Determination of Aluminum in a Sample
The Activity Series of the metals (from Exp. 5) shows that aluminum is a reactive metal. It reacts with strong acids to release hydrogen gas:
Al (s) + H+(aq) → Al3+ (aq) + H2 (g) [unbalanced equation]
When this equation is balanced, the stoichiometric relationship between the reactants and products gives a method for calculating the amount of aluminum metal that reacted with the acid. Hence the percentage of aluminum in a sample may be calculated by measuring the volume of the hydrogen produced. Fine aluminum powder will be used as the sample in this experiment.
The hydrogen gas produced by the reaction is collected over water and is considered to be “wet” due to the presence of water vapor. The pressure of the “dry” hydrogen has to be calculated by subtracting the vapor pressure of water from the total pressure, at the temperature at which the gas is collected. The ideal gas law is then used to calculate the moles of hydrogen gas collected.
EXPERIMENTAL PROCEDURE: This lab will be performed in the Yenka program.
- First, open Lab 7 Determination of Al Yenka file and click on the “cm” unit next to the Volume of Gases listed below the instructions on the simulation and change it to “mL”. Collect 0.020 g of fine Al solid and place it in a clean, 75-mm test tube.
- The apparatus is connected already for you in the Yenka lab file. It has been constructed to allow the user to observe the generated volume of H2 gas that will be produced from the reaction with a strong acid. The glass tubing connected to the stopper of the test tube will display bubbles as the reaction takes place within the test tube. The volume of gas that forms will be displayed above the test tube by the Volume of Gas listing. The figure is shown here.
- Apply one drop of 6 M HCl (0.05 mL) to the test tube by dragging the HCl bottle near the bottom of the test tube and releasing the HCl bottle. Observe the volume of gas increase as H2 gas forms in the test tube. Wait until the volume value stops fluctuating before proceeding.
- Another drop of 6 M HCl is applied to the test tube in the same fashion. Once again, notice the increase in volume of H2 gas that is formed. Once the volume value stops fluctuating, you can proceed.
- Repeat this process for two more drops and allow the volume value to stop fluctuating to ensure you can record an accurate value of H2 gas that forms.
- When the evolution of hydrogen gas is complete (no more bubbles will be formed), record the final volume of H2 gas that forms form the reaction of adding 6 M HCl to the fine Al solid.
- Do not forget to record the Temperature from the thermometer displayed at the bottom of the laboratory simulation. You will need this value to proceed with your calculations. Use the temperature chart located on the next page to calculate the vapor pressure of water.
- The Barometric pressure is also displayed near the bottom of the simulation. Observe the photo near the bottom of the laboratory simulation and utilize the value to complete your calculations.
Pressure of “dry” H2 gas can be found by subtracting the Vapor Pressure of Water with the Barometric Pressure.
Use PV=nRT in order to solve for the amount of moles of H2 produced. (make sure you are using the correct units for all of your values, i.e. atm, liters, and Kelvin).
Then, you will have to use the Balanced Equation to go from moles of H2 to moles of Al.
Percent by mass can be solved by taking the value obtained for mass of Al reacted and dividing it by the original mass of the aluminum sample and then multiplying by 100%.
Variation of Vapor Pressure of Water with Temperature
Determination of Aluminum NAME: _____________________________
Mass of aluminum sample
Volume of H2 collected
Temperature at gas-collection site
1. Vapor pressure of water (from table on page 3)
2. Pressure of “dry” H2 collected
3. Amount of H2 produced (mol)
4. Amount of Al reacted (mol)
5. Mass of Al reacted (g)
6. Percent by mass of Al reacted