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Chapter 3: Calculations with Chemical Formulas

Learn how to calculate molecular mass, formula mass, Avogadro's Number, moles to grams conversion, stoichiometry, limiting reagents, and percent composition in chemical formulas.

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Chapter 3: Calculations with Chemical Formulas

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  1. Chapter 3: Calculations with Chemical Formulas Renee Y. Becker Valencia Community College CHM 1045

  2. Atomic and Molecular Mass • The molecular mass is the sum of the masses of the atoms making up the molecule (units amu) (molecular compounds only!) calculate the molecular mass: H2O C2H4O2

  3. Formula Mass • Formula Massis the sum of atomic masses of all atoms in a formula unit of any compound, molecular or ionic (units amu) calculate the formula mass: NaCl MgCO3

  4. Avogadro • One mole of a substance is the gram mass value, molar mass, equal to the amu mass of a substance • Molar mass, (MM), is in units grams/mole (conversion factor for gmol and molg) • Avogadro’s Number (NA = 6.022  1023 particles) is the numerical value assigned to the unit, 1 mole

  5. Example 1: Molar Mass • Calc. the molar mass: • Fe2O3 • C6H8O7 • C16H18N2O4

  6. Example 2: Avogadro’s Number Li2SO4 1. How many Li2SO4 units are in 3 moles of Li2SO4? 2. How many Li atoms are in 3 moles of Li2SO4? 3. How many S atoms are in 3 moles of Li2SO4? 4. How many O atoms are in 3 moles of Li2SO4?

  7. Example 3: Molesg Calculate 2.35 moles C  g C 4.67 x10-3 moles H2O g H2O 83.2 moles CaO  g CaO

  8. Example 4: g Moles Calculate 45.8 g Fe  moles Fe 6.54 x104 g C2H4  moles C2H4

  9. Stoichiometry • Stoichiometry: Relates the moles of products and reactants to each other and to measurable quantities

  10. Example 5: Stoichiometry 2 NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l) • How many moles of Cl2 are needed to react with 2 moles of NaOH? • How many moles of Cl2 are needed to react with 3 moles of NaOH? • How many moles of NaCl are formed by this reaction? • If you start with an excess of Cl2 and 5 moles of NaOH, how many moles of H2O can you produce?

  11. Example 6: Stoichiometry 2 NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l) 1. How many moles of NaOH are needed to react with 25.0 g of Cl2? 2. How many grams of NaOH are needed to react with 25.0 g of Cl2?

  12. Example 7: Stoichiometry • Let’s make up some more examples 2 NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l)

  13. Stoichiometry • Yields of Chemical Reactions: If the actual amount of product formed in a reaction is less than the theoretical amount, we can calculate a percentage yield. • % Yield = Actual yieldX 100% • Theoretical yield

  14. Example 8: Stoichiometry • CH2Cl2 is prepared by reaction of CH4 with Cl2 giving HCl. How many grams of CH2Cl2 result from the reaction of 1.85 kg of CH4 if the yield is 43.1%? • CH4(g) + 2 Cl2(g)  CH2Cl2(l) + 2 HCl(g)

  15. Stoichiometry • Limiting Reagents: The extent to which a reaction takes place depends on the reactant that is present in limiting amounts--the limiting reagent. • I need to make a fruit salad that is 1/2 apples and 1/2 oranges. I have 10 apples but only 7 oranges. What is the limiting fruit? How many apples and oranges can I use?

  16. Example 9: Stoichiometry • Limiting Reagent Calculation: Lithium oxide is a drying agent used on the space shuttle. If 80.0 kg of water is to be removed and 65 kg of lithium oxide is available, which reactant is limiting? • Li2O(s) + H2O(l) 2 LiOH(s) • Li2OMM = 29.88 g/mol • H2OMM = 18.02 g/mol

  17. Example 10: Stoichiometry • K2PtCl4 + 2 NH3 Pt(NH3)2Cl2 + 2 KCl • If 10.0 g of K2PtCl4 and 10.0 g ofNH3 are allowed to react: • a) which is limiting reagent? • b) how many grams of the excess reagent are consumed? • c) how many grams of cisplatin are formed? • K2PtCl4 = 415.08 g/mol NH3 = 17.04 g/mol • Pt(NH3)2Cl2 = 300.06 g/mol

  18. Percent Composition • Percent Composition: Identifies the elements present in a compound as a mass percent of the total compound mass. • Empirical Formula: Determined from data about percent composition, tells only the smallest whole number ratio of atoms in a compound. • Molecular Formula: Tells actual numbers of atoms in a molecule, can be same as empirical formula or a multiple of it.

  19. Example 11: Percent Composition What is glucoses, C6H12O6, empirical formula, and what is the percentage composition of glucose?

  20. Example 12: Percent Composition • Saccharin has the molecular formula C7H5NO3S, what is its empirical formula and the percent composition?

  21. Example 13: Empirical Formula • A compound was analyzed to be 82.67% carbon and 17.33% hydrogen by mass. The molar mass is 58.11 g/mol. • What is the empirical formula and molecular formula?

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