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Chapter 3

Chapter 3. Mass Relationships in Chemical Reactions. Atomic Mass. What is atomic mass?. Mass of an atom in atomic mass units (AMU) 1 AMU is the mass of 1/12 of a carbon-12 atom Also called atomic weight. Relative Atomic Mass. What does the relative atomic mass of an element represent?.

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Chapter 3

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  1. Chapter 3 Mass Relationships in Chemical Reactions

  2. Atomic Mass • What is atomic mass? • Mass of an atom in atomic mass units (AMU) • 1 AMU is the mass of 1/12 of a carbon-12 atom • Also called atomic weight

  3. Relative Atomic Mass • What does the relative atomic mass of an element represent? • Average mass of all known existing isotopes of that element • It is “unitless” • Proportioned based on abundance • Ex. Carbon has a mass of 12.011 which is an average of all of the carbon isotopes

  4. Isotope Proportions A certain sample of element Z contains 60% of 69Z and 40% of 71Z. • What is the relative atomic mass of element Z in this sample?

  5. Isotope Proportions Chlorine exists as two isotopes, 35Cl and 37Cl. The relative atomic mass of chlorine is 35.45. • Calculate the percentage abundance of each isotope.

  6. Molar Mass and NA • What unit is used to measure the number of atoms/molecules? • Moles (mol) – the number of atoms in 12 grams of a carbon-12 sample • 1 mole = 6.02 x 1023 • Avogadro's number (NA) • We use it like we would a pair (2), dozen (12), or a gross (144) to simplify our math because it is freaking huge

  7. How Big Is A Mole? • If you would distribute 1 mole of pennies among the current population of the Earth, how much would everyone get? • How long would it take to spend this amount if you would spend $1,000/day? • How many people could you make from 1 mole of cells? • How long is 1 mole of seconds? • If we took a mole of regular 8.5 x 11 inch sheets of paper and started laying them out side by side on the planet’s surface. What percentage of the Earth would be covered?

  8. How Big Is A Mole? Answers • $900 million dollars • 2,500,000 years • 10 billion • 950,000 x age of the universe (14 billion yrs) • 100 % …and it would be 17 million sheets thick

  9. Molar Mass • How is molar mass different from relative atomic mass? • It is the mass of one mole of a substance • Abbreviated MM • Same number as atomic mass, but with different units • Units: • IB Units: g mol-1

  10. Molar Mass • What is the molar mass (MM) of the following elements? • Lithium • Plutonium • Bromine • 6.94 • 239.05 • 79.91

  11. Molar Mass • How do we determine the molar mass of a molecule? • What is the molar mass of water? • Add up the molar masses of each element within the compound • H2O • MM = 2(H) + 1(O) • MM = 2(1.01) + 1(16.00 ) • MM = 18.02

  12. Molecular Vs. Molar • What is the difference between atomic, molecular and molar mass? • Different units for different purposes • Atomic/Molecular Mass  AMU’s • Molar Mass  • Can be used interchangeably

  13. Molar Mass • What is the molar mass of each of the following species: • KBr • FeCl3 • Sodium Hydroxide • Lead (II) Oxide • 119.01 • 162.20 • 40.00 • 223.19

  14. Navigating • How can we navigate between mass (m), moles (n) and the number of atoms, molecules or particles? • We use molar mass (MM) and Avogadro's number (NA)

  15. Double Circles

  16. Double Triangles m mass N number NA Avogadro's number n moles n moles MM molar mass

  17. Mass-Mole Conversions • How many moles in 28 grams of CO2 ? • What is the mass of 5 moles of Fe2O3 ? • Find the number of moles of argon in 452 g of argon. • Find the grams in 1.26 x 10-4 mol of HC2H3O2 • Find the mass in 2.6 mol of lithium bromide.

