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Atomic Mass and Molecular Mass

Atoms and molecules are too small to be weighed. We refer to them as relative atomic masses. Chemists use a relative mass scale to compare the masses of different atoms. Hydrogen is chosen as standard and it was assigned a mass of unit (atomic mass unit—amu). It is defined as follows:

The atomic mass of an element is defined as the ratio of the mass of one atom of the element to the mass of one atom of hydrogen.
 
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Since it is a ratio, the relative atomic mass of an element is merely a number.
 
Atomic mass of carbon = 12
 
Atomic mass of oxygen = 16
 
C-12 Standard The atomic mass of an element is defined as the ratio of the mass of one atom of an element to Description: 26925.png th mass of one C-12 atom.
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The molecular mass of a substance is the ratio of the mass of one molecule of the substance to Description: 32351.pngth mass of one C-12 atom.
 
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For example, the molecular mass of hydrogen is 2. This indicates that the mass of a hydrogen molecule is twice that of Description: 27373.png th mass of C-12 atom.
 
The molecular mass of a substance is obtained by adding the atomic masses of all the atoms present in the molecule.
 
For example, for carbon dioxide (CO2):
 
Molecular mass of CO2 = (1 × atomic mass of C) + (2 × atomic mass of O)
= (1 × 12) + (2 × 16)
= 12 + 32 = 44





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