The Equivalent Weight of a Metal

The objective of this lab is to determine the equivalent weight of a metal by allowing it to react with hydrochloric acid (HCl) to produce hydrogen gas. By measuring the weight of the metal and the volume of hydrogen gas produced, the equivalent weight of the metal can be calculated. This lab aims to provide hands-on experience with basic scientific analysis and research, demonstrate safe practices in the chemical laboratory, understand electrons in atoms and the periodic table, chemical bonding, chemical reaction and quantities in chemical reactions, molecules, compounds, and chemical composition. Additionally, it emphasizes the proper use of exponential notation and significant figures, understanding the composition of matter and energy, recording investigation results, using scientific reasoning to evaluate phenomena, and identifying unifying themes in the scientific field of study.

In the discussion, it is explained that the equivalent weight of an element is the weight of that element that reacts with or liberates 8.00 grams of oxygen or 1.00 gram of hydrogen. The reaction between aluminum (Al) and hydrochloric acid is given as an example:

Al(s) + 3HCl → AlCl3 + 3/2H2(g)

By determining the weight equivalence between aluminum and hydrogen gas, it is possible to calculate the equivalent weight of aluminum.

The procedure involves assembling a molar volume apparatus similar to the one used in the Molar Volume of Gases Experiment, with the addition of a flexible rubber tubing between the gas generation tube and the water bottle to regulate hydrogen gas production. 10 ml of 6.0 M HCl solution is placed in a test tube, and aluminum foil (weighing no more than 0.25g) is tied with a string and attached to the test tube. Care should be taken to ensure that the acid does not come into contact with the metal until the reaction is ready to begin. The apparatus should be checked for air leaks, and a dry 600 ml beaker should be weighed. Once the reaction apparatus is ready, the test tube is tilted to allow the hydrochloric acid to react with the aluminum metal. After the reaction is complete, the temperature and barometric pressure are measured, and the displaced water in the beaker is weighed.

Calculations involve determining the volume of hydrogen gas produced using the molar volume of gases and stoichiometry. The weight of the metal and the volume of hydrogen gas are used to calculate the equivalent weight of the metal.

Through this lab, students gain practical experience in performing chemical reactions, measuring volumes of gases, and using stoichiometry to determine equivalent weights. The lab also highlights the importance of accurate measurements, understanding reaction stoichiometry, and using the concept of equivalent weights to determine atomic weights.

Objective

·         Use the tools and equipment necessary for basic scientific analysis and research

·         Demonstrate safe practices in the Chemical Laboratory

·         Demonstrate an understanding of Electrons in Atoms and the Periodic Table and Chemical Bonding

·         Demonstrate an understanding of Chemical Reaction and Quantities in Chemical Reactions

·         Demonstrate an understanding of Molecules, Compounds and Chemical Composition

·         Demonstrate the proper use of Exponential Notation and Significant Figures

·         Demonstrate an understanding of the composition of matter and energy

·         Record the results of investigation through writing

·         Use scientific reasoning to evaluate physical and natural phenomena

·         Identify the unifying themes of the scientific field of study

Materials

·         Ring Stand

·         Glass Bottle

·         400 mL Beaker

·         600 mL Beaker

·         Glass Tubing

·         Rubber Tubing

·         Pinch Clamp

·         Rubber Stoppers

·         6 M HCl

·         Test Tubes

·         String

Discussion

The objective of this experiment is to ascertain the equivalent weight of a metal. By utilizing approximate atomic weight data obtained from the previous Dulong-Petit experiment in combination with the equivalent weight data from this experiment, it becomes possible to determine the precise atomic weight of the metal.

The equivalent weight of an element is defined as the weight of that element which reacts with or releases 8.00 grams of oxygen or 1.00 gram of hydrogen. In this particular experiment, aluminum metal will undergo a reaction with HCl to generate hydrogen gas, as depicted by the equation:

Al(s) + 3HCl → AlCl3 + 3/2H2(g)

By establishing the weight equivalence between aluminum and hydrogen gas, it becomes feasible to calculate the equivalent weight of aluminum.

Procedure

Prepare a molar volume apparatus similar to the one used in the Molar Volume of Gases Experiment for generating oxygen. The only alteration required is the inclusion of a flexible rubber tubing between the gas generation tube and the water-filled bottle. This modification allows for the control of hydrogen gas production. Consult Figure 19 for reference. Take 10 ml of a 6.0 M HCl solution and transfer it to a test tube. Carefully weigh aluminum foil, ensuring it does not exceed 0.25g with an accuracy of ±0.1 mg. Tie a string around the foil and attach it to the test tube depicted in Figure 19. Ensure that the acid does not come into contact with the metal until you are ready to initiate the reaction. Adjust the apparatus for any potential air leaks, following the procedure outlined in the Molar Volume of Gases Experiment. Weigh a dry 600 ml beaker and proceed to set up the reaction apparatus. Tilt the test tube to allow the hydrochloric acid to react with the aluminum metal. Allow the reaction tube to cool down and adjust the pressure within the system to match the barometric pressure. Measure the amount of water displaced in the beaker, as well as the temperature of the displaced water and the barometric pressure. 

Report Sheet for THE EQUIVALENT WEIGHT of A METAL

Name_______________________________________                          Date___________________________

Lab / Section_________________________________

Unknown Number_________

Review Questions

1.       What is the purpose of determining the equivalent weight of a metal in this lab?

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2.       How does the reaction between aluminum and hydrochloric acid help in determining the equivalent weight of aluminum?

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3.       Explain the concept of equivalent weight and its relationship to the reaction with hydrogen gas.

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4.       What is the significance of the Dulong-Petit law in determining the approximate atomic weight of a metal?

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5.       How is specific heat related to the atomic weight of an element according to the Dulong-Petit law?

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6.       Describe the procedure for measuring the freezing point depression of a solvent to determine the molecular weight of a solute.

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7.       What colligative property is used in the freezing point depression experiment, and how does it depend on the number of particles in a solution?

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8.       How does the concentration of a solution relate to the molecular weight of the solute?

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9.       What is the purpose of using a calorimeter in the determination of the atomic weight of a metal?

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10.    Explain the principle of energy conservation in the calorimetry experiment.

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11.    How does the specific heat of a metal affect the heat exchange in the calorimeter?

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12.    Describe the setup and procedure for generating hydrogen gas in the molar volume experiment.

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13.    What is the role of hydrochloric acid in the reaction with aluminum in the molar volume experiment?

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14.    How does the weight of aluminum and the volume of hydrogen gas relate to the equivalent weight of aluminum?

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15.    What is the significance of accurate measurements of temperature and barometric pressure in the molar volume experiment?

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16.    Explain the relationship between the atomic weight and equivalent weight of a metal.

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17.    How does the molar volume of gases concept help in calculating the volume of hydrogen gas produced?

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18.    What precautions should be taken to ensure the reliability of the experimental results in determining atomic weight?

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19.    Discuss the sources of error in each of the experiments conducted in the lab.

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20.    How can the results obtained in these experiments contribute to our understanding of chemical composition, atomic weights, and the behavior of matter?

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