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The Mole

Learn how average atomic masses are calculated and explore the concept of stoichiometry in chemistry. Understand the concept of a mole and its significance in chemical calculations.

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The Mole

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  1. The Mole 6.02 X 1023

  2. Average atomic Masses • Atomic masses on the Periodic Table are not whole numbers since they are weighted averages of Mass Numbers (P+N) • Average atomic mass depends on two factors- mass and relative abundance of each isotope. • Example- Cu-63 (69.17% and 62.94amu) and Cu-65(30.83% and 64.93amu) • Average atomic mass= (69.17x62.94)+(30.83x64.93) divided by 100=63.55amu • Do Practice Q1-2, p133.

  3. STOICHIOMETRY - the study of the quantitative aspects of chemical reactions.

  4. Launch lab p 319- How much is a mole? • Decades or dozens are counting units. The Mole is also a counting unit. • Measure a small item (paperclip, eraser, penny) in cm using a ruler. • If a mole is 6.02 X1023 items, how far will a mole of your items, placed end-to-end lengthwise, extend into space? Express your answer in meters. • Convert this answer into light years (ly). • (1 ly=9.46 X1015 m).

  5. How much is a mole? • Compare the distance you calculated with these astronomical distances: • Distance to the nearest star (other than the sun)=4.3 ly • Distance to the center of the galaxy= 30,000 ly • Distance to the nearest galaxy=2X106 ly

  6. The Mole • A counting unit • Similar to a dozen or gross, except instead of 12, it’s 602 billion trillion 602,000,000,000,000,000,000,000 • 6.02 X 1023 (in scientific notation) • This number (termed Avogadro’s number) is named in honor of Amedeo Avogadro (1776 – 1856), who studied quantities of gases and discovered that no matter what the gas was, there were the same number of molecules present

  7. Just How Big is a Mole? • Enough soft drink cans to cover the surface of the earth to a depth of over 200 miles. • If you had Avogadro's number of unpopped popcorn kernels, and spread them across the United States of America, the country would be covered in popcorn to a depth of over 9 miles. • If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

  8. The Mole ･Atoms and molecules are extremely small making it very difficult to measure their masses individually. ﾐIt is easier to weigh a large collection of these. ･A mole (abbreviated as mol) is defined as the number of C atoms in exactly 12 grams of pure C-12. ･This number is called Avogadro’s number.

  9. A Mole of ParticlesContains 6.02 x 1023 particles 1 mole C 1 mole H2O 1 mole NaCl = 6.02 x 1023 C atoms = 6.02 x 1023H2O molecules = 6.02 x 1023NaCl “molecules” (technically, ionics are compounds not molecules so they are called formula units) 6.02 x 1023 Na+ ions and 6.02 x 1023Cl– ions

  10. Avogadro’s Number as Conversion Factor 6.02 x 1023 particles 1 mole or 1 mole 6.02 x 1023 particles Note that a particle could be an atom OR a molecule!

  11. Learning Check 1. Number of atoms in 0.500 mole of Al a) 500 Al atoms b) 6.02 x 1023 Al atoms c) 3.01 x 1023 Alatoms 2.Number of moles of S in 1.8 x 1024 S atoms a) 1.0 mole S atoms b) 3.0 mole S atoms c) 1.1 x 1048 mole S atoms

  12. Molar Mass ･The mass in grams of one mole of atoms of any pure substance is known as its molar mass. The unit used is g/mol (gmol-1). ･Molar mass of sodium (Na) = mass of exactly one mole of Na atoms = 22.99 g/mol = mass of 6.022 x 1023atoms of Na. ･Molar mass of Uranium (U) = 238.03 g/mol. ･The atomic mass number of an element in a periodic table = molar mass for the element

  13. Practice p 322 • Determine the number of Zn atoms in 2.5 mol of Zn. • Calculate the number of molecules in 11.5 mol of water (H2O). • How many molecules of AgNO3 are there in 3.25 mol of it? • Calculate the number of oxygen atoms in 5 mol of oxygen molecules.

