Regents ChemistryMolecular WeightThe molecular weight of a substance is the weight in atomic mass units of all the atoms in a given formula. An atomic mass unit is defined as 1/12 the weight of the carbon-12 isotope. The old symbol was amu, while the most correct symbol is u (a lower case letter u). NYS Chemistry will use amu.
By the way, carbon-12 is defined as weighing exactly 12 amu. This is the starting point for how much an atom weighs. For example, if you weigh 1/2 as much as C-12, you weigh 6. If you weigh twice as much, you weigh 24. The molecular weight of a substance is needed to tell us how many grams are in one mole of that substance. The mole is the standard method in chemistry for communicating how much of a substance is present. You should have a periodic table for looking up atomic weights and a calculator. Point #1 - You need to know how many atoms of each element are in a substance in order to calculate its molecular weight. For example H2O has two atoms of hydrogen and one atom of oxygen. H2O2 has two atoms each of oxygen and hydrogen. Mg(OH)2 has one atom of magnesiun and two each of oxygen and hydrogen. If a subscript follows an atom with no parenthesis, that number tells you how many of that atom are present. If parentheses are involved, you must multiply each subscript inside by the one which is outside. How many of each element are in the following examples (answers just below the problems, so scroll carefully): KCl Fe2O3 Al(NO3)3 NH4NO3 Al2(SO4)3 Answers K - one; Cl - one Fe - two; O - three Al - one; N - three; O - nine N - two; H - four; O - three Al - two; S - three; O - twelve
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