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EXPT 1. Atomic Weight of Barium [Key Contents] - atomic theory, atomic weight, valency, equivalent weight - precipitation equilibrium, gravimetric analysis - precipitate chloride ion in barium chloride as silver chloride, determine atomic weight of barium - discovery of radium by Marie Curie, Nobel Prize-winning work of Richards [References] Principles of Modern Chemistry, 6th Ed. (Oxtoby et al.) Ch 2. Chemical Formulas, Chemical Equations, and Reaction Yields Ch 16. Solubility and Precipitation Equilibria Chemistry for Life, Chemistry for Better Life (Kim et al.) Ch 3. Atoms and Molecules Ch 9. Equilibrium Reactions [Goal] - to determine atomic weight of barium following the procedure used by Marie Curie in her discovery of radium and, thereby, learn principles and techniques involved in the historic experiment - to observe images of atoms using scanning tunneling microscope (STM) [Background] The world around us is the world of atoms. In the world of atoms, atomic weight is a fundamental quantity that distinguishes the atom of one element from the atom of another. In other words, different elements differ in atomic weight. And the atomic weight of an element is a constant over time and space. In a sense, the number of protons in an atom (atomic number) is more fundamental than the atomic weight. Creation of particles with different number of protons either in the big bang universe or in stars implies creation of nuclei of different chemical elements. Neutrons make heavy elements possible by binding repelling protons together through strong nuclear force. Thus the number of neutrons could differ in a given element leading to isotopes of the same element. Changes in the number of electrons lead to ions of different charge; however, identity of the element remains unchanged. Changes in the number of protons deep in the atomic nucleus results in the creation of entirely different elements with new chemical and physical properties. Hydrogen made in the big bang universe and heavy elements made later in stars and spewed out into the interstellar space by the supernova explosion encounter each other and are used collectively to make the solar system including the earth inhabited by living creatures. Consider the hydrogen bond in DNA. There is no that life on earth is a result of 13.7 billion years evolution of the universe. In the evolution of the universe, isotopes of an element are mixed uniformly giving rise to the observed atomic weight, which is the weighted average of isotopic masses. Atomic weights of many elements were known with a fairly good accuracy even before the discovery of the subatomic particles and Mosely's determination of the atomic number, which is why Mendeleev was able to come up with the periodic table arranging the elements in the order of increasing atomic weight. Two different methods were available to the 19th century scientists. In the first method, Avogadro's principle proposed in 1811 played a key role. Based on Avogadro's principle, one could determine molecular weight from density measurements. Knowing how atoms combine, than one could determine atomic weight of the combining atoms. Thus, Cannizzaro determined atomic weight of several elements based on the molecular weight of hydrogen as 2. Avogadro's principle could not be used for metallic elements that do not readily form gaseous compounds. An empirical law discovered by Dulong and Petit turned out to be quite useful in this case. Dulong and Petit showed in 1819 that the constant volume molar heat capacity, Cv, of solid elements is about the same. dE/dT ≒ 25 J․mol-1․K-1 The observed value of 25 J․mol-1․K-1 turned out to be 3R, which comes from the fact that the sum of the kinetic energy and the potential energy in a 3-dimensional network of atoms connect by springs is 3RT. There would have been no Mendeleev without Dulong and Petit. Majority Richards of of the current Harvard and atomic his weights associates were determined primarily by the by AgCl precipitation method. Marie Curie and her husband, Pierre Curie, determined the atomic weight of radium before they announced the new element. Marie Curie (1867-1934, 1903 Nobel Physics Prize, 1911년 Nobel Chemistry Prize) and Pierre Curie (1859-1906, 