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Moles Atoms are very, very small. 1 atom of hydrogen weighs approximately 1.67 x 10-27 kg. As a result, it’s not very practical to do chemical reactions by counting out the number of atoms or molecules that will be reacting, because we’ll be counting for a very long time! You’ve seen this before, because when working with a large number of objects, it’s frequently handy to use units that are easier to work with. 2 shoes = 1 pair 12 eggs = 1 dozen 144 pencils = 1 gross 500 sheets of paper = 1 ream 6.02 x 1023 atoms or molecules = 1 mole The idea behind moles is the same as the idea behind “dozens”, except that the number is much bigger. Definition: 1 mole = 6.02 x 1023 of anything. 6.02 x 1023 is referred to as “Avogadro’s number” in honor of the dude who first worked with it. If moles is such a handy number, why haven’t you used it before now? 1 mole of most objects that you work with on a daily basis is very, very large. For example, 1 mole of M&M’s would cover the continental United States to a depth of 125 km. Although we could use moles to describe numbers of things that we work with everyday, it’s not really very practical. Finding Molar Mass Molar mass (also called “molecular weight” or “molecular mass”): The weight of one mole of a chemical compound. The unit is “g/mol”. For elements, the mass of one mole of atoms is called the “atomic mass” and is found on the periodic table. How to calculate the molar mass of a compound: For elements, the molar mass is the same thing as the atomic mass. For chemical compounds, it’s the sum of the masses of all of the atoms in the molecule. Example: NaCl Na: Cl: 23 grams x 1 atom = 35 grams x 1 atom = Total: 23 grams/mol 35 grams/mol 58 grams/mol Solve these: o o o MgCl2 Fe(OH)2 Be3(PO4)2 For these last two examples, tell them that the molar mass for compounds like this is found by multiplying everything in the parentheses by the number outside the parentheses. ____g/mol ____g/mol ____g/mol Mole calculations: How do you count out a mole of atoms? You don’t. Even if it were possible to count out individual atoms in a reasonable period of time, the equipment we have only measures “grams.” As a result, we need to be able to convert between atoms/molecules, moles, and grams. Use the diagram below with the t-chart method of doing calculations to help you convert between grams, moles, and molecules/atoms. (This is on your EOC Periodic Table) grams molar mass moles 6.20 x 1023 molecules or atoms Handy hint: In conversion factors, always write “1” in front of “moles”! Quick recap of the t-chart method: 1. Make a T 2. Put what you’ve got in the top left 3. Put the units of what you’ve got in the bottom right 4. Put the units of what you want in the top right 5. Put in the conversion factors 6. [If needed, add another step to get to where you’re going] 7. Multiply the stuff on the top together and divide by the stuff on the bottom. Examples: (Go over the T-chart with the first few): How many grams are in 2.1 moles of Be? How many molecules are in 6.3 moles of CH4? How many molecules are there in 11.1 grams of carbon dioxide? How many grams are in 4.1 x 1023 molecules of H2O? Molar Mass Practice Worksheet Find the molar masses of the following compounds: 1) NaBr 2) PbSO4 3) Ca(OH)2 4) Na3PO4 5) (NH4)2CO3 6) C6H12O6 7) Fe3(PO4)2 8) (NH4)2S 9) Zn(C2H3O2)2 10) AgF Mole Calculation Practice Worksheet Answer the following questions: 1) How many moles are in 25 grams of water? 2) How many grams are in 4.5 moles of Li2O? 3) How many molecules are in 23 moles of oxygen? 4) How many moles are in 3.4 x 1023 molecules of H2SO4? 5) How many molecules are in 25 grams of NH3? 6) How many grams are in 8.2 x 1022 molecules of N2I6?