Molecular Weight
The molecular weight of a molecule indicates how heavy that molecule is relative to an atom of carbon (with six protons and six neutrons). Saying that the molecular weight of water is 18 means that the water molecule is 18/12 as heavy as a carbon-12 atom, or 18/16 as heavy as an oxygen-16 atom. In general the molecular weight of a molecule is the sum of the atomic weights of its constituent atoms.
The gram molecular weight (GMW) of a substance is defined as the weight in grams of one mole (6 x 1023 molecules, known as Avogadro's number). The molecular weight of water is 18.02; its gram molecular weight is 18.02 g. Given the gram molecular weight of a substance, the weight of an individual molecule can be calculated by dividing that weight by Avogadro's number.
The first direct approach to determining molecular weight was proposed by two French scientists in 1819, Pierre Louis Dulong (1785-1838) and Alexis-Thérèse Petit (1791-1820). They suggested that the amount of heat required to raise the temperature of an atom of a solid material by a given amount should be independent of the type of atom, with the result that the gram atomic weight should be inversely proportional to the material's specific heat. Although the law of Dulong and Petit proved a fair approximation for many elements, it was far from exact for many others, and it was not at all helpful in determining the atomic weights of gaseous elements.
In 1811, the Italian physicist Lorenzo Romano Amadeo Carlo Avogadro di Quaregua e di Cerreto, known to posterity as Avogadro, concluded that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Unfortunately, Avogadro's ideas had little influence on the work of his contemporaries, and it was not until 1860 that another Italian scientist, Stanislao Cannizzaro, pointed out that Avogadro's hypothesis could be used as a basis for determining atomic and molecular weights.
The classical method of determining the molecular weight of a gas is to use density data. Experimentally, a few milliliters of a volatile liquid are placed in a stoppered flask containing a small orifice. The flask is then heated to a temperature above the boiling point of the liquid. As the liquid evaporates, its vapor replaces the air in the flask. The flask is then allowed to cool, and the vapor condenses as air re-enters the flask. The mass of the vapor is then calculated, and used in conjunction with the ideal gas law to determine the molecular weight of the liquid. This method works quite well for many gases and volatile liquids, but it cannot be used for substances that decompose on heating, such as urea.
The molecular weights of such substances have been determined by measuring those physical properties of solutions that depend primarily on the concentrations of solute particles and their molecular weight. Such properties include the lowering of vapor pressure, the elevation of boiling point, and the lowering of freezing point and osmotic pressure. Freezing point depressions have often been used because they are comparatively easy effects to observe. Osmotic pressure measurements have been used for solutes of high molecular weight.
Modern methods for measuring molecular weight include chromatographic methods and mass spectrometry.
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