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UNIT 4

UNIT 4. Stoichiometry Balancing Equations The Mole. Chemical Equations. special information goes here: Δ ( heat) , catalyst, spark, etc. Mg(OH) 2 (s) + 2HCl( aq )  MgCl 2 ( aq ) + 2H 2 O(l). reactants. products. the arrow in the equation means: “produce”, “yield”, “react to form”.

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UNIT 4

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  1. UNIT 4 Stoichiometry Balancing Equations The Mole

  2. Chemical Equations special information goes here: Δ (heat), catalyst, spark, etc. Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l) reactants products the arrow in the equation means: “produce”, “yield”, “react to form” • Letters in parentheses show the physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous (in water) • Correctly written formulas for all reactants and products are a “must.” • Coefficients give the relative amounts of reactants used and products formed. Do NOT balance equations by changing the subscripts!!!

  3. Balancing Chemical Equations • Gaseous nitrogen and hydrogen react to form ammonia: • Write the correct formulas for the reactants and products. • N2(g) + H2(g)  NH3(g) • Coefficients (numbers in front of the formulas) are used to balance the numbers of atoms of each element. • N2(g) + H2(g)  NH3(g) • 2 atoms N 2 atoms H 1 atom N, 3 atoms H • N2(g) + H2(g)  2NH3(g) • 2 atoms N 2 atoms H 2 atoms N, 6 atoms H • N2(g) + 3H2(g)  2NH3(g) • 2 atoms N 6 atoms H 2 atoms N, 6 atoms H • The equation is now balanced. • The Law of Conservation of Mass has been satisfied.

  4. Balancing Chemical Equations • Tip: Balance first the elements which appear least often. • NaOH(aq) + H3PO4(aq)  Na3PO4(aq) + H2O(l) • sodium hydroxide phosphoric acid sodium phosphate • Balance Na or P first. • 3NaOH(aq) + H3PO4(aq)  Na3PO4(aq) + H2O(l) • 3NaOH(aq) + (1)H3PO4(aq)  (1)Na3PO4(aq) + H2O(l) 6 atoms of H, 7 atoms of O 2 atoms of H, 5 atoms of O • 3NaOH(aq) + (1)H3PO4(aq)  (1)Na3PO4(aq) + 3H2O(l) • Balanced equation: • 3NaOH(aq) + H3PO4(aq)  Na3PO4(aq) + 3H2O(l)

  5. Balancing Chemical Equations • CaC2 (s) + H2O(l)  Ca(OH)2 (aq) + C2H2 (g) • calcium carbide calcium hydroxide acetylene • CaC2 (s) + 2H2O(l)  Ca(OH)2 (aq) + C2H2 (g) • PCl3(g) + H2O(l)  H3PO3 (aq) + HCl (aq) • phosphorus trichloride phosphorous acid hydrochloric acid • PCl3 (g) + 3H2O(l)  H3PO3 (aq) + 3HCl (aq) • H2S(g) + Fe(OH)3 (s)  Fe2S3 (s) + H2O(l) • hydrogen iron(III) iron(III) • sulfide hydroxide sulfide • 3H2S(g) +2Fe(OH)3(s)  Fe2S3(s) +6H2O(l)

  6. Stoichiometry • The application of the laws ofdefinite proportionsand theconservation of massto chemical reactions is calledstoichiometry. Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l) Applications: • Computing the formula mass or molar massof a compound from its chemical formula • Computing the% composition of a compoundfrom its chemical formula • Computing the amounts of reactants required and products formed (the yield)for a specific chemical reaction

  7. Stoichiometry and the Mole Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l) 2 molecules 2 molecules 1 formula unit 1 formula unit • In the laboratory, it is virtually impossible to work with single molecules or formula units. Instead, we generally work with amounts of chemicals large enough to see and manipulate. • AMOLEis anumber (Avogadro’s number)large enough to give appreciable amounts of material. • A MOLEis6.022 x 1023of anything…anything!!! • The mass of a mole of any element is its atomic mass in grams. • 1moleof magnesium has a mass of 24.305g(see periodic table). • 1atomof magnesium has a mass of 24.305amu.

