volume of the resulting solution is kept constant, as well as the total number of moles of reactants. The optimum ratio, which is the stoichiometric ratio of the equation, should consume the greatest amount of reactants, form the greatest amount of product and generate the most change in the property being measured. For this experiment the property being measured is temperature change.

Procedure:

1. Commercial bleach contains NaClO (Sodium Hypochlorite). Most are listed at 5.25% NaClO by mass. This is approximately 0.67 M. Carefully prepare 0.50 M NaClO solution by diluting 372 mL of bleach to a total solution volume of 500 mL. The bleach should be fresh. We will later determine the molarity of our bleach in a separate experiment. Stir to mix well.

2. Prepare potassium iodide, 0.50M in sodium hydroxide, 0.20 M. Dissolve 41.5 g of potassium in approximately 200 mL of distilled water. Add 4 g of NaOH and stir until dissolved. Dilute to 500 mL.

3. Bring both solutions to the same temperature by placing both solutions in a room temperature water bath.

4. Measure the temperature of both solutions. Record. Use the same thermometer for both measurements. Be sure to clean and dry in between readings.

5. Pour 5.0 mL of the diluted bleach into a Styrofoam cup, and then add 45.0 mL of the potassium iodide solution. Stir with a thermometer and record the highest temperature reached by the mixture. Pour the solution out, rinse the cup and thermometer, and repeat the process using a different ratio of the two substances, always keeping the total volume at 50.0 mL. Continue testing various ratios until you have at least three measurements on each side of the one that gave the maximum temperature.

6. Plot your data. Place the change in temperature on the y axis and label the x axis with ml NaClO and with mL of potassium. Draw two straight lines that best fit your data, and determine where they intersect. Be sure to include the points at the 0:50 mL and 50:0 mL ratios. Find the stoichiometric mole ratio of reactants from the point of intersection on your graph.

7. In your laboratory report include answers to the following questions.

a. Explain how this method allows you to find the mole ratio of reactants.

b. Why must you keep a constant volume of reactants?

c. Is it necessary that the concentrations of the two solutions be the same?

d. What is meant by the term limiting reagent?

e. Does the measurement of temperature of the measurement of volume limit the precision of your data? Explain.

f. Which reactant is the limiting reagent along the upward sloping line of your graph? Which is the limiting reagent along the downward sloping line?

g. Why is it more accurate to use the point of intersection of the two lines to find the mole ratio rather than the ratio associated with the greatest temperature change?

h. If the two solutions used are not at the same initial temperature, a correction must be made to find the correct change in temperature. How should this be done?

Experiment Three: Reaction of Sodium Hypochlorite with sodium sulfite in Sodium Hydroxide Solution

Objective: To use the method of continuous variations to determine the mole ratio of two reactants.

Introduction: First, solutions of the reactants are prepared in which the concentrations are known. Second, the solutions are mixed a number of times using different ratios of reactants. Third, some property of the reaction that depends on the amount of product formed or on the amount of reactant that remains is measured. This property may be the color intensity of a reactant or product, the mass of a precipitate that forms, the volume of gas evolved or the change of temperature. The volume of the resulting solution is kept constant, as well as the total number of moles of reactants. The optimum ratio, which is the stoichiometric ratio of the equation, should consume the greatest amount of reactants, form the greatest amount of product and generate the most change in the property being measured. For this experiment the property being measured is temperature change.

Procedure:

1. Commercial bleach contains NaClO (Sodium Hypochlorite). Most are listed at 5.25% NaClO by mass. This is approximately 0.67 M. Carefully prepare 0.50 M NaClO solution by diluting 372 mL of bleach to a total solution volume of 500 mL. The bleach should be fresh. We will later determine the molarity of our bleach in a separate experiment. Stir to mix well.

2. Prepare sulfur sulfite, 0.50M in sodium hydroxide, 0.20 M. Dissolve 31.5 g of sodium sulfite in approximately 200 mL of distilled water. Add 4 g of NaOH and stir until dissolved. Dilute to 500 mL.

