For reasons similar to those applying in the case of chromium, the atomic weight of molybdenum is three times the equivalent of the metal in the molybdic salts, or six times that in the molybdates. Molybdenum may be di-, tri-, or hexa-valent.
The determinations made before 1858 may be ignored as of no modern significance. Those made since that date yield three ratios, from which the atomic weight of molybdenum may be calculated, namely: - Na2MoO4:2NaCl = 100:56.744 ± 0.0017;
- 2AgCl:MoO3 = 100:50.204 ± 0.0017;
- MoO3:M = 100:66.708 ± 0.0010.
The values so obtained are (1) 96.06, (2) 95.92, (3) 96.04, the general mean being 96.006.
It is to be observed that the low results of Seubert and Pollard may be accounted for by loss of molybdic anhydride by volatilisation during ignition. Special precautions were taken by subsequent investigators to prevent this. Miiller first oxidised the pure metal to the trioxide by heating in oxygen, using a quartz vessel, and then reduced the oxide by heating in a current of pure dry hydrogen. He made eight experiments, using three different samples of molybdenum, and his results, as shown in the table, are practically identical with those of Vandenberghe, and of Smith and Maas. From these considerations the atomic weight of molybdenum appears to approximate to the value 96.04 ± 0.01. The International Committee (1925) gives the value
Mo = 96.0. |