  8. Stoichiometry and the Mole Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l) 1 mole 2 moles 1 mole 2 moles • One way to tell if we have a mole of a chemical is to weigh it. • The mass of a mole (aka the molar mass) of anyelementis its atomic mass in grams. • The mass of a mole (aka the molar mass) of anycompoundis its formula mass in grams.

  9. Stoichiometry and the Mole Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l) 1 mole 2 moles 1 mole 2 moles 1 formula unit of Mg(OH)2 has: 1 ion Mg2+ 24.3050 amu 2 atoms O 2 x 15.9994 = 31.9988 amu 2 atoms H 2 x 1.00794 = 2.01588amu formula mass 58.3197 amu

  10. Stoichiometry and the Mole Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l) 1 mole 2 moles 1 mole 2 moles 1 mole of Mg(OH)2 has: 1 mole of Mg2+ ions 24.3050 g 2 moles of O atoms 2 x 15.9994 = 31.9988 g 2 moles of H atoms 2 x 1.00794 = 2.01588 g molar mass 58.3197 g

  11. Stoichiometry and the Mole Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l) 1 mole 2 moles 1 mole 2 moles 58.320 g 2 x 36.461 g = 72.922 g 95.211 g 2 x 18.015 g = 36.030 g • Balanced chemical equations tell us theproportionsin which chemicals react. These are generally inMOLES. • One way tomeasure molesin the lab is toweigh the chemical. Molar masses: Mg(OH)2 58.320 g HCl 36.461 g MgCl2 95.211 g H2O 18.015 g

  12. Converting from Mass to Moles Once we know the molar mass of a compound, we can calculate the number of moles present in ANY MASS of that compound.Simply treat the molar mass as a conversion factor. 1 mol Mg(OH)2 = 58.320 g • How many moles of Mg(OH)2are in 29.16 g of the compound? 29.16 g x 1 mol Mg(OH)2 = 0.5000 mol Mg(OH)2 58.320 g • How many moles of Mg(OH)2 are in 4.07 x 10-3 g of the compound? 4.07 x 10-3 g x 1 mol Mg(OH)2 = 6.98 x 10-5 mol Mg(OH)2 58.320 g

  13. Converting from Moles to Mass If we can convert from mass to moles,we can also convert from moles to mass. What is the mass in grams of 1.5 moles of Mg(OH)2 ? 1.5 mol Mg(OH)2 x 58.320 g = 87 g 1 mol Mg(OH)2

  14. Examples 1. What is the mass in grams of 0.250 mol of MgCl2? 2. How many moles of NH4Cl are in 76.5 g? 3. How many moles of nitrate ions are present in 2.5 mol of Al(NO3)3? 4. How many grams of nitrate ions are present in 10.0 g of Al(NO3)3?

  15. Converting from Moles to Numbers of Formula Units, Molecules, or Atoms Moles can be converted into numbers of formula units (for ionic compounds) OR numbers of molecules OR numbers of atoms. How manyformula unitsof Mg(OH)2are in 1.5 moles of the compound? 1.5 mol Mg(OH)2 x 6.022 x 1023 formula units = 9.0 x 1023 formula units 1 mol Mg(OH)2 How manyhydroxide ions (OH-) are in 1.5 moles of Mg(OH)2? 1.5 mol Mg(OH)2 x 2 mol OH- ions x 6.022 x 1023 ions = 1.8 x 1024 ions 1 mol Mg(OH)2 1 mol OH- ions

  16. The Map Number of things Moles Mass conversion factor: 1 mol = molar mass conversion factor: 1 mol = 6.022 x 1023

  17. Examples 1. How many molecules of HCOOH are present in 0.0772 mol of the compound? 2. Calculate the mass of 6.626 x 1026 formula units of NH4Cl.

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