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THE MOLE A MOLE IS: • 6.02 x 1023 of anything. • Na = Avogadro’s Number = 6.02 x 1023 In chemistry, we talk about moles of representative particles. So what’s a representative particle ? ? ? Substance Examples Representative Particle Element C, Mg, Fe Atoms Diatomic element H2, O2, Br2, F2, I2, N2, Cl2 Molecules Molecular compound H2O, CH3OH Molecules Ionic compound NaCl, MgBr2 Formula Units SO . . . • One mole of carbon contains 6.02 x 1023 atoms of carbon. • One mole of nitrogen contains 6.02 x 1023 molecules of nitrogen • On mole of NaI contains 6.02 x 1023 formula units of sodium iodide. The mole is important because: • The mole concept allows us to weigh atoms in the lab. • The mole concept allows us to compare amounts of atoms in chemical reactions. 1 MOLAR MASS the mass of a mole of any element or compound in grams 6 The mass of one mole of carbon atoms C Carbon 12.011 HOW TO CALCULATE THE MOLAR MASS OF A COMPOUND: 1. You need to know how many atoms of each element are in a substance in order to calculate its molecular weight. For example, H2O has two atoms of hydrogen and one atom of oxygen. 2. You need to know the molar mass of each element in order to calculate the molar mass of the compound. You can find these values on your periodic table. 3. Multiply each element's molar mass by how many atoms are present in the formula, then add the answers. To calculate the molar mass of H2O: Hydrogen: two atoms, molar mass = 1.0079g/mol Oxygen: one atom, molar mass = 15.9994g/mol (2 x 1.0079g) + (1 x 15.9994g) = 18.0152g/mol Here’s another example: Calculate the molar mass of Al2(SO4)3: two atoms of aluminum and the molar mass of Al is 26.98g/mol three atoms of sulfur and the molar mass of S is 32.06g/mol twelve atoms of oxygen and the molar mass of O is 15.9994g/mol (2 x 26.98g/mol) + (3 x 32.06g/mol) + (12 x 15.9994g/mol) = 342.1328g/mol Calculate the molar mass of: 1) (NH4)2S 2) Fe2O3 3) KClO4 4) SF6 5) (NH4)2SO4 6) 2 BASIC MOLE CALCULATIONS Now we’re ready to do some basic calculations. Before we begin, let’s review the process we use to convert from one unit to another in chemistry, called the factor-label method. • In this process, numbers and units are equally important! • Conversion factors are relationships that convert one quantity to another. • Multiply what is given by a conversion factor set up as a fraction. Flip the conversion factor fraction if necessary to eliminate the units you’re trying to get rid of. • Get into the habit of putting a line through the units as they “divide out”. The only units that are not “canceled” should be the correct units of the final answer. When all units have been canceled except those needed for the final answer, you are ready to pick up your calculator and find the number. Example: • Convert 1000 grams to pounds. It is known that one pound is equal to 454 grams. This becomes the conversion factor needed to work the problem. Factor-label tips: Setting up the factor-label problem is just as important as the answer Form the habit of working neatly, canceling units, and circling the answer Remember, units are just as important as numbers in the answer CONVERSIONS HIGHWAY Number of particles: MASS Single atoms MOLES Molecules Formula Units ATOMS 3 We can convert between MOLES and MASS by means of the MOLAR MASS a. What is the mass of 3 moles of H2O? b. How many moles of O2 are in 4.00 g of O2? We can convert between MOLES and NUMBER OF PARTICLES by means of AVOGADRO’S NUMBER a. How many atoms are in 5 x 10-7 mol of gold, Au? b. If we have 5 x 108 molecules of O2, how many moles of O2 do we have? We can convert between MOLECULES/FORMULA UNITS and ATOMS by counting the number of atoms. a. How many atoms are in 1 molecule of CO2? b. How many atoms are in 7.8 moles of CO2? c. How many atoms are in 50.0 g of O2? 4 PRACTICE PROBLEMS 1. How many moles are present in the following samples? a. 25.0 g of carbon b. 0.353 g of MgO c. 400.2 g of water 2. What is the mass in grams of the following: a. 0.00100 mol of K2CO3 b. 3.50 mol of iron (II) chloride c. 5 x 10-4 mol of hydrogen sulfide 3. Make the following conversions: a. 1.00 mol of ammonium chloride to formula units b. 2.5 mol of O3 to molecules c. 0.003 mol of cadmium to atoms 4. Make the following conversions: a. 200 x 1023 formula units of AgCl to moles b. 6.02 x 1025 atoms of nitrogen to moles c. 120.2 x 1015 molecules of H2 to moles 5. How many atoms are contained in one molecule of: a. HC2H3O2 b. Ammonium chloride c. (CH3)2CO d. ammonium sulfate 6. Make the following conversions: a. 3.655 mol of K2CO3 to atoms b. 2.5 mol of O2 to atoms c. 1.35 mol of ammonium chloride 7. Make the following conversions a. 5.0 g of KClO3 to moles b. 2.5 mol of HC2H3O2 to grams c. 4.2 mol of NaCl to formula units d. 1.2 x 1022 formula units of KI to moles e. 7.8 mol of CH3OH to atoms 5