The mole is the standard method in chemistry for communicating how much of a substance is present.
Here is how the International Union of Pure and Applied Chemistry (IUPAC) defines "mole:"
The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
This is the fundamental definition of what one mole is. One mole contains as many entities as there are in 12 grams of carbon-12 (or 0.012 kilogram).
In one mole, there are 6.022 x 1023 atoms. Here’s another way: there are 6.022 x 1023 atoms of carbon in 12 grams of carbon-12.
Let’s say that real clearly: one mole of ANYTHING contains 6.022 x 1023 entities.
The word "entities" is simply a generic word. For example, if we were discussing atoms, then we would use "atoms" and if molecules were the subject of discussion, the word entities would be replaced in actual use by "molecules."
Avogadro’s Number has been very carefully measured in a number of ways over many decades. The symbol for mole is "mol." Why does a four-letter word have a three-letter symbol? That’s really the wrong question. Here’s why.
Here it is again: one mole of ANY specified entity contains 6.022 x 1023 of that entity. For example:
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One mole of donuts contains 6.022 x 1023 donuts
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One mole of H2O contains 6.022 x 1023 molecules
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One mole of nails contains 6.022 x 1023 nails
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One mole of Fe contains 6.022 x 1023 atoms
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One mole of dogs contains 6.022 x 1023 dogs
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One mole of electrons contains 6.022 x 1023 electrons
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One mole of ChemTeam members contains 6.022 x 1023 poor, suffering (I mean happy, joyful) high school students |
Get the idea?
6.022 x 1023 is so important in chemistry that it has a name. It is called Avogadro’s Number and has the symbol N. It is so named in honor of Amedeo Avogadro, an Italian chemist, who, in 1811, made a critical contribution (recognized only in 1860 after his death) which helped greatly with the measurement of atomic weights. (Someday, the ChemTeam hopes to tell the story for the WWW.)
However, counting atoms or molecules is very difficult since they are so small. However, we can "count" atoms or molecules by weighing large amounts of them on a balance.
When we weigh one mole of a substance on a balance, this is called a "molar mass" and has the units g/mol (grams per mole). This idea is very critical because it is used all the time.
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A molar mass is the weight in grams of one mole.
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One mole contains 6.022 x 1023 entities. |
Therefore, a molar mass is the mass in grams of 6.022 x 1023 entities.
OK. How does one calculate a molar mass? Get ready, because you already know how to calculate a molar mass.
The molar mass of a substance is the molecular weight in grams.
All you need to do is calculate the molecular weight and stick the unit "g/mol" after the number and that is the molar mass for the substance in question.
Calculate the molar mass of Al(NO3)3
(1 x 26.98) + (3 x 14.007) + (9 x 16.00) = 213.00 g/mol
213.00 grams is the mass of one mole of aluminum nitrate.
213.00 grams of aluminum nitrate contains 6.022 x 1023 entities of Al(NO3)3
Practice ProblemsCalculate the mass of one mole of each of these substances. (Optional: try naming each.) 1. AlCl3 14. Ba(SCN)2 27. LiH 40. Ba(BrO3)2 53. AlBr3 66. HCl
2. TeF4 15. K2S 28. CO 41. Hg2Cl2 54. P2O5 67. K2SO4
3. PbS 16. NH4Cl 29. SnI4 42. Cr2(SO3)3 55. NH4NO3 68. NaCl
4. Cu2O 17. KH2PO4 30. KOH 43. Al(MnO4)3 56. Ba(OH)2 69. LiI
5. AgI 18. C2H5NBr 31. K2O 44. CoSO4 57. PbSO4 70. Hg2O
6. N2O 19. Ba(ClO3)2 32. H2SO4 45. Ca(NO3)3 58. Ba3(PO4)2 71. HF
7. MoCl5 20. Fe(OH)3 33. Hg3N2 46. NaH2PO4 59. NaC2H3O2 72. FeCl3
8. Hg2Br2 21. (NH4)2S 34. SiF4 47. (NH4)3PO4 60. Ba(OH)2 73. NaHSO4
9. Ta2O5 22. CoCl2 35. NH4OH 48. KAl(SO4)2 61. NaHCO3 74. Ag2O
10. HgF2 23. KMnO4 36. N2O5 49. Hg2SO4 62. Al(OH)3 75. Pb(ClO2)2
11. KCl 24. CaSO4 37. SnCrO4 50. Al2(SO4)3 63. NH4MnO4 76. CoF3
12. KF 25. H2CO3 38. Al2O3 51. FePO4 64. Fe2O3 77. Al(C2H3O2)3
13. ZnO 26. CO2 39. CuCO3 52. Ca(C2H3O2)2 65. CaCO3 78. Na2Al2(SO4)4
79. (HOOCCH2)2NCH2CH2N(CH2COOH)2
80. (NH4)2CH(CH2)5COOH
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