  18. Mole-Number Conversions • How many moles of magnesium is 3.01 x 1022 atoms of magnesium? • How many molecules are there in 4.00 moles of glucose, C6H12O6? • How many moles are 1.20 x 1025 atoms of phosphorous? • How many atoms are in 0.750 moles of zinc? • How many oxygen atoms are in 0.400 moles of N2O5?

  19. To the right, To the right • How many atoms are in 20.0 grams of gold? • Mass  20.0 g • Divide by molar mass to get to moles • 0.102 moles • Multiply by Avogadro's number to get to atoms • 6.14E22 Au atoms

  20. To the left, To the left • What is the mass of 1.00E24 carbon atoms? • Number  1.00E24 atoms • Divide by Avogadro's number to get to moles • 1.66 moles • Multiply by molar mass to get mass • 19.9 grams

  21. Combined • Find the mass in grams of 2.00 x 1022 molecules of diatomic fluorine. • Find the number of molecules in 60.0 g of N2O. • How many molecules are in 5 mg of aspartame, C14H18N2O5?

  22. Combined

  23. PRELAB 1 Gum

  24. Make sure… • Calculating and Analyzing • Mass of sugar with the correct uncertainty • Percent mass of sugar in your piece of gum • Percent Error

  25. Reflection • Please write the questions in the box. What mathematical relationship/formula did you use to determine percent by mass? Why?

  26. Extension Question • Please write the question in the box. If you were given one molecule of sucrose, C6H12O6, what information do you think you would need to figure out the percent mass of carbon? Why?

  27. Percent Composition • What is percent composition? • What two pieces of info do we need? • Relative measure of the mass of each different element present in the compound • Part/Whole x 100 • Mass of the element and the compound

  28. Percent Composition • How do we calculate it? • Calculate the mass of the compound • Calculate the mass of each element

  29. Percent Composition • What is the percent mass of hydrogen in water? • 2 hydrogens • 2(1.01) = 2.02 g/mol • Water is H2O • 18.02 g/mol • 2.02/18.02 x100 = • 11.2% (tenths place)

  30. Percent Composition • Calculate the percent composition of oxygen in sodium sulfate. • Na2SO4 142.04 g/mol • 4 oxygens • 4(16.00) = 64.00 g/mol • (64.00/142.04) x 100 = • 45.1%

  31. Percent Composition • Tartrazine, or yellow dye # 5, is commonly found in many foods. Its molecular formula is C16H9N4Na3O9S2 . What is the percent composition of Carbon and Sulfur? The molar mass of Tartrazine is 534.4 g/mol.

  32. Working in Reverse • If we start with %’s, what can that possibly lead us to? • Working the formula in reverse and using mole ratios, we can get the empirical and possibly the molecular formula

  33. Determination of the Empirical Formula • How do we determine it? • Assume that you have 100% of the element = mass • Divide each element by its molar mass • Divide by the smallest amount of moles • (Optional) Multiply by a coefficient

  34. Butane’s Empirical Formula • Butane is made up of only carbon and hydrogen. If hydrogen makes up 17.34% of butane’s mass and carbon the remaining 82.66%, what is the empirical formula of butane? • C2H5 • What is the molecular formula for butane if its molecular mass is 58.14 g/mol? • Multiply each subscript by 2 • C4H10

  35. Determine the Empirical Formula • Determine the empirical formula of a compound having the following percent composition by mass… • K: 24.75% • Mn: 34.77% • O: 40.51% Empirical Formula: KMnO4 • What is the molecular formula if its molecular mass is 158.04 g/mol? ANSWER: KMnO4 (potassium permanganate)

  36. What is the difference between a chemical reaction and equation? • A chemical reaction is the process of making a new substance(s) • Ex. Iron oxidizing • A chemical equation is a symbolic representation of the reaction • 4Fe + 3O2 → 2Fe2O3 Chemical Reactions and Equations

