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Understanding Stoichiometry: Moles, Molar Mass, and Formulas

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Explore the concept of moles, molar mass, and formula calculations in chemistry through examples and practical applications. Learn about balancing reactions and empirical formulas. Video lessons included.

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Understanding Stoichiometry: Moles, Molar Mass, and Formulas

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  1. UNIT 2 STOICHIOMETRY

  2. Video 2.1 Moles and Molar Mass

  3. Stoichiometry • The Mole represents a specific amount of any substance. • Specifically it represents 6.02x1023 particles such as atoms and molecules.

  4. The Mole = 6.02x1023 • The mole is based on the fact that 12 grams of Carbon-12 has a mole of atoms. • A mole means you have 6.02x1023 particles.

  5. Molar Mass • Molar Mass (aka gram formula mass or molecular mass) is the mass of one mole of a substance. • Element’s molar masses are reported on the periodic table.

  6. Molar Mass Examples: Elements • What is the molar mass of iron? • What is the molar mass of copper? 55.8 g/mol 63.5 g/mol

  7. Molar Mass Examples: Compounds • What is the molar mass of water? • H2O = 2(1.0) + 16.0 = • 18.0g/mol • What is the gram-formula-mass of calcium chloride? • CaCl2 = 40.1 + 2(35.5) = • 111.1 g/mol

  8. Video 2.2 Calculating Moles

  9. What if you have more than one mole of a substance? Use the formula on table T: number of moles = given mass (g) _ gram-formula mass (Given mass will be your answer.)

  10. Let’s try these together… • Calculate the mass of 6.70 moles of carbon. 6.70 = x _ 12.og/mol x = 80.4 g

  11. Example 2 Calculate the mass of 0.023moles of lithium. 0.023 = x _ 6.9 g/mol x = 0.16 g

  12. Example 3 Calculate the mass of 25.02 moles of calcium phosphate. 25.02 = x _ 310.3/mol x = 7764 g

  13. The gfm formula • Use the same formula to calculate the moles, placing the number in the question on the numerator: number of moles = given mass (g) gram-formula mass

  14. Let’s try these together… 4. Determine the number of moles in 8.0 grams of Boron. x = 8.0 g _ 10.8g/mol x = 0.74 moles

  15. Example 5 Determine the number of moles in 0.567 grams of Helium. x = 0.567 g _ 4.00 g/mol x = 0.142 moles

  16. Example 6 Find the number of moles in 1230 grams of magnesium sulfate. x = 1230 g _ 120.4 g/mol x = 10.2 moles

  17. Video 2.3 Moles to Coefficients

  18. Reactions Mix + 2 Eggs + 1 cup water = Cake Reactants Products 2 Mix = 2 Cakes 6 Eggs = 3 Cakes

  19. Coefficients 4Al + 3O2 2Al2O3 reactants products Coefficients: How many moles of the substance are needed in a reaction.

  20. Relating Moles • To relate moles of one substance to another, simply create a proportion: 4Al + 3O2 2Al2O3 • If 3 moles of oxygen react, how many moles of Aluminum oxide form? 2 • If 4 moles of aluminum react, how many moles of oxygen are needed? 3 • If 4 moles of aluminum oxide are formed, how many moles of oxygen were used? 6 • If 8 moles of aluminum react, how many moles of oxygen are needed? 6

  21. Relating Moles 16 Al + 3S8 8Al2S3 • If 2.50 moles of sulfur react, how many moles of aluminum sulfide form? 2.50 mole S = x moles Al2S3 3 8 x = 6.67 moles

  22. Example 2 16 Al + 3S8 8Al2S3 If 3.75 moles of aluminum react, how many moles of sulfur are needed? 3.75 mole Al = x moles S8 16 3 x = 0.703 moles

  23. THINK Why do chemists use moles to measure substances? Why aren’t grams, liters and molecules enough?

  24. Video 2.4 Balancing

  25. Conservation of Mass • In a reaction, atoms and molecules cannot appear or disappear. Mass must stay constant from the beginning to the end of the reaction. • H2 + O2  H2O • ___H2 + ___O2  ___ H2O

  26. Balancing Reactions ___ N2 + ____H2  ____ NH3 ___Li + ____O2  ___Li2O

  27. Balancing __Pb(NO3)2 +__K2CrO4___PbCr2O4 + ___KNO3 ___C4H8 + ___O2  ___CO2 + ___H2O

  28. Types of Reactions • Synthesis: A + 2B  AB2 • Decomposition: AB2 A + 2B • Combustion: CH4 + O2 CO2 + H2O • Single Replacement: AB + C  CB +A • Double Replacement: AB + CD  AD + CB * Notice synthesis and decomposition are opposites. Also, combustion can have any carbon compound as a reactant.

  29. Video 2.5 Empirical and Molecular Formulas

  30. Empirical Formulas • Empirical formula refers to any molecular formula in it’s reduced form. Are these empirical? If not, reduce them: • C2H2 • C6H12O6 • NO2 • Na2(OH)2 • CH • CH2O • NO2 • NaOH

  31. Molecular Formulas • Molecular Formulas are some multiple of the empirical formula. • If the empirical formula is CH4 a molecular formula could be CH4, C2H8, C3H12 etc. • A compound whose empirical formula is NH3 has a mass of 34 g/mol. What is the empirical formula? NH3 = 17.0 g/mol 34/17 = 2 2NH3 = N2H6

  32. Steps: • Find the mass of the empirical formula. • Divide the mass given by the empirical mass. • Distribute your answer through the empirical formula. • If a compound has a mass of 45 g/mol and an empirical formula of CH3, what is the molecular formula? CH3 = 15 g/mol 45/15 = 3 3CH3 = C3H9

  33. Video 2.6 Percent Composition

  34. Percent Composition • Nutrition Facts on foods can tell you just how much of a substance you are consuming and how that relates to how much you should eat in a day. • It is equally important to know how much of a element or compound is in a mixture.

  35. Percent Composition % By mass: mass part x 100 total mass • Find the % by mass of phosphoric acid. H3PO4 3(1.0) x 100 3(1.o)+32.1+4(16.0) = 3.02%

  36. Example 2 Find the percent by mass of Ca in Ca(OH)2. 40.0 x 100 40.0+2(16.0)+2(1.0) = 54.1%

  37. Find the percent by mass of Na: 23.0/74.5 *100 = 30.9% • NaClO • NaCl • NaOH 23.0/58.5 *100 = 39.3% 23.0/40.0 *100 = 57.5%

  38. Percent Composition Examples A sample of a substance containing only magnesium and chlorine was tested in the laboratory and found to be composed of 74.5% chlorine by mass. If the total mass of the sample was 190.2 grams, what is the mass of the magnesium? 0.745 (190.2) = 141.7 grams

  39. For teacher use  Class slides

  40. THINK I went apple picking and they charge per dozen apples. I was worried it would take forever to count all the apples when I was on line. But the were smart: they weighed the apples. A dozen apples weighs 180 grams. • I have 540 grams of apples, how many dozen do I have? • How much does 2 dozen weigh? • How much does 0.25 dozen weigh? • How is this similar for moles?

  41. Mole Quiz Review • Find the gram formula mass of: • Ne c. NH3 • NaCld. Mg(OH)2 • Find the number of moles is 230g Mg(OH)2 • Find the mass of 2.50 moles of NH3 • Given: 2NaCl + Mg(OH)2 2NaOH + MgCl2 Find the number of moles of NaCl that will react with 3.75 moles of Mg(OH)2.

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