STOICHIOMETRY

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STOICHIOMETRY

• The Mole
• Atomic and Molecular Masses
• Chemical Formula
• Stoichiometry

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ATOMIC MASS

• Atomic mass standard is based on the atom
  Carbon-12 or 12C (6 p, 6 n0, 6 e-).
• One C-12 atom weighs exactly 12 amu
• 1 amu atomic mass unit 1.661E-27 kg

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ATOMIC WEIGHT

• Atomic weight (mass) of an element is defined as
  a weighted average over all naturally occurring
  isotopes of the element this is the number on
  bottom on each element box on the Periodic Table.

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Calculate the Atomic Weight of Boron

• Boron has two naturally occurring isotopes. 10B
  has a mass of M1 10.0129 amu and abundance of
  A1 19.78. 11B has a mass of M2 11.0093 amu
  and abundance of A2 80.22.
• At. Wt S (Mi x Ai)/100
• 10.0129 x 19.78 11.0093 x 80.22/100
  10.81 amu (see PT)

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MOLE

• The number of C-12 atoms in exactly 12 grams of
  pure C-12
• 6.022E23 items
• Avogadros Number, N
• A mole of an element has a mass equal to its
  average atomic weight (mass).
• 1 mol of naturally occurring Boron has a mass of
  10.81 g.

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MOLECULE and MOLAR MASS

• Molecule arrangement of atoms chemically bonded
  together.
• Molar Mass Sum of atomic masses of constituent
  atoms in one molecule (amu) or one mole of
  molecules (gram).
• Use atomic and molar masses to the 1/100 place.

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CHEMICAL FORMULA

• Qualitative description of the constituent
  elements in a molecule or ion.
• C12H22O11 contains C, H and O
• SO42- contains S and O
• Quantitative description of the relative numbers
  (subscripts) of atoms of each element.
• Can be used to determine composition or mass .

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TYPES OF CHEMICAL FORMULAS

• Chemical - shows type and number of atoms
  (shorthand notation)
• Structural - shows chemical bonds (Fig 2.16)
• Ball and Stick - shows spatial arrangement, 3D
  (Fig 3.7 and 2.18)
• Space filling - shows space atoms fill, 3D (Fig
  2.17, also p 95)

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Figure 3.7 The Two Forms of Dichloroethane
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Computer-Generated Molecule of Caffeine
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CONVERSIONS

• Grams to Moles Divide by Molar Mass
• Atomic or Molar Mass
• Moles to Grams x by Molar Mass
• Grams to amu Divide by N
• amu to Grams x by N
• Moles to Units x by N
• Units to Moles Divide by N
• Atoms or Molecules

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DETERMINATION OF A CHEMICAL FORMULA

• A chemical formula can be determined from the
• Mass of each element in the formula
• Mass of each element in the formula (
  Composition)
• Number of moles of each element in the formula
• Elemental analysis by combustion

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CHEMICAL FORMULAS

• EMPIRICAL - includes all atoms in molecule in
  correct smallest integer ratios
• MOLECULAR - includes all atoms in molecule in
  actual numbers and correct ratios can be
  determined from the empirical formula and molar
  mass.

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CHEMICAL REACTION

• A chemical reaction involves rearrangements of
  atoms breaking initial chemical bonds (in the
  reactants) and making new chemical bonds (in the
  products).
• R1 R2 ? P1 P2 P3
• Methane burns in oxygen to form carbon dioxide
  and water

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CHEMICAL EQUATION

• Shorthand symbolic notation for a chemical
  reaction
• CH4(g) O2 (g) ? H2O(l) CO2(g) Note that
  this reaction is NOT BALANCED
• Qualitative aspect
• identity of reactants R and products P use
  study of nomenclature to write equations
• Identify the state of matter for each R and P
• identify reaction type

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CHEMICAL EQUATION (2)

• Quantitative aspect
• how much reactant is consumed and how much
  product is formed
• coefficients must be consistent with the Law of
  Conservation of Mass atoms are neither created
  nor destroyed in a chemical reaction.
• i.e. chemical equation must be balanced
• CH4(g) 2O2 (g) ? 2H2O(l) CO2(g) Note
  that this reaction is BALANCED

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STOICHIOMETRY

• Quantitative relationships in a chemical reaction
  based on a BALANCED chemical equation.
• Relationships between R(eactant)1 and R2 or R1
  and P(roduct)2 or P1 and P2

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C(s) 2H2(g) ? CH4(g)Formation of methane

• One atom of solid carbon reacts with two
  molecules of gaseous hydrogen to produce one
  molecule of gaseous methane.
• One mole of solid carbon reacts with two moles of
  hydrogen gas to produce one mole of methane gas.
• 12.0 g of C reacts with 4.0 g of H2 to produce
  16.0 g of CH4. Note conservation of mass 124
  16

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STOICHIOMETRIC COEFFICIENTS

• We will use mole interpretation for
  stoichiometric coefficients (SC), the
  coefficients in front of Rs and Ps. I.e., SCs
  represent of moles of each R and P
• Provide quantitative (i.e. mole) relationships
  between R and P.
• Can be used to determine amount of mass of each R
  and P (using mol to g conversion)

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MOLE RATIOS

• A mole ratio is a ratio of Stoichiometric
  Coefficients from a balanced chemical eqn.
• These ratios are conversion factors from amt of
  R1 to amt of R2, amt of P2 to amt of R1, etc

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C(s) 2H2(g) ? CH4(g)1 mol 2 mol ? 1
mol12.0 g 4.0 g ? 16.0 g

• How many g of carbon are needed to react with
  10.0 g of hydrogen? How much CH4is formed
• g-H2 ? mol-H2 ? mol-C ? g-C
• 10.0g H2 /2.0g H2/molx1 molC/2 mol H2 x12.0g
  C/mol 30.0 g C
• 10.0g H2 /2.0g H2/mol1 molCH4/2 mol H2
  16.0g CH4/mol 40.0 g CH4
• Is mass conserved?

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Calculating Mass of Reactants and Products
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REACTION YIELD

• In the previous example, say that only 32.0 g of
  CH4 were produced due to side reactions and
  waste.
• We define the percent or reaction yield as
  actual yield/theoretical yieldx100
• This gives yield 32.0/40.0 x 100 80.0

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LIMITING REACTANT

• Find the actual moles of each reactant. Use the
  balanced chem eqn to determine how many mol of R2
  is required to react completely with R1. Do you
  have enough R2? If not, R2 limiting reactant
  LR and R1 reactant in excess XS.
• Always use the LR to solve the stoichiometric
  problem to find the amount of product formed.
• Calculate the amount of XS left over.
• Calculate the grams of methane formed when 18.5 g
  carbon and 2.9 g hydrogen react.

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Solving a Stoichiom. Problem Involving Masses of
Reactants and Products