Chemistry

Atoms and Molecules


Formula Mass


Since the ionic compounds do not consist of molecules, the use of term "molecular mass" for them is not very correct. So, we use the term 'formula mass' for ionic compounds in which individual molecules do not exist.The formula mass of an ionic compound is the relative mass of its 'formula unit' as compared with the mass of a carbon-12 atom taken as 12 units.In other words, formula mass is the sum of the atomic masses of the atoms (or ions) represented by its formula. In order to calculate the formula mass of an ionic compound, we should know the formula of the ionic compound as well as the atomic masses of all the atoms (or ions) present in the formula. The formula mass is then calculated in a way similar to the calculation of molecular mass. Please note that the atomic mass of an atom and its ion is just the same (because the electrons which convert an atom into an ion have negligible mass).For example, the formula mass of sodium chloride (NaCl) will be the sum of atomic masses of sodium (Na) and chlorine (Cl). Now, the atomic mass of Na is 23 u and the atomic mass of Cl is 35.5 u, so the formula mass of sodium chloride (NaCl) will be 23 + 35.5 = 58.5 u. We will now solve one problem based on the calculation of formula mass.
Sample Problem. Calculate the formula mass of potassium carbonate (K2CO3) (Given : Atomic masses :K = 39 u ; C = 12 u ; O = 16 u)
Solution. We know that:
Formula mass = Mass of + Mass of + Mass of
of K2CO3 2 K atoms one C atom 3'O'atoms
= 2 x 39 + 12 + 3 x 16
= 78 + 12 + 48
= 138 u
Thus, the formula mass of potassium carbonate is 138 u.

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