  14. Practice p 324 • How many moles contain each of the following? • 5.75 X 1024 atoms Al • 2.50 X 1020 atoms Fe • How many moles contain each of the following molecules? • 3.75 X 1024 CO2 • 3.58 X 1023 ZnCl2

  15. Practice p 328 • Determine the mass in grams of each of the following • 3.57 mol Al • 42.6 mol Si • Convert each given quantity in scientific notation to mass in grams expressed in scientific notation • 3.45 X 102 mol Co • 2.45 X 10-2 mol Zn

  16. Practice p 329 • Determine the number of moles in each of the following • 25.5 g Ag • 300 g S • Convert each mass to moles. Express the answer in scientific notation. • 1.25 X 103 g Zn • 1 kg Fe

  17. Learning Check! Find the molar mass (usually we round to the tenths place) = 79.9 g/mole • 1 mole of Br atoms • 1 mole of Sn atoms = 118.7 g/mole

  18. Molar Mass of Molecules and Compounds ･1 mole of any compound = 6.022 x 1023units of the compound. ･Molar masses of compounds are calculated by adding the molar masses of the atoms present in the compound. ･Calculate the molar mass of Ca3(PO4)2 --Need to look the molar masses of each of the element present and multiply by the number of the atoms present.

  19. Learning Check! • Molar Mass of K2O = ? Grams/mole B. MolarMass of antacid Al(OH)3 = ? Grams/mole

  20. Practice p 335 (bottom) • Determine the molar mass of each ionic compound- • NaOH, CaCl2, KC2H3O2 • Calculate the molar mass of each molecular compound- C2H5OH, HCN, CCl4 • Sr(NO3)2, (NH4)3PO4, C12H22O11

  21. Practice p 335 (top) • Determine the moles of Cl- ions in 2.5 moles of ZnCl2. • Calculate the number of moles of each element in 1.25 mol C6H12O6. • Determine the number of moles of sulfate ions present in 3 mol of Fe2(SO4)3. • How many moles of oxygen atoms are present in 5 mol of P2O5? • Calculate the number of moles of hydrogen atoms in 1.15 X101 mol of water. Express the answer in scientific notation.

  22. Learning Check Prozac, C17H18F3NO, is a widely used antidepressant that inhibits the uptake of serotonin by the brain. Find its molar mass.

  23. Calculations with Molar Mass molar mass Grams Moles

  24. Converting Moles and Grams Aluminum is often used for the structure of light-weight bicycle frames. How many grams of Al are in 3.00 moles of Al? 3.00 moles Al ? g Al

  25. 1. Molar mass of Al 1 mole Al = 27.0 g Al 2. Conversion factors for Al 27.0g Al or 1 mol Al 1 mol Al 27.0 g Al 3. Setup3.00 moles Al x 27.0 g Al 1 mole Al Answer = 81.0 g Al

  26. Learning Check! The artificial sweetener aspartame (Nutra-Sweet) formula C14H18N2O5 is used to sweeten diet foods, coffee and soft drinks. How many moles of aspartame are present in 225 g of aspartame?

  27. Practice p 336 • What is the mass of 3.25 mol of H2SO4? • What is the mass of 4.35 X 101 mol of ZnCl2? • Write the chemical formula for potassium permanganate, and then calculate the mass in grams of 2.55 mol of the compound.

  28. Practice p 337 • Determine the number of moles present in each compound-22.6 g AgNO3, 6.5 g ZnSO4, 35.0 g HCl. • Identify each as an ionic and molecular compound and convert the given mass to moles. Express your answers in scientific notation- 2.5 kg Fe2O3, 25.4 mg PbCl4.

  29. Atoms/Molecules and Grams • Since 6.02 X 1023 particles = 1 mole AND1 mole = molar mass (grams) • You can convert atoms/molecules to moles and then moles to grams! (Two step process) • You can’t go directly from atoms to grams!!!! You MUST go thru MOLES. • That’s like asking 2 dozen cookies weigh how many ounces if 1 cookie weighs 4 oz? You have to convert to dozen first!

  30. Calculations molar mass Avogadro’s numberGrams Moles particles Everything must go through Moles!!!

  31. Atoms/Molecules and Grams How many atoms of Cu are present in 35.4 g of Cu? 35.4 g Cu 1 mol Cu 6.02 X 1023 atoms Cu 63.5 g Cu 1 mol Cu = 3.4 X 1023 atoms Cu

  32. Learning Check! How many atoms of K are present in 78.4 g of K?

  33. Learning Check! What is the mass (in grams) of 1.20 X 1024 molecules of glucose (C6H12O6)?

  34. Learning Check! How many atoms of O are present in 78.1 g of oxygen? 78.1 g O2 1 mol O2 6.02 X 1023molecules O2 2 atoms O 32.0 g O2 1 mol O2 1 molecule O2

  35. Practice p 339 • A sample of ethanol C2H5OH has a mass of 45.6 g.- How many C atoms does the sample contain? How many H atoms are present? How many O atoms are present? • A sample of Na2SO3 has a mass of 2.25 g.- How many Na+ ions are present? How many SO32-ions are present?What is the mass in grams of one molecule of Na2SO3?

