Determining Marble Purity: Laboratory Report

Abstract

In this lab experiment, we aimed to determine the percentage purity of marble (CaCO3) using a titration method. The calculated purity values exhibited a notable difference, with a deviation of 4.74% between two specific measurements. However, this deviation falls within the uncertainty range of ±4.84%. We identified potential sources of random and systematic errors contributing to the variance in results. To improve accuracy, we suggested practical measures to minimize these errors.

Introduction

The objective of this lab experiment was to determine the percentage purity of marble (CaCO3) by reacting it with hydrochloric acid (HCl) and titrating the excess acid with sodium hydroxide (NaOH).

The reaction between CaCO3 and HCl produces calcium chloride (CaCl2), water (H2O), and carbon dioxide (CO2). By quantifying the amount of unreacted HCl, we can calculate the purity of the marble sample.

The purity of the marble sample was expressed as a percentage, and two independent measurements were obtained, which showed a deviation of 4.74%. However, it's important to consider the uncertainty associated with these measurements, which was ±4.

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84%. To account for this deviation, we investigated the potential sources of error, including random and systematic errors, and proposed improvements to minimize them in future experiments.

Materials and Methods

The following materials were used in the experiment:

• Marble (CaCO3) sample
• Hydrochloric acid (HCl)
• Sodium hydroxide (NaOH)
• Phenolphthalein indicator
• Pipette dispenser
• Conical flask
• Weighing balance
• Butter paper
• Fans and windows for ventilation

The experimental procedure consisted of the following steps:

1. Weigh out a specific quantity of marble sample using a weighing balance and record the mass.
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2. Transfer the marble sample to a conical flask.
3. Add a known volume of hydrochloric acid (HCl) to the conical flask containing the marble sample.
4. Observe the effervescence as carbon dioxide (CO2) is evolved during the reaction between CaCO3 and HCl.
5. Add a few drops of phenolphthalein indicator to the solution.
6. Titrate the excess HCl with sodium hydroxide (NaOH) until the solution turns pale pink, indicating neutralization.
7. Record the volume of NaOH used in the titration.
8. Repeat the experiment for multiple trials.

Data Analysis

The percentage purity of marble was calculated using the following formula:

Purity (%) = (Volume of NaOH used in titration * Molarity of NaOH * 100) / Mass of marble sample

Two independent measurements of purity were obtained, resulting in values with a deviation of 4.74%. However, it's crucial to consider the uncertainty associated with these measurements, which was ±4.84%. The deviation falls within the range of uncertainty, suggesting that the results are consistent within the experimental limitations.

Results

The calculated percentage purity values for the marble sample are as follows:

The deviation between Measurement 1 and Measurement 2 is 4.74%. However, considering the uncertainty of ±4.84%, the deviation falls within the acceptable range of experimental error.

Discussion

The deviation in the calculated purity values can be attributed to various sources of error, including random and systematic errors:

Random Errors

1. Loss of marble after weighing: Some marble particles were lost when transferring the sample from the weighing paper, leading to disparities in the masses used.
2. HCl left in the pipette: In manual pipetting, it's challenging to ensure that every drop of acid is transferred to the conical flask, affecting the volume of acid used.
3. Air bubbles in pipette dispenser: Air bubbles trapped in the pipette dispenser introduced errors in the volume of HCl delivered.

Systematic Errors

1. Unevenly crushed marble: The marble sample provided had uneven particle sizes, resulting in variations in the surface area available for the acid to react with, leading to differing purity values.
2. Excess NaOH added in titration: The endpoint of titration, indicated by the color change to pale pink, is subject to qualitative judgment and may result in excess NaOH being added, affecting the accuracy of the titration.
3. Unevenly distributed impurities: As a natural material, marble contains impurities that are unevenly distributed, contributing to variations in purity among different samples.

To minimize these errors in future experiments, the following improvements are recommended:

• Keep fans and windows closed and cover the marble sample during transportation to reduce particle loss due to wind.
• Practice careful and precise technique when using laboratory apparatus to minimize errors such as air bubbles in pipettes and parallax in measurements.
• Ensure that the marble sample is uniformly crushed to maintain consistent surface area for reactions.
• Consider using a different indicator, such as Universal Indicator, to achieve a more precise endpoint in titrations and reduce the margin of error.

Conclusion

In this lab experiment, we determined the percentage purity of marble using a titration method. Despite a deviation of 4.74% between two independent measurements, the results fell within the uncertainty range of ±4.84%, indicating consistency within experimental limitations. We identified sources of random and systematic errors contributing to the deviation and proposed practical measures to minimize these errors in future experiments. The experiment provided valuable insights into the challenges of purity determination and the importance of error analysis in scientific research.

Recommendations

For future experiments involving the determination of purity, it is essential to implement the recommended improvements to enhance accuracy and reliability of results. Additionally, exploring alternative indicators and refining techniques can further reduce errors and improve the precision of purity measurements. Continuous attention to error sources and mitigation strategies is crucial for achieving more accurate experimental outcomes.