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| Molecules of Compounds |
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| A molecule of a compound consists of two or more atoms of different elements joined together in a fixed ratio. |
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| Examples: CuSO4 contains Cu - 1 atom, S - 1 atom, O - 4 atoms |
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| A chemical formula represents the composition of a molecule of the substance in terms of the symbol of the elements present in the molecule. It is also called molecular formula. |
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The molecular mass of a substance is the relative mass of its molecule as compared with the mass of a12C atom taken as 12-units. It indicates the number of times, one molecule of the substance is  |
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| Table 2: Molecular Formulae of important Acids |
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| Table-3: Molecular formulae of important bases |
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| Molecular formulae of important salts |
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| Table- 4: (A) Molecular formulae of Oxides |
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| Molecular mass is equal to sum of the atomic masses of all atoms present in one molecule of the substance. |
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| Example: |
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| H2O |
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| Mass of H atom = 1 |
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| Mass of 2H atoms = 2 |
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| Mass of O atom = 16 |
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| Molecular mass = 2 + 16 = 18gms |
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| Molecular mass expressed in grams is numerically equal to gram molecular mass of the substance. |
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| Molecular mass of O2 = 32 |
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| Gram molecular mass of O2 = 32gms |
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 It represents name of the substance |
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 It represents one molecule of substance |
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 It gives the names of all the elements present in one molecule of the substance. It also gives the number of atoms of each element present. |
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 It represents a definite mass and refers to the presence of 6.023 x 1023 molecule. |
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| It is very difficult to accurately weigh the number of atoms as they are extremely light and small and they cannot be seen. Indirect methods have been utilized to determine the absolute mass of an atom. |
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| Relative atomic mass of elements |
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| Mass of a hydrogen atom = 16.735 x 10-24g |
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| Mass of a oxygen atom = 26.565 x 10-24g |
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| We all know that atoms are too small and weigh too little to express their masses in grams or kilograms. Therefore it was agreed upon that the mass of a particular atom should be considered as a standard unit and the masses of other atoms to be related to the standard. The resulting masses are considered to be relative atomic masses as they are relative to the standard. |
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| Oxygen and hydrogen were initially chosen as the standard units. But later C12 atom [carbon isotope] was considered the standard for comparison with other atoms or molecules. |
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| Relative atomic mass (RAM) of an element is the number of times one atom of an element is heavier than 1/12 the mass of an atom of carbon [C12]. |
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| Gram atomic mass is the relative atomic mass of an element expressed in grams. |
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