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CH 3: Stoichiometry

CH 3: Stoichiometry. Moles. Molar Mass. Molar mass – mass in grams of one mole of a substance Aka – molecular weight Calculate molar mass by summing the masses of the component atoms. Molar Mass Related Calculations:. Molar mass of a substance. Moles present in a given mass of substance.

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CH 3: Stoichiometry

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  1. CH 3: Stoichiometry Moles

  2. Molar Mass • Molar mass – mass in grams of one mole of a substance • Aka – molecular weight • Calculate molar mass by summing the masses of the component atoms.

  3. Molar Mass Related Calculations: • Molar mass of a substance. • Moles present in a given mass of substance. • Mass of a given number of moles of a substance. • Number of particles in a given mass or a given number of moles of a substance.

  4. What is the molar mass of glucose. The formula for glucose is C6H12O6. • What is the mass of 0.023 moles of glucose?

  5. How many glucose molecules are present in 0.023 moles of the compound? • How many moles of glucose are present in 3.5 x 10-3 grams of the substance?

  6. Mass Percent • Compounds are typically described by either their chemical formula or their percent by mass of the component elements. Mass % X = n (molar mass X) x 100% molar mass of compound

  7. Mass Percent Calculations • Calculate the mass percent of each element in C6H12O6 • Assume one mole of the substance.

  8. Empirical and Molecular Formulas • Molecular formula – ratio of atoms in a molecule • Empirical formula – simplest ratio of elements in a molecule • Glucose: • Molecular formula: C6H12O6 • Empirical Formula: • Molecular formula = n (empirical formula)

  9. Mass %  Empirical Formula Given mass percent data: • “Calculate” mass in grams of each element in 100 g of compound. • Convert each mass into moles of the element. • Divide each molar answer by the smallest of the values. • If the numbers obtained in step 3 are not whole numbers, multiply all by an integer so the results are whole numbers • Whole numbers obtained in step ¾ are the subscripts in the empirical formula.

  10. Empirical Formula  Molecular Formula Molar mass needed • Calculate mass of one mole of the empirical formula. Molar Mass = integer (n) empirical formula weight (n)(empirical formula) = molecular formula

  11. Caffeine • Molar mass = 194 g/mol • 49.49 % C • 5.19 % H • 28.85 % N • 16.48 % O What is the empirical and molecular formula of caffeine?

  12. Acetaminophen (Tylenol) Molar mass = 151 g/mol • 63.56 % C • 6.00 % H • 9.27 % N • 21.17 % O What is the empirical and molecular formula of acetaminophen?

  13. #72 page 120 Urea: Molar Mass 60 g/mol (I looked this up – not in question) 1.121 g N 0.161 g H 0.480 g C 0.640 g O What is the empirical and molecular formula of urea?

  14. 3.7/3.8 Chemical Reactions • Writing chemical reactions • Write the correct chemical formula for each reactant and product. • Use a subscript after each formula to indicate the state of each substance • (s) – solid • (l) – liquid • (g) – gas • (aq) – aqueous (dissolved in water)

  15. Chemical equations must obey the law of conservation of matter – balancing does this. • Balance the equation by adding coefficients in front of reactants and products as needed • DO NOT CHANGE THE FORMULAS FOR THE COMPOUNDS

  16. One of my favorite chemical reactions: Mg(s) + HCl(aq) MgCl2 (aq) + H2 (g) • As written the reaction does not obey the law of conservation of matter – reaction needs to be balanced

  17. Mg(s) + 2 HCl(aq) MgCl2 (aq) + H2 (g) • Now the reaction is balanced!

  18. Balance the Reactions N2 (g) + H2 (g) NH3 (g) Na(s) + Cl2 (g) NaCl(s) Pb(NO3)2(aq) + NaCl(aq) PbCl2(s) + NaNO3(aq)

  19. Meaning of the Balanced Reaction • Atomic level • Molar level • Stoichiometry - relationship between moles, the balanced reaction and mass • Page 104 outlines the needed steps

  20. 2 Na + Cl2 2 NaCl 2 mol Na + 1 mol Cl2 2 mol NaCl

  21. Cookie example! 2 eggs + 1 bag chips  50 cookies This example was much more enjoyable in person!

  22. Molar Ratios • Balanced reactions lead to molar ratios N2 + 3 H2  2 NH3

  23. N2 (g) + 3 H2 (g) 2 NH3 (g) • If 3.5 grams of N2 to react, how many moles and how many grams of NH3 would be made?

  24. N2 (g) + 3 H2 (g) 2 NH3 (g) • How many grams of H2 are needed to make37.8 grams of NH3?

  25. 2 LiOH + CO2 Li2CO3 + H2O • How many grams of LiOH are needed to react 750. grams of CO2? • How many grams of Li2CO3 will be made if 750. grams of CO2 react?

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