Solution to the beryllium valence challenge

Analytical and Bioanalytical Chemistry, May 2010

Juris Meija

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Solution to the beryllium valence challenge

Juris Meija 0 ) Institute for National Measurement Standards, National Research Council Canada , 1200 Montreal Road, Ottawa , ON K1A 0R6, Canada In both cases the amount of water is 90.0 g/18.0 g mol1 = 5.00 mol. If x = 2, the first solution has 200 mol water per mol BeCl2. If, however, x = 3, the second solution has exactly 200 mol water per mol BeCl3. Now we need to compare the densities of these solutions with those of KCl (M=75), MgCl2 (M=93), CaCl2 (M=111), and MnCl2 (M=126), all containing 200 mol (3600 g) water per mol salt. The mass fraction of chlorides in such solutions is: - The quest in this column is to elucidate the valence of beryllium from the density measurements of the soluble chlorides, as suggested by Mendeleyev in 1889 [1, 2]. From the composition of beryllium oxide we gather that the atomic weight of beryllium is either 9 or 13.5 Da, depending on the valence of beryllium, or simply 4.5x Da where x is the valence [2]. Now, it is known that solutions of beryllium chloride, prepared by dissolving 2.00 g and 3.00 g anhydrous salt in 90.0 g water, have densities 1.0138 and 1.0211 g mL1, respectively at 15C. Because dilute equimolar solutions of metal chlorides will have densities increasing in the order of their molecular weights it is necessary to calculate the amount of beryllium chloride in these two solutions: 2:00 g n1BeClx 4:5x35:5x g mol 1 0:050 x 1 mol; 3:00 g n2BeClx 4:5x35:5x g mol 1 0:075 x 1 mol: The densities of these solutions (15 C) can be obtained from chemistry handbooks, and are 1.012, 1.020, 1.024, and 1.029 g mL1, respectively. If beryllium is divalent (BeCl2, M = 80), the density of BeCl2 solution with n(H2O): n(BeCl2) = 200:1 must be slightly above that of KCl yet below that of MgCl2which is the case. Alternatively, for trivalent beryllium (BeCl3, M = 120) the density of its chloride solution must be above that of CaCl2which is not the case. Thus, density measurements of aqueous chlorides are (correctly) in support of divalent beryllium.

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Juris Meija. Solution to the beryllium valence challenge, Analytical and Bioanalytical Chemistry, 2010, 3-3, DOI: 10.1007/s00216-010-3559-1