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Formula and Molecular Weights

# Formula and Molecular Weights - amu Ionic substances are...

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Formula and Molecular Weights The formula weight of a substance is the sum of the atomic weights of each atom in its chemical formula . For example, water (H 2 O) has a formula weight of: 2*(1.0079 amu ) + 1*(15.9994 amu ) = 18.01528 amu If a substance exists as discrete molecules (as with atoms that are chemically bonded together) then the chemical formula is the molecular formula , and the formula weight is the molecular weight . For example, carbon, hydrogen and oxygen can chemically bond to form a molecule of the sugar glucose with the chemical and molecular formula of C 6 H 12 O 6 . The formula weight and the molecular weight of glucose is thus: 6*(12 amu ) + 12*(1.00794 amu ) + 6*(15.9994 amu ) = 180.0
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Unformatted text preview: amu Ionic substances are not chemically bonded and do not exist as discrete molecules. However, they do associate in discrete ratios of ions. Thus, we can describe their formula weights, but not their molecular weights . Table salt (NaCl), for example, has a formula weight of: 23.0 amu + 35.5 amu = 58.5 amu Percentage composition from formulas In some types of analyses of it is important to know the percentage by mass of each type of element in a compound. Take for example methane: CH 4 Formula and molecular weight: 1*(12.011 amu ) + 4*(1.008) = 16.043 amu %C = 1*(12.011 amu )/16.043 amu = 0.749 = 74.9% %H = 4*(1.008 amu )/16.043 amu = 0.251 = 25.1%...
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