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Chemistry 111

Avogardo's Number

Avogadro's Number

Avogadro's Number is an important conversion factor that helps us to convert between the microscopic (atomic) scale and the macroscopic scale at which we work in the laboratory. For example, it would be impossible to measure 10 molecules of water for an experiment, but easy to measure 10 grams. which would contain considerably more than 10 molecules!.

Avogadro's Number is 6.022 x 1023sup> things per mole. the value can be used to determine the number of objects in a sample of a given number of moles, or the number of moles for a given number of objects.

Some Examples

Problem Solution
How many moles of O2 are there in a sample that contains 3.53 x 1024 O2 molecules? 
                           1 mol
3.53 x 1024 molec * ------------------ = 5.86 mol
                    6.022 x 1023 molec
How many protons are there in an O2 molecule? (Oxygen is atomic number 8.) 
             2 O atom    8 proton
1 O2 molec * --------- * -------- = 16 proton
             O2 molec     O atom
How many mol protons are there in a sample that contains 3.53 x 1024 O2 molecules?
(Note: There are two solutions to this problem shown to the right!)		 
                          1 mol O2       16 mole protons
3.53 x 1024 molec * ------------------ * ---------------- = 93.8 mol protons
                    6.022 x 1023 molec      1 mole O2
 
                    16 protons           1 mol 
3.53 x 1024 molec * ---------- * --------------------- = 93.8 mol
                     1 molec     6.022 x 1023 protons

One of the hardest concepts for beginners in chemistry is conversion between a small number of things (like moles or grams) and a large number of things (like atoms or protons.) The following examples convert from moles to number of atoms, molecules, or ions.

Problem Solution
How many sodium atoms are there in 2.30 mol of Na? 
           6.022 x 1023 atom
2.30 mol * ----------------- = 1.39 x 1024 atom
                  mol
How many nitrate ions (NO3-) are there in 0.853 mol of Cu(NO3)2?
(Note: There are two solutions to this problem shown to the right!)		 
                     6.022 x 1023 Cu(NO3)2   2 NO3- ion
0.853 mol Cu(NO3)2 * --------------------- * ------------- = 1.03 x 1024 NO3- ion
                            mol               Cu(NO3)2
 
                      2 mol NO3-    6.022 x 1023 NO3- ion
0.853 mol Cu(NO3)2 * ------------ * --------------------- = 1.03 x 1024 NO3- ion
                        Cu(NO3)2        mol NO3-

Now it's your turn!

Problem 1
How many Ar atoms are ther in 3.89 mol of Ar?
3.89 atom 2.34 x 1024 atom 4.68 x 1024 atom 6.46 x 10-24 atom
Problem 2
How many H atoms are there in 1.35 mol of H2O?
1.35 atom 8.13 x 1023 atom 1.63 x 1024 atom 2.70 atom
Problem 3
How many mol of CCl4 will contain 4.23 x 1022 molecules?
0.351 mol 0.0702 mol 2.55 x 1048 mol 14.2 mol
Problem 4
How many mol of CaCO3 will contain 2.99 x 1024 O atoms?
1.66 mol 4.97 mol 14.9 mol 0.552 mol