Experiment Report: Stoichiometry of a Precipitation Reaction

Abstract

The purpose of the lab, "Stoichiometry of a Precipitation Reaction," was to calculate the amount of a second reactant required to react with the first reactant. Stoichiometry was used to determine the necessary amount of the second reactant for a complete reaction. The calculated amount of the second reactant, Na2CO3, was found to be 0.72 grams. When the two reactants, CaCl2·2H2O and Na2CO3, were mixed, a precipitate of calcium carbonate (CaCO3) formed. The resulting precipitate, when dried and weighed, measured 1.

6 grams.

Experiment and Observations

In the course of this experiment, I followed a series of steps to carry out the precipitation reaction. The key steps and observations were as follows:

1. Retrieve all necessary equipment from the lab kit.
2. Weigh out 1.0 gram of CaCl2·2H2O on a digital scale and place it in a 100 mL beaker.
3. Add 25 mL of distilled water to the beaker containing CaCl2·2H2O and stir the mixture.
4. Calculate the amount of the second reactant, Na2CO3, needed to make a precipitate of calcium carbonate using stoichiometry.
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   The calculated amount was 0.72 grams.

5. Weigh out 0.7 grams of Na2CO3 and add it to 25 mL of distilled water in a separate container.
6. Both reactants mixed separately with distilled water appeared clear.
7. Upon adding the Na2CO3 solution to the CaCl2·2H2O solution, the color of the mixture changed to a cloudy white.
8. Weigh and record the mass of the paper filter provided.
9. Pour the mixed solution into the paper filter to separate the precipitate (CaCO3) from the liquid.
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10. Dry the filter with the precipitate and weigh the mass of the dried precipitate, which was found to be 1.6 grams.

Calculations and Errors

The experiment involved several calculations and the determination of key values. Some potential sources of error in the experiment were identified, including:

1. Measurement of Distilled Water: Accurately measuring 25 mL of distilled water can be challenging using a graduated cylinder, leading to potential errors in the experiment.
2. Rounding: The need to round the calculated amount of Na2CO3 to the nearest tenth of a gram due to limitations in the digital scale, resulting in potential inaccuracies.
3. Residue on Glass: Using the beaker as a mixing container may have left some residue on the glass, which could affect the accuracy of measurements.

Discussion

"Stoichiometry of a Precipitation Reaction" was a laboratory experiment that differed from the previous experiments in that it required more extensive mathematical calculations to determine the quantities needed for a complete reaction between two compounds. While it involved complex calculations, it was similar to mathematical formulas in terms of dividing, converting, and canceling units. The experiment introduced me to the concept of stoichiometry, which involves using mathematical relationships to balance chemical reactions and calculate reactant quantities.

One notable aspect of the experiment was the unexpected visual transformation that occurred when the two clear solutions were mixed. Initially, I did not anticipate any visible changes because both solutions were clear. However, upon mixing, a precipitate of calcium carbonate (CaCO3) formed, giving the solution a cloudy white appearance. This unexpected reaction underscored the principle that in chemistry, even when solutions appear clear, reactions can yield surprising results.

While the experiment involved mathematical calculations, it also emphasized the importance of careful measurements and precision. Small errors in measuring distilled water or rounding calculated values could affect the accuracy of the results. It was necessary to adapt to the limitations of laboratory equipment, such as the digital scale, which did not provide measurements to the hundredth place.

Conclusion

"Stoichiometry of a Precipitation Reaction" introduced the concept of stoichiometry and required detailed mathematical calculations to determine the quantities of reactants needed for a chemical reaction. Despite the challenges posed by rounding and measurement limitations, the experiment successfully demonstrated the principles of stoichiometry and the unexpected visual changes that can occur during a chemical reaction.

Questions

1. From your balanced equation what is the theoretical yield of your product?
   The theoretical yield of the product, CaCO3, based on the balanced equation, is 0.6808 grams of CaCO3.
2. According to your data table, what is the actual yield of the product?
   The actual yield of the product is 0.6 gram of CaCO3.
3. What is the percent yield?
   The percent yield is calculated as follows:

Percent Yield (%) = (Actual Yield / Theoretical Yield) × 100

Percent Yield = (0.6 g / 0.6808 g) × 100 ≈ 88%

The percent yield for the reaction is approximately 88%.

How could these errors be reduced in the future?

To reduce errors in future experiments, the following measures can be considered:

• Use of a More Precise Scale: Employ a digital scale that can measure to the hundredth place to improve the accuracy of measurements.
• Alternative Mixing Container: Use a funnel with a plug/unplug mechanism to facilitate efficient mixing and ensure minimal residue on the glass.

Additional Experiment

If the experiment were to be repeated with 1.0 gram of CaCl2·2H2O and 1.0 gram of Na2CO3, the following calculations would apply:

1. From the balanced equation: Na2CO3 + CaCl2·2H2O → CaCO3 + 2NaCl + 2H2O
2. Molecular mass of Na2CO3 + CaCl2·2H2O = 147.01 g/mol
3. Moles of Na2CO3 + CaCl2·2H2O = 1 / 147.01 ≈ 6.8 × 10^-3 mol
4. Molecular mass of Na2CO3 = 105.99 g/mol
5. Moles of Na2CO3 = 1 / 105.99 ≈ 9.43 × 10^-3 mol
6. Theoretical Yield of CaCO3:

Theoretical Yield = Moles of CaCO3 = Moles of Na2CO3 + CaCl2·2H2O

Theoretical Yield = (9.43 × 10^-3 mol - 6.8 × 10^-3 mol) × 105.99 g/mol ≈ 0.28 grams of CaCO3

The theoretical yield of CaCO3 would be approximately 0.28 grams.

Limiting Reactant

The limiting reactant in the reaction between 1.0 gram of CaCl2·2H2O and 1.0 gram of Na2CO3 is CaCl2·2H2O. To determine the grams of the excess reactant (Na2CO3) remaining in solution, we can calculate as follows:

Grams of Excess Reactant = (Moles of Na2CO3 - Moles of Na2CO3 + CaCl2·2H2O) × 105.99 g/mol ≈ 0.28 grams of Na2CO3

Approximately 0.28 grams of Na2CO3 would remain as the excess reactant in solution.

Historical Context

The experiment highlights the historical use of natural remedies, such as willow bark tea containing salicylic acid, for pain relief. Salicylic acid was a key component of early pain-relieving medicines, with its use dating back to 400 BCE. It was also utilized by the Lewis and Clark expedition in the early 1800s. This historical context demonstrates how natural substances have been used throughout history for medicinal purposes.

Conclusion

The experiment provided valuable insights into stoichiometry and the practical application of chemical calculations. It also emphasized the importance of precision in measurements and the potential for unexpected reactions in chemistry. The calculations for theoretical and percent yield allowed for a deeper understanding of the reaction's efficiency. By addressing potential sources of error and using more precise equipment, future experiments can aim for even more accurate results.