Stoichiometric Double Check

Description

A reaction in which it is possible to determine the masses of several reactants and products is conducted. A gaseous product is determined by mass lost from solution. One reactant mass and one product mass are determined directly.

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• Students must wear eye protection; there is a possibility of spattering during the addition of the acid to the bicarbonate. Add the acid slowly so that the gas release is controlled. Control the amount of acid used. Wash spills immediately. Wear aprons and old clothing.
• The test tube removed from the oven will be hot for quite a while and must not be touched or weighed until cool.

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The reaction is: NaHCO₃(c) + HCl(aq) --> CO₂(g) + H₂O(l) + NaCl(aq)

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1. Lab Hint--Be sure that all of the initial apparatus weighed must be present on the pan when a mass is taken. Also, a major source of error is the spraying of material out as the bicarbonate effervesces.
2. Place a plastic weighing pan on the balance.
3. Prepare a wire from which to hang a test tube. Wrap a loop of wire around the top of a 16-mm test tube near the lip. Shape the other end of the wire into a loop. This loop should be about 3-4 cm from the tube. Determine the mass of a test tube by hanging it from the loop of wire attached to the balance hook.
4. Remove the tube from the balance.
5. Add approximately 2.5 g of sodium bicarbonate to the test tube
6. Weigh the sodium bicarbonate and test tube.
7. Measure 8.0 mL of 6.0 M HCl and place in a beaker containing a dropping pipet.
8. Determine the total mass of the following on the balance pan at one time: 8.0 mL of 6.0 M HCl in a 50-mL beaker; the test tube and sodium bicarbonate; and a dropping pipet.
9. Remove the dropping pipet and slowly add a dropperful of HCl to the bicarbonate in the test tube.
10. Note the appearance of any reaction. Note any change in mass.
11. Add more acid until no further bubbling occurs or until the entire sample of acid is used up.
12. Replace the dropping pipet on the balance and determine the total mass of the entire apparatus (pipet, beaker, and test tube).
13. Place the test tube in a drying oven set to 85°C (or alternate drying apparatus), and allow to dry overnight.
14. Remove the test tube. Glassware in the drying oven will be hot. Use care in removing the test tube.
15. Allow it to cool, and determine its mass.
16. Clean the test tube at the sink.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 021 Stoichiometric Double Check

1. Write an equation for the reaction that occurs between hydrochloric acid and sodium bicarbonate.
2. Predict the effect on the amount of NaCl recovered from the experiment if some material spattered out of the test tube while the gas was being liberated.
3. How will you be able to tell when the bicarbonate has completely reacted?
4. Which reactant is considered the limiting reactant in this experiment?

Mass of test tube and sodium bicarbonate =

Mass of empty test tube =

Mass of sodium bicarbonate =

Initial mass of total apparatus =

Final mass of total apparatus =

Mass of carbon dioxide formed =

Mass of test tube and sodium chloride =

Mass of empty test tube (same as above) =

Mass of sodium chloride formed =

________________________________________________

Predict the effect of replacing dilute HCl with dilute H₂SO₄.

1. List several possible sources of error in your procedure indicating whether the final amount of solid sodium chloride formed would be increased or decreased as a result.
2. Determine the number of moles of each of the following that were produced or consumed in the reaction:
   1. NaCl
   2. CO₂
   3. NaHCO₃
3. Calculate the following mole ratios from your data (use whole numbers by rounding)
   1. NaCl : CO₂
   2. NaHCO₃ : NaCl
   3. NaHCO₃ : CO₂
4. How do the whole number ratios above compare to the coefficient ratios in the balanced equation?
5. Why did we not attempt to determine experimentally the amount of water produced?
6. From your data, calculate the amount of water that was formed.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 021 Stoichiometric Double Check

Answer these questions before watching the movie.

1. Write an equation for the reaction that occurs between hydrochloric acid and sodium bicarbonate.
2. Predict the effect on the amount of NaCl recovered from the experiment if some material spattered out of the test tube while the gas was being liberated.
3. How will you be able to tell when the bicarbonate has completely reacted?
4. Which reactant is considered the limiting reactant in this experiment?

Watch the movie.

Use the following data for your calculations if you are doing a makeup lab.

Mass of test tube and sodium bicarbonate = 22.09 g

Mass of empty test tube = 19.70 g

Mass of sodium bicarbonate = ______ g

Initial mass of total apparatus = 64.08 g

Final mass of total apparatus = 62.64 g

Mass of carbon dioxide formed =_________ g

Mass of test tube and sodium chloride = 22.29 g

Mass of empty test tube (same as above) = 19.70 g

Mass of sodium chloride formed = __________ g

Predict the effect of replacing dilute HCl with dilute H₂SO₄.

1. List several possible sources of error in your procedure indicating whether the final amount of solid sodium chloride formed would be increased or decreased as a result.
2. Determine the number of moles of each of the following that were produced or consumed in the reaction:
   1. NaCl
   2. CO₂
   3. NaHCO₃
3. Calculate the following mole ratios from your data (use whole numbers by rounding)
   1. NaCl : CO₂
   2. NaHCO₃ : NaCl
   3. NaHCO₃ : CO₂
4. How do the whole number ratios above compare to the coefficient ratios in the balanced equation?
5. Why did we not attempt to determine experimentally the amount of water produced?
6. From your data, calculate the amount of water that was formed.

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Purpose

• To perform a metathesis reaction.
• To determine the quantitative relationship between reactants and products in a chemical reaction.