  37. How do we read a chemical equation? • 4Fe(s) + 3O2(g) → 2Fe2O3(s) • Reactants (left) • Products (right) • → Goes in one direction • ↔ Reversible reaction • Coefficients represent the mole ratios • Notations (s, l, g, aq) represent phases/states • Conservation of mass Chemical Reactions and Equations

  38. What is a balanced equation? • The number of atoms on the left has to equal the number on the right • Use coefficients to balance them • Unbalanced: • CH4 + O2 → CO2 + H2O • Balanced: Chemical Reactions and Equations

  39. Chemical Reactions and Equations • How do we balance the equations? • KClO3 → KCl + O2 • 2KClO3 → 2KCl + 3O2 • Al + O2 → Al2O3 • 4Al + 3O2 → 2Al2O3 • C2H6 + O2 → CO2 + H2O • 2C2H6 + 7O2 → 4CO2 + 6H2O

  40. Chemical Reactions and Equations Balance the following chemical equations: • K2CO3 + HCl KCl + H2O + CO2 • K2CO3 + 2HCl  2KCl + H2O + CO2 • CaCO3 + HNO3 Ca(NO3)2 + H2O + CO2 • CaCO3 + 2HNO3 Ca(NO3)2 + H2O + CO2 • Pb(NO3)2 + NaI PbI2 + NaNO3 • Pb(NO3)2 + 2NaI  PbI2 + 2NaNO3 • Al2(SO4)3 + NaOH Al(OH)3 + Na2SO4 • Al2(SO4)3 + 6NaOH  2Al(OH)3 + 3Na2SO4

  41. Writing Equations Determine and balance the following equations: • Zinc metal reacts with copper (II) sulfate solution to produce solid copper metal and zinc (II) sulfate solution. • Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq) • Solid calcium hydroxide reacts with solid ammonium chloride on heating to produce solid calcium chloride, steam and ammonia gas. • Ca(OH)2(s) + 2NH4Cl(s)  CaCl2(s) + 2H2O(g) + 2NH3(g) • Solid silicon tetrachloride reacts with water to produce solid silicon dioxide and hydrogen chloride gas. • SiCl4(s) + 2H2O(l)  SiO2(s) + 4HCl(g)

  42. What is stoichiometry? The study of any numerical observation within a chemical reaction Using the mole method to determine how much reactant is needed (or product created) based on the number of moles Amounts of Reactants and Products

  43. What can be said about the reaction: • H2 + O2 → H2O • Reactants: Hydrogen and Oxygen • Products: Water • Unbalanced • Balanced: 2H2 + O2→2H2O • Proportion: 2 moles of hydrogen and 1 mole of oxygen create 2 moles of water Amounts of Reactants and Products

  44. How many moles of water should be formed with 8 moles of oxygen? What happens when our measurements are not in moles? 1:2 Ratio 8 moles of oxygen is equivalent to 16 moles of water Convert them into moles before starting using molar mass Amounts of Reactants and Products

  45. Amounts of Reactants and Products

  46. Amounts of Reactants and Products • I have the formula: Mg + HCl → MgCl2 + H2 • What is the mass of MgCl2 that is formed when 20.0 g of HCl reacts with Mg? • Balance it • Mg + 2HCl → MgCl2 + H2 • Convert into moles • (20.0 g)/(36.45 g/mol) = 0.549 molHCl • Use the mole ratio method • 2:1 ratio → 0.275 mol MgCl2 • Convert back into mass • 0.275 mol MgCl2 → 26.2 g

  47. Amounts of Reactants and Products • What mass of silver nitrate as a solution in water would need to be added to 5.0 g of sodium chloride to ensure a complete precipitation of the chloride? • AgNO3(aq)+ NaCl (aq)  AgCl(s) + NaNO3(aq)

  48. Amounts of Reactants and Products • What mass of barium sulfate would be produced from 10.0 g of barium chloride in the following reaction? • BaCl2 + H2SO4 BaSO4 + HCl • Balanced: BaCl2 + H2SO4 BaSO4 + 2HCl

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