Tuesday, January 20, 2009

Molecular Weight

Molecular weight of a compound is defined as the weight of a molecule of the compound relative to a carbon atom, the atomic weight of which is supposed to be exactly 12. The molecular weight when expressed in grams is called gram molecular weight. The molecular weight in grams is, in fact, the weight of 1 mole of molecules, e.g., molecular weight of oxygen is 3 and 32 g is the weight of 1 mole of oxygen molecules. Mathematically,

molecular weight = weight of molecules in grams/number of moles of molecules

Molecular weight is measured in atomic mass unit (amu). Atomic mass unit is defined as 1/12 of the mass of the carbon isotope. There are various methods to determine the molecular weight of compounds viz,. vapour-density method. depression-in-freezing-point method, elevation-in-boiling-point method, gravimetric method, volumetric method, etc. The problems on molecular weight based on depression-in-freezing-point and elevation-in-boiling-point methods shall be discussed in Next blogs.
The mole method is found to be very useful in tackling the problems on molecular weight based on the aforesaid methods.

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