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THE STOICHIOMETRY AND ATOMIC MASS OF ALUMINUM
Prior to the invention of the mass spectrogram by the British scientist, Francis William Aston in 1919, the atomic mass of an element was determined by stoichiometry. That is, measured masses of various substances were reacted. The atomic mass of the elements in the material was then deduced from the data obtained. The concepts of atoms and atomic “weight” were first introduced by another Englishman, John Dalton, in his atomic theory published in 1803. Dalton made three assertions which are the basis for much of the chemistry we learn today.
(1) The chemical elements are composed of very minute individual particles of matter, called atoms, which preserve their individuality in all chemical changes.
(2) All the atoms of the same element are identical in all respects, particularly in weight. Different elements have atoms differing in weight. Each element is characterized by the weight of its atom. (Dalton didn’t know about isotopes. Their existence wasn’t discovered until 1919 by F. W. Astor.)
(3) Chemical composition occurs by the union of its atoms of the elements in simple numerical ratios.
As the quality of balances and the understanding the combining numbers improved, so did the accuracy of the atomic weights. In 1826, a Swedish chemist, John J. Berzelius, published a list of the atomic masses of fifty elements after more than ten years of work. While a number of atomic weights were published during that period, Berzelius’ values are the most consistent with the values given today. His atomic weight of aluminum changed from 54.88 grams in 1814, to 54.77 grams in 1818, and finally to 27.43 grams in 1826. Today’s value for the atomic mass of aluminum is 26.9815 grams.
The purpose of this experiment is to determine both the atomic mass of aluminum and the combining ratio between aluminum and hydrogen in much the same way as Berzelius did, with a gas measuring tube and a balance. This will be accomplished by studying the single replacement reaction that takes place when aluminum metal reacts with hydrochloric acid giving hydrogen gas as one of the products.
The gas generated will be collected by the downward displacement of water. This volume will be then converted into moles. Next, you will compare the ratio of the moles of hydrogen to aluminum in the balanced equation with the ratio determined experimentally. The atomic mass of aluminum will then be calculated from the mass of aluminum reacted and the number of moles of hydrogen collected.
For example, in a similar experiment, a student reacted a piece of aluminum foil with a mass of 0.041 g. When the reaction ceased, he recorded a volume of 54.0 ml of hydrogen at a water temperature of 21°C with an atmospheric pressure of 102.2 kPa. The student measured the volume of gas by displacing the water in a 60-ml syringe which required a correction of +1.2 ml. With only that information, he was able to determine both the stoichiometry of the reaction and the atomic mass of aluminum as follows:
- The next step in the calculation is to determine the partial pressure of only the hydrogen gas in the mixture of hydrogen and water vapor collected. The pressure exerted by the hydrogen gas alone is then converted into atmospheres. The water temperature was 21°C and atmospheric pressure was 102.2 kPa. The vapor pressure of water is listed in the table to be 2.5 kPa at 21°C. The term “partial pressure” refers to the part of the total pressure exerted by each of the gases in a mixture. The relationship known as Dalton’s Law of Partial Pressures will be needed in this calculation. Where,
total hydrogen water vapor hydrogen hydrogen
Total Pressure = Sum of the partial pressures of the individual gases in the mixture Total Pressure, P P P 102.2 kPa = P 2.5 kPa P = 102.2 kPa - 2.5 kPa = 99.7 kP
a
2.. The most important step in this calculation is to determine the number of moles of dry hydrogen gas generated by the reaction between aluminum metal and hydrochloric acid. This is accomplished using the ideal gas equation,
PV = nRT
Where, P = partial pressure of the hydrogen gas in kPa V = volume of H 2 collected in the syringe in liters (volume of hydrogen in ml ÷ 1000 ml/liter) n = number of moles of hydrogen in the gas measuring tube (the unknown here) R = ideal gas constant, 8.31 J/deg⋅mol T = the temperature of the gas in degrees Kelvin, K (°C + 273 = °K)
Using this student’s data, P = the partial pressure of the hydrogen was calculated to be 99.7 kPa. V = volume of hydrogen collected , 54.0 ml + the 1.2 ml correction for the syringe = 55.2 ml or 0.0552 liter (55.2 ml ÷1000 ml/liter) T = temperature in degrees Kelvin, 21°C + 273 = 294 K
Substituting in the ideal gas equation,
Materials:
tennis ball can or tall beaker 60-ml syringe syringe tip cap 250 ml beaker gas generating apparatus
thermometer large (18 x 150 mm) test tube ruler 4 M HCl aluminum wire or foil
scissors or single edge razor blade small hypodermic syringe or calibrated transfer pipet
Procedure:
- Tear a 4 to 5 inch length of aluminum foil from the roll of aluminum foil. Using scissors, a utility knife, or a single edge razor blade, cut the foil into roughly 8 cm^2 pieces (2 cm x 4 cm).
