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 (

K

₂CO₃) (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

K

₂CO₃ 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

2. Monovalent Cations (Cations Having a Valency of 1+)

3. Divalent Cations (Cations Having a Valency of 2+)

4. Trivalent Cations (Cations Having a Valency of 3+)

5. Dalton’s Symbols of Elements

6. an example, let us give the significance of symbol C

7. Formulae of Some Molecular Compounds

8. An Important Discussion

9. Molecular Formulae of Some Common Elements

10. Explanation of the Law of Conservation of Mass

11. 2. A Negatively Charged Ion is Known as Anion

12. Molecules

13. As an example, let us give the significance of the formula H₂O

14. Simple Ions and Compound Ions (Polyatomic Ions)

15. Molecules of Compounds

16. Formulae of Elements

17. Chemical Formulae

18. Some Ionic Compounds

19. Symbols of Elements

20. Molecular Mass

21. Gram Atomic Mass And Gram Molecular Mass

22. Atoms

23. Formulae of Compounds

24. Formula Mass

25. Atoms, Molecules and Ions

26. 1. Molecules of Elements

27. Mole of Atoms

28. 1. A Positively Charged Ion is Known as Cation

29. Ions

30. Mole of Molecules

31. Dalton's Atomic Theory

32. Laws Of Chemical Combination

33. Calculation of Molecular Mass

34. Divalent Anions (Anions Having a Valency of 2-)

35. Molecular Masses of Some Common Elements

36. Some Common Ions

37. Explanation of the Law of Constant Proportions

38. John Dalton

39. Significance of the Formula of a Substance

40. Formula Unit of Ionic Compounds

41. Ionic Compounds

42. Symbols Derived from Latin Names of the Elements

43. Modern Symbols of Elements

44. Atomic Mass Of An Element

45. Valencies of Some Common Metal Elements Valencies of Some Common Non-Metal Elements

46. Atomic Masses of Some Common Elements

47. Writing Of Formulae Of Molecular Compounds

48. Laws Of Conservation of Mass

49. Problems Based On Moles Of Atoms

50. Writing Of Formulae Of Ionic Compounds