An important tool for determining the formula of an unknown compound or predicting the behavior of one that is known is molecular weight, the relative masses of the elements in a compound. This quantity is also known as molecular mass. Since most conventional units of mass are too large for this purpose, molecular weight is measured in atomic mass units (amu), or Daltons. The amu is defined as 1/12 the mass of one atom of carbon-12, or about the weight of a proton or neutron. Molecular weight is determined by adding the weights of the atoms present in one molecule multiplied by the number of atoms of each type. These weights can be found on the periodic table usually at the bottom of each cell. In addition to giving the weight of one molecule of a compound in amu, the molecular weight is also numerically equal to the molar mass, or weight in grams of one mole of a compound.
Example:
Lithium ethyl is an organometallic compound which bursts into flame when exposed to air. Its molecular formula is Li What is its molecular weight and molar mass?
The compound has one atom of Li, two of C and four of H. The molecular weight of the compound is therefore (atomic mass of Li) + 2(atomic mass of C) + 4(atomic mass of H), or (6.941) + 2(12.011) + 4(1.0079). The answer is 34.99 amu.
The mass of 1 mole of Li (the molar mass) is 34.99g/mol.