1903 Nobel Physics Prize) You will be repeating what Curies did in their atomic weight determination of radium. In April, 1902, they isolated about 120 mg of radium from about a ton of pitchblend and announced its atomic weight in July. In the purification, barium (which is right above radium in the periodic table and thus have almost identical chemical properties) went with radium. Barium was finally removed by fractional crystallization. Then the atomic weight of radium was determined by measuring the weight of chlorine as AgCl in a known amount of RaCl2. Curie's lab notebook shows several key figures such as 0.10925 g RaCl2, 0.10647 g AgCl, Ra/Cl = 3.154. The atomic weight of radium was reported as 223.3. Let's consider the significance of Ra/Cl = 3.154. The weight of Cl in 0.10925 g RaCl2 must have been derived from the weight of Cl in AgCl. The difference from 0.10925 g should be the weight of Ra. weight of Ra in 0.10925 g RaCl2 : 0.10925 x (3.154)/(4.154) = 0.08295 weight of Cl in 0.10925 g RaCl2 : 0.10925 x (1)/(4.154) = 0.02630 What is the atomic weight of Cl Curies used in their determination of Ra atomic weight as 223.3? 0.08295/(223.3/2) = 0.02630/X X = 35.40 The weight of Cl in 0.10925 g RaCl2 should be the same as the weight of Cl in 0.10647 g AgCl. What is the atomic weight of Ag Curies used? 0.02630/35.40 = (0.10647 - 0.02630)/Y Y = 107.91 The same principles apply in the atomic weight determination of Ba. The key reaction is the following. Ag+ + Cl- → AgCl(s) You will be determining the atomic weight of Ba from known atomic weights of Ag and Cl. Thereby, you will learn principles and techniques involved in the historic experiment by Curie. [Apparatus and Chemicals] oven, two 100 mL beakers, 10 mL pipette, pipette filler, glass rod, filter paper, Buchner funnel, Buchner flask, vacuum pump, forceps, spatula BaCl2·2H2O solution (to be provided by TA, make 1 L solution containing about 12 g BaCl2·2H2O weighed to 0.01 g), AgNO3 (169.87 g/mol) [Procedure] Expt 1. Atomic Weight of Barium 1) Using a pencil, mark a filter paper with your group number and dry in a 120oC oven for more than 10 minutes. Then, weigh it to 0.001 g accuracy. 2) Transfer 10.0 mL of the barium chloride solution to a 100 mL beaker. 3) Weigh about 0.5 g AgNO3 and dissolve with distilled water in a 100 mL beaker. 4) Without delay, slowly add all of the AgNO3 solution into the barium solution constantly stirring with a glass rod. 5) Stir for about 10 min. In the mean time, set up the evacuation system with the Buchner funnel, Buchner flask, and the vacuum pump. 6) Place the oven-dried filter paper on the Buchner funnel, turn on the vacuum pump, and use distilled water to fix the paper on the funnel. 7) Pour the solution containing AgCl precipitate to the filter paper on the funnel. Let not the precipitate collect on the wall of the funnel. Use spatula and wash bottle to transfer all of the precipitate. Incomplete transfer is the major source of error. 8) If the filtrate is cloudy or any precipitate is visible in the filtrate, repeat filtration. 9) Wash the precipitate several times with distilled water. Make sure that all precipitate remain on the filter paper. 10) Dry the precipitate together with the filter paper in a 120oC oven and weigh to 0.001 g accuracy after brief cooling. [Caution] - Avoid skin contact with the AgNO3 solution. - Do not hold the filter paper with your finger. Use forcept. [Data Analysis] 1) Calculate the weight of Ba and Cl in the BaCl2 used. 2) Calculate the atomic weight of Ba and compare againt the book value. The moisture content of the filter paper is about 4%. Take this into account when calculating the weight of AgCl from the total weight of the precipitate and the dried filter paper if the filter paper is not dried initially. [Additional Material] Assignment for Internet Search 1. Nobel Prize for Theodore Richards Theodore Richards (1868-1928) Professor, Harvard University 1914 Nobel Prize in Chemistry 2. Methods for Atomic Weight Determination ♦ gas density Cannizzaro (1826-1910) relative weight oxygen -------------------------------------------------------H2O 18 16 NO 30 16 N2O 44 16 NO2 46 32 SO2 64 32 CO2 44 32 O2 32 32 O3 48 48 ♦ Dulong-Petit's law ♦ gravimetric analysis 3. Scanning Tunneling Microscopy