Don't ruin a good today by thinking about a bad yesterday. Let it go. Anonymous
Idea Transcript
The Mole We have been talking about matter at the microscopic level, in terms of atoms, ions and molecules. However, in most cases the measurements and observations we make are of macroscopic quantities of matter. In order to relate the macroscopic and microscopic worlds it is necessary for us to experimentally determine the number of atoms or molecules contained in a macroscopic sample. How can we do this? Normally, if I asked you to determine the quantity of a specific item you would count. However, there are two serious drawbacks to this approach when it comes to atoms and molecules. First of all we can’t see individual atoms and molecules. Secondly, the numbers of atoms in everyday quantities of matter are astronomical. For example if we took a teaspoon of water molecules and expanded each molecule to the size of a marble, it would cover the entire surface of the earth to a depth of 3 miles. In order to deal with the huge numbers of atoms involved in everyday chemical processes, chemists defined a quantity known as a mole. 1 mole is the number of atoms contained in exactly 12 g of 12 C 1 mole = 6.0221367 1023 This quantity is known as Avagadro’s number. We will generally round this figure off to 6.02 1023 It is worthwhile to keep in mind that a mole is a quantity used for counting things, like a dozen or a gross, it can be applied to anything. Example Lets assume there are 55,000 (5.5 104 ) students at Ohio State University. How many dozens of students are there? How many moles of students are there? 5.5 104 students (1 dozen students/12 students) = 4.5 103 dozen students 5.5 104 students (1 mole students/6.02 1023 students) = 9.1 10-20 mole students Because of the way the mole has been defined (1 mole of 12 C atoms has a mass of 12 g) we have a very convenient relationship between microscopic and macrosopic weights. Avagadro’s number of atoms of any element has a mass in grams numerically equal to its atomic weight in amu’s. 1 mole 12 C = 12.000 g 1 mole C = 12.011 g 1 mole S = 32.066 g If we reverse the preceding definitions you can see how one can calculate the number of atoms in a sample from a measurement of the mass of the sample. Example How many sulfur atoms are contained in a 1.56 g sample of elemental sulfur? 1.56 g S (1 mol S/32.066 g S) = 4.86 10-2 mol S 4.86 10-2 mol S (6.02 1023 atoms S/1 mol S) = 2.93 1022 atoms S Given one of the three quantities; mass, number of molecules and number of moles, you should be able to calculate the other two quantities.