  36. Contd- • A sample of CO2 has a mass of 52 g.- How many C atoms are present? How many O atoms are present? What is the mass in grams of 1 molecule of CO2? • A sample of silver chromate has a mass of 25.8 g.- Write the formula for silver chromate. How many cations are present? How many anions are present? What is the mass in grams of 1 molecule of silver chromate?

  37. Percent Composition of Compounds • Composition of compounds can be described in 2 ways: By the number of its constituent atoms. By the mass (%) of each element present. • ･The mass percents of elements are obtained by comparing the mass of each element present in 1 mole of the compound to the total mass of the compound. • ･Useful today with unknown compounds.

  38. Percent Composition What is the percent carbon in C5H8NO4 (the glutamic acid used to make MSG monosodium glutamate), a compound used to flavor foods and tenderize meats? a) 8.22 %C b) 24.3 %C c) 41.1 %C

  39. Practice p 344 • What is the percent composition of H3PO4? • Which has the larger percent by mass of sulfur H2SO3 or H2S2O8? • Calculate the percent by mass of each element in CaCl2. • Sodium sulfate is used in the manufacture of detergents- Identify each of the elements of sodium sulfate, and write the formula. - identify the compound as ionic or covalent- calculate the percent by mass of each element in sodium sulfate.

  40. Chemical Formulas of Compounds • Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio (the law of definite proportions). NO2 2 atoms of O for every 1 atom of N 1 mole of NO2 : 2 moles of O atoms to every 1 mole of N atoms • If we know or can determine the relative number of moles of each element in a compound, we can determine a formula for the compound.

  41. Types of Formulas • Empirical Formula The formula of a compound that expresses the smallest whole number ratio of the atoms present. (Ionic formulae are always empirical formulae) • Molecular Formula The formula that states the actual number of each kind of atom found in one molecule of the compound. Example- CH is the empirical formula for benzene, C6H6).

  42. To obtain an Empirical Formula 1. Determine the mass in grams of each element present, if necessary. 2. Calculate the number of moles of each element. 3. Divide each by the smallest number of moles to obtain the simplest whole number ratio. • If whole numbers are not obtained* in step 3), multiply through by the smallest number that will give all whole numbers *Be careful! Do not round off numbers prematurely

  43. A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. Determine a formula for this substance. require mole ratios so convert grams to moles moles of N = 2.34g of N = 0.167 moles of N 14.01 g/mole moles of O = 5.34 g = 0.334 moles of O 16.00 g/mole Formula:

  44. Calculation of the Molecular Formula A compound has an empirical formula of NO2. The colourless liquid, used in rocket engines has a molar mass of 92.0 g/mole. What is the molecular formula of this substance?

  45. Empirical Formula from % Composition A substance has the following composition by mass: 60.80 % Na ; 28.60 % B ; 10.60 % H What is the empirical formula of the substance?

  46. Practice p 346 • A compound has 63.16% O and 36.84%N. What is the empirical formula of this compound? • Determine the empirical formula of a compound containing 35.98% Al and 64.02%S. • Propane contains 81.82% C and 18.18% H. What is the empirical formula ? • Aspirin has 60% C, 4.44% H, and 35.56% O. Determine the empirical formula of this compound.

  47. Practice p 350 • A compound has 49.98 g C and 10.47g H. It’s Molar mass is 58.12g. Determine it’s molecular formula. • A liquid has 46.68% N and 53.32% O, molar mass is 60.01g. What is the molecular formula? • An oxide of potassium has 19.55g K and 4 g of O. What is it’s empirical formula? • A compound has 65.45 % C, 5.45%H and 29.09% O. If the molar mass is 110g, what is its molecular formula? • Morphine has 17.9g C, 1.68g H, 4.225g O and 1.228 g N. Determine the empirical formula of morphine.

  48. Formulas of hydrates • Hydrate is a compound with a fixed number of water molecules bound to it. • Na2CO3.10H2O- sodium carbonate decahydrate • Hydrate+ heat= anhydrous compound (without water)

  49. How to find out the formula of a hydrate • Need to know how many moles of H20 attached to the molecule- BaCl2.xH2O • Mass sample • Heat it, mass again. • The difference in mass= mass of water • Divide it by molar mass of H20 (18g)= moles of water • Determine moles of compound as well. • Determine ratio of water molecules to compound molecules.

  50. Try it! • A mass of 2.5 g of blue, hydrated copper sulfate (CuSO4.xH2O) is placed in a crucible and heated. After heating, 1.59 g of white anhydrous copper sulfate (CuSO4) remains. What is the formula for the hydrate?

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