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Stoichiometry

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  1. Stoichiometry Molar mass, Percent composition, Moles, Conversions, Empirical formulas, Molecular formulas

  2. The Mole • SI unit for amount • A counting unit for measuring large quantities of small items • Atoms • molecules • Particles 1 mole = 6.022 x 10 23 atoms = molar mass

  3. Molar Mass • The amount of mass (grams) in one mole of a substance • For elements • Average atomic mass  periodic table • For compounds • Must add up each element’s mass • Subscripts are factored in by multiplication • Units are g/mol • May be called formula mass or molecular mass

  4. Steps to solve • Find elements on periodic table • Write down the mass • Multiply the mass by the subscript • Add together

  5. Sample: Determine Molar Mass Calculations are FUN! 40.08 g/mol 2. 55.847 g/mol • Ca • Fe • NaCl Na: 22.989 g/mol Cl: 35.453 g/mol 58.443 g/mol

  6. Molar Mass (cont.) 4. Fe2O3 5. Na2O • Fe: 55.847 x (2) = 111.694 g/mol • O: 15.999x (3)= 47.997 g/mol • 159.691 g/mol • Na: 22.989 x(2) =  45.978 g/mol • O: 15.999= 15.999 g/mol • 61.977 g/mol

  7. Percent Composition • Steps to solve • Find the total mass of the COMPOUND • Take the mass of each element and divide it by the total mass • Multiply by 100

  8. Sample: Calculate the percent Composition C: 12.011 44.009 O: 31.998 44.009 CO2 C: 12.011 g/mol O: (2)15.999 g/mol = 31.998 g/mol 44.009 g/mol .2730 100 27.3 % 100 72.7% .727

  9. Sample 2 H2SO4 H: (2)1.008 g/mol = 2.016 g/mol S: 32.064 g/mol O: (4)15.999 g/mol = 63.996 g/mol 98.076 g/mol H: 2.016 98.076 S: 32.064 98.076 O: 63.996 98.076 .0206 100 2.06 % .327 100 32.7 % .653 100 65.3%

  10. Composition of Hydrates Hydrate: crystal contains water within • Water can fit into the salt crystal • Happens in fixed ratios • Each salt that forms a hydrate has a DIFFERENT water ratio • Each salt crystal is unique Anhydrous: water has been removed • Achieved through drying • Steps to solve • Calculate the total mass of the compound INCLUDING THE water • Divide the water mass by total mass • Multiply by 100

  11. That was nothing. Time for Coco! Sample Problem: Hydrate • What percentage of water is found in CuSO4 •5H2O • Cu: 63.54 • S: 32.064 • O: (4)15.999= 63.996 • H2O: (5)18.01= 90.05 • 249.65 90.05 249.65 100 .361 36.1% H2O

  12. The Mole conversion Mass (grams) Particles Ions Atoms Molecules Formula units ÷ by molar mass X by 6.022 x 10 23 Mole ÷ by 6.022 x 10 23 X by Molar mass ÷ by 22.4 L x by 22.4 L Volume (Liters) Gases at STP

  13. Stoichiometric Conversions: Mass to Moles • Steps to convert • Identify what you start with • Determine what units you end in • Might have to do side calculations • Set up conversion factor • Evaluate • Do the Math! • You can relate any unit to another by this method • Relate one unit to another • conversion factors

  14. Example • How many moles are in 28 grams of CO2? 28 grams CO2 .636 _______ moles CO2 C: 12.011g/mol O: (2)15.999 g/mol = 31.998 g/mol 44.009 g/mol

  15. Empirical Formula • When the elements are in the smallest whole number ratio within compound Steps to solve • When given a compound • Divide out by the common multiple Steps to solve • When given percentages • Change percent sign to grams (NO MATH) • Convert masses to moles • Using conversions • Re-divide ALL moles amounts by the smallest mole amount • Multiply if not whole numbers • Write compound with subscripts

  16. Example: What’s my Empirical Formula Piece of Cake! • C6H6 • C8H18 • C2H6O2 • X39Y13 • CH • C4H9 • CH3O • X3Y

  17. Example: with percent A compound is found to contain 36.48% Sodium, 25.41% Sulfur, and 38.11% Oxygen. Find its empirical formula. Na2SO3

  18. Molecular Formula Multiple of an empirical formula So many compounds with the same Empirical formula

  19. Steps to solve • Complete an empirical formula process if needed • Find the mass of the EMPIRICAL FORMULA • Divide the Empirical formula mass by the molecular mass • Molecular mass is normally given in the problem • Answer is the common multiple • Multiple the SUBSCRIPTS by the common multiple

  20. All done! Time to go sledding! Yippee!! Example • A compound with an empirical formula of C4H4O and a molecular mass of 136 grams. What is the molecular formula of this compound? • C: 4 (12.011) = 48.044 • H:4 (1.009 ) = 4.036 • O: 15.999 • 68.079 C4H4O x 2 = C8H8O2

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