3. Bring both solutions to the same temperature by placing both solutions in a room temperature water bath.

4. Measure the temperature of both solutions. Record. Use the same thermometer for both measurements. Be sure to clean and dry in between readings.

5. Pour 5.0 mL of the diluted bleach into a Styrofoam cup, and then add 45.0 mL of the sulfur sulfite solution. Stir with a thermometer and record the highest temperature reached by the mixture. Pour the solution out, rinse the cup and thermometer, and repeat the process using a different ratio of the two substances, always keeping the total volume at 50.0 mL. Continue testing various ratios until you have at least three measurements on each side of the one that gave the maximum temperature.

6. Plot your data. Place the change in temperature on the y axis and label the x axis with ml NaClO and with mL of sodium thiosulfate. Draw two straight lines that best fit your data, and determine where they intersect. Be sure to include the points at the 0:50 mL and 50:0 mL ratios. Find the stoichiometric mole ratio of reactants from the point of intersection on your graph.

7. In your laboratory report include answers to the following questions.

a. Explain how this method allows you to find the mole ratio of reactants.

b. Why must you keep a constant volume of reactants?

c. Is it necessary that the concentrations of the two solutions be the same?

d. What is meant by the term limiting reagent?

e. Does the measurement of temperature of the measurement of volume limit the precision of your data? Explain.

f. Which reactant is the limiting reagent along the upward sloping line of your graph? Which is the limiting reagent along the downward sloping line?

g. Why is it more accurate to use the point of intersection of the two lines to find the mole ratio rather than the ratio associated with the greatest temperature change?

h. If the two solutions used are not at the same initial temperature, a correction must be made to find the correct change in temperature. How should this be done?

Experiment Four: Reaction of Ferric Nitrate with Sodium Hydroxide

Objective: To use the method of continuous variations to determine the mole ratio of two reactants.

Introduction: First, solutions of the reactants are prepared in which the concentrations are known. Second, the solutions are mixed a number of times using different ratios of reactants. Third, some property of the reaction that depends on the amount of product formed or on the amount of reactant that remains is measured. This property may be the color intensity of a reactant or product, the mass of a precipitate that forms, the volume of gas evolved or the change of temperature. The volume of the resulting solution is kept constant, as well as the total number of moles of reactants. The optimum ratio, which is the stoichiometric ratio of the equation, should consume the greatest amount of reactants, form the greatest amount of product and generate the most change in the property being measured. For this experiment the property being measured is mass of precipitate formed.

Procedure:

1. The iron in iron nitrate acts as a Lewis acid in solution. When combined with sodium hydroxide, the precipitate formed remains insoluble as long as iron nitrate is not in excess of the stoichiometric mole ratio. When iron nitrate is in excess, the precipitate will begin to dissolve. The larger the excess, the greater the amount of precipitate that dissolves. Your plot of the data will reflect this.

2. Prepare 500 mL of 0.01 M ferric nitrate solution and 500 mL of 0.01 M sodium hydroxide solution.

3. Label seven 100- mL graduated cylinders 1-7.

4. Using a clean, 50-mL graduated cylinder, add the appropriate volume of ferric nitrate solution to each 100 mL graduated cylinder, as shown in the Table.

5. Use a second 50-mL graduated cylinder to add the appropriate volume of sodium hydroxide solution to each 100 mL graduated cylinder as shown in the table.

|Cylinder |1 |2 |3 |4 |5 |6 |7 |

|Fe(NO3)3, 0.1 M, mL|5 |10 |12 |15 |17 |20 |24 |

|NaOH |55 |50 |48 |45 |43 |40 |36 |

|0.1 M, mL | | | | | | | |

|Fe:OH ratio | | | | | | | |

|Volume of ppt | | | | | | | |

6. Use a large stirring rod to thoroughly mix the reactants. Observe the signs of chemical reaction in each cylinder. Mixing the yellow-orange solution of ferric nitrate with the colorless sodium hydroxide solution gives a rust-colored precipitate and a pale yellow supernatant.

