Chemistry

Atoms and Molecules


Calculation of Molecular Mass


If the molecular formula of a substance is known, its molecular mass can be calculated, because the molecular mass is equal to sum of the atomic masses of all the atoms present in one molecule of the substance. For example, one molecule of water (H2O) contains 2 atoms of hydrogen and 1 atom of oxygen.So,the molecular mass of water will be equal to the sum of the masses of 2 hydrogen atoms and 1 oxygen atom. Knowing that the atomic mass of hydrogen is 1 u and that of oxygen is 16 u, the molecular mass of water can be calculated as follows :
Mass of H atom = 1 u
Mass of 2H atoms = 2x1
= 2 u
Mass of O atom = 16 u
Now, Molecular mass of H2O = Mass of 2H atoms + Mass of O atom
= 2 + 16
= 18 u
Thus, the molecular mass of water (H2O) is 18 u.
In the case of ionic compounds like sodium chloride which consist of ions (and not molecules) the term formula mass is sometimes used in place of molecular mass. The molecular masses of some common elements, which exist as molecules, are given below :

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Notes
Atoms and Molecules - Notes
1. Drawbacks of Dalton’s Atomic Theory
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2. Monovalent Cations (Cations Having a Valency of 1+)
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3. Divalent Cations (Cations Having a Valency of 2+)
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4. Dalton’s Symbols of Elements
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5. Trivalent Cations (Cations Having a Valency of 3+)
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6. Formulae of Some Molecular Compounds
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7. an example, let us give the significance of symbol C
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8. Chemical Formulae
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9. An Important Discussion
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10. Molecular Formulae of Some Common Elements
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11. 2. A Negatively Charged Ion is Known as Anion
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12. Molecules
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13. As an example, let us give the significance of the formula H2O
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14. Explanation of the Law of Conservation of Mass
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15. Molecules of Compounds
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16. Atoms
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17. Molecular Mass
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18. Some Ionic Compounds
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19. Atoms, Molecules and Ions
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20. Formulae of Elements
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21. Simple Ions and Compound Ions (Polyatomic Ions)
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22. Formulae of Compounds
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23. Formula Mass
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24. Symbols of Elements
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25. 1. A Positively Charged Ion is Known as Cation
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26. Gram Atomic Mass And Gram Molecular Mass
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27. Mole of Molecules
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28. Mole of Atoms
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29. 1. Molecules of Elements
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30. Ions
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31. John Dalton
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32. Dalton's Atomic Theory
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33. Explanation of the Law of Constant Proportions
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34. Significance of the Formula of a Substance
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35. Some Common Ions
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36. Molecular Masses of Some Common Elements
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37. Laws Of Chemical Combination
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38. Divalent Anions (Anions Having a Valency of 2-)
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39. Ionic Compounds
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40. Calculation of Molecular Mass
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41. Formula Unit of Ionic Compounds
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42. Atoms And Molecules
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43. How Do Atoms Exist
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44. Writing Of Formulae Of Ionic Compounds
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45. Gram Molecular Mass
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46. Problems Based On Moles Of Atoms
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47. Problems Based On Moles Of Molecules
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48. Laws Of Conservation of Mass
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49. Experiment to Verify Law of Conservation of Mass
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50. Atomic Mass Of An Element
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