- Determine the mass of the piece of aluminum foil. If you have access to an analytical balance with a sensitivity of 0.001 gram or better, mass the metal directly. If your balance lacks milligram (0.001gram) or better sensitivity, you must calculate the mass of your sample. See the special calculation work sheet. The mass of your foil must not exceed 0. gram.
- Fill the tennis ball can with tap water leaving a 1-2 cm space at the top. Then fill the syringe with water by pushing it below the surface of the water allowing the air to escape. Push a tip cap onto the open end to seal the syringe. Do not place the “J” tube into the water at this time.
- Roll the foil into a loose coil and place it in a test tube followed by 3.0 ml of 4 M HCl. Press the stopper assembly firmly into the test tube with a twist to seal the system. Place the tip of the “J” tube into the bottom of the syringe. Check to be certain that all of the connections are tight to avoid a loss of hydrogen gas.
- Swirl the test tube containing the acid and aluminum metal until the reaction begins to proceed rapidly. This may take 3- minutes depending on the acid temperature. Cool the reaction by placing the test tube into the water in the tennis ball can. Continue to swirl the test tube while it is in the water to cool the mixture and slowing the reaction. It may be necessary to hold the “J” tube against the syringe with one hand while swirling the test tube with the other. Wash any residual specks of un-reacted aluminum metal from the sides of the test tube by tilting the test tube slightly and then shaking the solution until it contacts the metal. Record your observations of the contents of the test tube.
Tip Cap
60-ml syringe
Tennis ball can
Aluminum foil and hydrochloric acid reacting in a test tube
- When all the aluminum has been reacted, you must equalize the pressure inside the syringe and test tube with the atmosphere. With the tip of “J”tube still inside the syringe, move both the syringe and the “J” tube together until the open tip of the “J” tube is just above the level of the water in the tennis ball can. Now you can remove the “J” tube from the syringe. When recording the volume of the gas in the syringe, always move the syringe up or down until the level of the water inside is nearly the same as the level of the water in the tennis ball can. Record the volume of hydrogen gas to the nearest 0.5 ml. Be certain that you have read the volume of the hydrogen correctly. 60-ml syringes are calibrated in milliliters with the “0" at the top. Finally, record both the water temperature and the atmospheric pressure.
- Repeat the experiment as time permits.
- Clean up by rinsing all glassware and returning the common materials to their designated areas.
Calculations and Questions
Q1. (a) Determine the vapor pressure of water at the water temperature from a Table of the Vapor Pressures of Water at Various Temperatures.
(b) Calculate the partial pressure of just the hydrogen in the gas measuring tube from Dalton's Law of Partial Pressures.
Q2. (a) Correct the recorded volume of hydrogen in the syringe by adding 1.2 ml to your value.
Corrected volume of hydrogen = recorded volume of hydrogen + 1.2 ml
(b) Calculate the number of moles, n, of hydrogen gas contained in the syringe from the ideal gas equation: Q3. (a) Write the word equation and balanced formula equation for the reaction between the aluminum metal and the hydrochloric acid.
(b) Calculate the actual number of moles of aluminum reacted from the mass of foil reacted and the atomic mass of aluminum given on the Periodic Table of the Elements.
(c) One goal in this experiment is to compare the actual ratio of moles of hydrogen to moles of aluminum with the expected ratio predicted from the balanced chemical equation. Calculate the actual ratio of moles of hydrogen produced to the moles of aluminum metal reacted.
Always equalize the pressure inside the syringe by moving the syringe up or down until the level of the water inside the syringe is the same as the water level outside syringe the before removing the “J” tube.
Data:
Trial #1 Trial #2 Trial # Mass of aluminum metal reacted g. g. g. Atmospheric pressure kPa kPa kPa Water temperature °C °C °C Volume of hydrogen collected ml ml ml Corrected Volume of hydrogen ml ml ml Vapor pressure of water (from the table below) [Q1a] kPa kPa kPa Partial pressure of the hydrogen [Q1b] kPa kPa kPa Moles of hydrogen collected [Q2] mol mol mol Moles of aluminum reacted [Q3b] mol mol mol Ratio of the moles of hydrogen collected to the moles aluminum reacted [Q3c]
mole H 2 mole Al
mole H 2 mole Al
mole H 2 mole Al Percent error [Q3d] % % % Moles of Al reacted calculated from the moles of H 2 collected [Q4a] mol mol mol Calculated atomic mass of aluminum [Q4b] g/mol g/mol g/mol
Vapor Pressure of Water at Various Temperatures
°C kPa °C kPa °C kPa °C kPa °C kPa
13 1.5 17 1.9 21 2.5 25 3.2 29 4.
14 1.6 18 2.1 22 2.6 26 3.4 30 4.
15 1.7 19 2.2 23 2.8 27 3.6 31 4.
16 1.8 20 2.3 24 3.0 28 3.8 32 4.