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(per 10 students working in pairs)

• 12.5 g sodium bicarbonate
• 5 16-mm test tube
• 5 wire test tube hanger
• 40 mL 6.0 M HCl
• 5 50-mL beaker
• 5 centigram balance
• 5 weighing pan
• 5 10-mL graduated cylinder
• 5 hot water bath
• 5 dropping pipet
• oven or sand bath or hot plate or lamp

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• Be sure that students understand that all of the initial apparatus weighed must be present on the pan when a mass is taken. Also warn that a major source of error is the spraying of material out as the bicarbonate effervesces.
• The loss of CO₂ is less than expected due to water solubility.
• The apparent production of NaCl will probably be larger than the calculated stoichiometric amount due to trapped water that is not completely removed from the sample.
• Use a plastic weighing pan to protect the balance pan.

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Teacher preparation: 20 min

Class Time: 2 periods

• 45-50 minutes first period
• 10 minutes second period

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A gas will be evolved rapidly. The 6 M HCl is very hazardous. Fumes during the evaporation present a danger.

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• Students must wear eye protection; there is a possibility of spattering during the addition of the acid to the bicarbonate. Add the acid slowly so that the gas release is controlled. Control the amount of acid used. Wash spills immediately. Wear aprons and old clothing.
• Warn students that the test tube removed from the oven will be hot for quite a while and must not be touched or weighed until cool.
• If many tubes are used, ventilation of the fumes from the oven may be a problem. Place the oven in an operating fume hood, or use one of the recommended alternative heat sources in a fume hood.

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All materials can be safely disposed of at the sink.

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• Students have difficulty visualizing what happens in a chemical reaction that they are observing in the lab. This exercise allows them to see the reaction and to measure the mass change that results. The reaction is:

  NaHCO₃(c) + HCl(aq) --> CO₂(g) + H₂O(l) + NaCl(aq)

• Furthermore, by isolating the NaCl through evaporation, students obtain data to study the stoichiometric relationships between the products and reactants.

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Set Questions:

1. Write an equation for the reaction that occurs between hydrochloric acid and sodium bicarbonate.
2. Predict the effect on the amount of NaCl recovered from the experiment if some material spattered out of the test tube while the gas was being liberated.
3. How will you be able to tell when the bicarbonate has completely reacted?
4. Which reactant is considered the limiting reactant in this experiment?

Answers to Set Questions:

1. NaHCO₃ + HCl --> NaCl + CO₂ + H₂O
2. Less NaCl is recovered than would be produced by the reaction.
3. An additional drop of acid no longer causes bubbling.
4. NaHCO₃ will be completely consumed in the reaction.

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Presentation Question:

• Predict the effect of replacing dilute HCl with dilute H₂SO₄.
  • The salt weighed at the end will be Na₂SO₄ instead of NaCl. Excess H₂SO₄ is much more difficult to remove than excess HCl, however. Vinegar might be a safer alternative to HCl, but acetates are deliquescent.

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Mass of test tube and sodium bicarbonate = 22.09 g

Mass of empty test tube = 19.70 g

Mass of sodium bicarbonate = 2.39 g

Initial mass of total apparatus = 64.08 g

Final mass of total apparatus = 62.64 g

Mass of carbon dioxide formed = 1.44 g

Mass of test tube and sodium chloride = 22.29 g

Mass of empty test tube (same as above) = 19.70 g

Mass of sodium chloride formed = 2.59 g

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Closure Questions:

1. List several possible sources of error in your procedure indicating whether the final amount of solid sodium chloride formed would be increased or decreased as a result.
2. Determine the number of moles of each of the following that were produced or consumed in the reaction:
   1. NaCl
   2. CO₂
   3. NaHCO₃
3. Calculate the following mole ratios from your data (use whole numbers by rounding)
   1. NaCl : CO₂
   2. NaHCO₃ : NaCl
   3. NaHCO₃ : CO₂
4. How do the whole number ratios above compare to the coefficient ratios in the balanced equation?
5. Why did we not attempt to determine experimentally the amount of water produced?
6. From your data, calculate the amount of water that was formed.

Answers to Closure Questions: (based on sample data )

1. Spattering from the tube will lower the amount of NaCl; faulty weighing can increase or decrease experimental amount versus actual amount of NaCl; adding insufficient HCl will lead to too little NaCl; and not removing all water from NaCl will lead to too high an NaCl value.
2. 1. 2.59 g NaCl x (1 mol NaCl/ 58.5 g NaCl) =

      0.0443 mol NaCl

   2. 1.44 g CO₂ x (1 mol CO₂/ 44.0 g CO₂) =

      0.0327 mol CO₂

   3. 2.39 g NaHCO₃ x (1 mol NaHCO₃ / 84.0 g NaHCO₃) = 0.0284 mol NaHCO₃
3. 1. (0.0443 mol NaCl/ 0.0327 mol CO₂) = 1.35 ≅ 1
   2. (0.0284 mol NaHCO₃ / 0.0443 mol NaCl) = 0.64 ≅ 1
   3. (0.0284 mol NaHCO₃ / 0.0327 mol CO₂)= 0.87 ≅ 1
4. In each case, the predicted ratio is 1.00.
5. Additional water came from the HCl solution.
6. NaHCO₃ + HCl --> NaCl + CO₂ + H₂O

   (0.0284 mol NaHCO₃) x (1 mol H₂O/1 mol NaHCO₃ )x

   (18.0 g H₂O/1 mol H₂O) = 0.511 g H₂O

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• mole
• mole ratio
• balanced equation
• replacement reaction
• double replacement reaction
• acid-base reaction
• metastasis

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