Using the Mole Concept |
One mole of any substance can be calculated from its formula mass. Since this is true it is absolutely essential that when you are using the mole concept that the correct formula be used. It is not enough to say "use 1 mole of nitrogen". Do we mean atomic elemental nitrogen or nitrogen gas? There is a difference! One mole of N consists of Avogadro's number of nitrogen atoms (and has a mass of 14.01 g), whereas 1 mole of N_{2} consists of Avogadro's number of molecules, each molecule having two nitrogen atoms. One mole of N_{2} molecules would have a mass of 2 X 14.01 g = 28.02 g. |
One of the advantages of the mole concept is that it lets us think about formulas on two levels at the same time. One level is that of atoms or molecules or ions, and the other level is that of lab-sized practical quantitites, such as moles and grams. Look at the equation below: |
H_{2}O consists of
2 H
+ O 1 molecule of H_{2}O 2 atoms of H 1 atom of O 1 dozen H_{2}O molecules 2 dozen atoms of H 1 dozen O atoms 6.02 X 10^{23} H_{2}O molecules 12.04 X 10^{23} H atoms 6.02 X 10^{23} O atoms 1 mole of H_{2}O molecules 2 moles of H atoms 1 moles of O atoms 18.0 g of H_{2}O 2.0 g of H 16.0 g O atoms |
When we think about H_{2}O at the first level, we can easily see that a dozen of its molecules are made from two dozen atoms of H and one dozen atoms of O. However, if we switch to the more practical lab-sized level, it is just as easy to think about one mole of H_{2}O and to view this quantity as consisting of two moles of H and one mole of O. |
The numbers in all but the last row are in the same ratio regardless of the scale, whether we deal with single particles or with moles of them. After planning an experiment at the mole level, it is easy to convert numbers of moles into corresponding masses of chemicals to meet any desired needs. |