7. Let the reaction mixtures sit undisturbed for at least 10 minutes to allow the precipitates to settle.

8. After the precipitates have settled, record the volume of precipitate in each graduated cylinder.

9. On graph paper, plot the milliliters of Fe(NO3)3 vs. volume of precipitate. Draw the best-fit line through the ascending data, and a smooth curve through the descending data. Determine their intersection point. From the point of intersection, determine the stoichiometric mole ratio for each reaction. Write out the correct balanced equation for each reaction.

10. In your laboratory report include answers to the following questions.

a. Explain how this method allows you to find the mole ratio of reactants.

b. Why must you keep a constant volume of reactants?

c. Is it necessary that the concentrations of the two solutions be the same?

d. What is meant by the term limiting reagent?

e. Which reactant is the limiting reagent along the upward sloping line of your graph? Which is the limiting reagent along the downward sloping line?

f. Why is it more accurate to use the point of intersection of the two lines to find the mole ratio rather than the ratio associated with the greatest temperature change?

Experiment Five: Reaction of Cupric Chloride with Sodium Phosphate

Objective: To use the method of continuous variations to determine the mole ratio of two reactants.

Introduction: First, solutions of the reactants are prepared in which the concentrations are known. Second, the solutions are mixed a number of times using different ratios of reactants. Third, some property of the reaction that depends on the amount of product formed or on the amount of reactant that remains is measured. This property may be the color intensity of a reactant or product, the mass of a precipitate that forms, the volume of gas evolved or the change of temperature. The volume of the resulting solution is kept constant, as well as the total number of moles of reactants. The optimum ratio, which is the stoichiometric ratio of the equation, should consume the greatest amount of reactants, form the greatest amount of product and generate the most change in the property being measured. For this experiment the property being measured is mass of precipitate formed.

Procedure:

1. Prepare 500 mL of 0.05 M cupric chloride solution.

2. Prepare 500 mL of 0.05 M sodium phosphate (Na3PO4) solution.

3. Label seven 100- mL graduated cylinders 1-7.

4. Using a clean, 50-mL graduated cylinder, add the appropriate volume of cupric chloride solution to each 100 mL graduated cylinder, as shown in the Table.

5. Use a second 50-mL graduated cylinder to add the appropriate volume of sodium phosphate solution to each 100 mL graduated cylinder as shown in the table.

|Cylinder |1 |2 |3 |4 |5 |6 |7 |

|CuCl2, 0.05M, mL |10 |20 |24 |30 |36 |40 |50 |

|Na3PO4, 0.05M, mL |50 |40 |36 |30 |24 |20 |10 |

|Vol of ppt | | | | | | | |

6. Use a large stirring rod to thoroughly mix the reactants. Observe the signs of chemical reaction in each cylinder. (Mixing the blue solution of cupric chloride with the colorless sodium phosphate solution gives an aqua-colored precipitate and a colorless supernantant)

7. Let the reaction mixtures sit undisturbed for at least 10 minutes to allow the precipitates to settle.

8. After the precipitates have settled, record the volume of precipitate in each graduated cylinder.

9. On graph paper, plot the milliliters of CuCl2 vs. volume of precipitate.

10. Draw the best-fit line through the ascending data, and the best-fit line through the descending data. Determine their intersection point. From the point of intersection, determine the stoichiometric mole ratio for each reaction. Write out the correct balanced equation for each reaction.

11. In your laboratory report include answers to the following questions.

a. Explain how this method allows you to find the mole ratio of reactants.

b. Why must you keep a constant volume of reactants?

c. Is it necessary that the concentrations of the two solutions be the same?

d. What is meant by the term limiting reagent?

e. Which reactant is the limiting reagent along the upward sloping line of your graph? Which is the limiting reagent along the downward sloping line?

f. Why is it more accurate to use the point of intersection of the two lines to find the mole ratio rather than the ratio associated with the greatest temperature change?

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