Relationship between mass, moles and molecules in a compound

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Relationship between mass, moles and molecules in
a compound
X molar mass (__g__ mole)
Mass (g)
X 6.022 x 1023 (units mole)
Grams moles moles gram
moles units units or molecules
mole
moles grams grams mole
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Molar mass

• Molar mass of a substance mass in grams of one
  mole of the substance.
• A compounds molar mass is NUMERICALLY equal to
  its formula mass.
• 2 mol H x 1.01 g H 2.02 g H
• 1 mol H
• 1 mol O x 16.00 g O 16.00 g O
• 1 mol O molar mass H2O
• 18.02 g/mol
• Formula mass H2O 18.02 amu
• Molar mass H2O 18.02 g/mol

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Molar Mass Example

• What is the molar mass of K2SO4?
• 2 mol K x 39.10 g K 78.20 g K
• 1 mol K
• 1 mol S x 32.10 g S 32.07 g S
• 1 mol S
• 4 mol O x 16.00 g O 64.00 g O
• 1 mol O
• molar mass K2SO4
• 174.27 g/mol
• How many moles of each element are present in
  this compound?
• 2 mol K, 1 mol S, 4 mol O

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• What is the molar mass of C6H12O6?
• 6 mol C x 12.01 g C 72.06 g C
• 1 mol C
• 12 mol H x 1.01 g H 12.12 g H
• 1 mol H
• 6 mol O x 16.00 g O 96.00 g O
• 1 mol O
• molar mass C6H12O6
• 180.18 g/mol
• How many moles of each element are present in
  this compound?
• 6 mol C, 12 mol H, 6 mol O

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Converting to grams from moles

• How many moles of glucose are in 4.15x10-3 g
  C6H12O6?
• 4.15x10-3 g x 1 mol C6H12O6 2.30 x 10-5 mol
  C6H12O6
• 180.18 g
• How many molecules of glucose are in 4.15x10-3 g
  C6H12O6?
• 2.30 x 10-5 mol C6H12O6 x 6.022 x 10 23
  molecules
• 1 mol
• (2.30 x 6.022)(10(-523)) 13.90 x 10 18
  molecules
• 1.39 x 10 19 molecules

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• What is the mass in grams of 6.25 moles copper
  (II) nitrate?
• Cu 2 NO3 - formula is Cu(NO3)2
• Find molar mass of Cu(NO3)2 first.
• 1 mol Cu x 63.55 g Cu 63.55 g Cu
• 1 mol Cu
• 2 mol N x 14.01 g N 28.02 g N
• 1 mol N
• 6 mol O x 16.00 g O 96.00 g O
• 1 mol O
• molar mass Cu(NO3)2 187.57 g/mol
• Now find mass in grams of 6.25 moles
• 6.25 moles x 187.57 g 1172 g Ans. 1170 g
  Cu(NO3)2
• 1 mol

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Atoms and Ions Within Compounds

• How many carbon atoms are in one mole of C2H6?
• 1 mole C2H6 2 moles C 6.022 x 1023 atoms
  1.204 x 1024 atoms
• 1 mole C2H6 1 mole C
• How many MOLES of carbon atoms are in one mole of
  C2H6?
• 1 mole C2H6 2 moles C 2 moles C atoms
• 1 mole C2H6
• How many moles of hydroxide ions are in one mole
  of calcium hydroxide? How many moles of Ca2?
• 1 mole Ca(OH)2 2 moles OH-- 2 moles
  hydroxide ions
• 1 mole Ca(OH)2
• 1 mole Ca(OH)2 1 mole Ca2 1 mole calcium
  ions
• 1 mole Ca(OH)2

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Percentage Composition

• Composition is the by mass of each element
  in a compound.
• Find the percentage composition of sodium
  chloride.
• Na Cl - formula is NaCl
• 1 mol Na x 22.99 g Na 22.99 g Na
• 1 mol Na
• 1 mol Cl x 35.45 g Cl 35.45 g Cl
• 1 mol Cl molar mass NaCl 58.44
  g/mol
• 22.99 g Na x 100 39.34 Na
• 58.44 g NaCl
• 35.45 g Cl x 100 60.66 Cl
• 58.44 g NaCl

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• Find the percentage composition of sodium
  nitrate.
• Na NO3 - formula is NaNO3
• 1 mol Na x 22.99 g Na 22.99 g Na
• 1 mol Na
• 1 mol N x 14.01 g N 14.01 g N
• 1 mol N
• 3 mol O x 16.00 g O 48.00 g O
• 1 mol O molar mass NaNO3
  85.00 g/mol
• 22.99 g Na x 100 27.05 Na
• 85.00 g NaNO3
• 14.01 g N x 100 16.48 N
• 85.00 g NaNO3
• 48.00 g O x 100 56.47 O
• 85.00 g NaNO3

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Percentage Composition

• Why mass instead of moles?
• Isnt 2/3 of the water molecule hydrogen?
• Moles indicate the amounts of each atom needed to
  make the molecule stable from an electron
  standpoint.
• 2.02 g H x 100 11.21 H
• 18.02 g H2O

O
H
H
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Empirical Formula

• Use composition to convert to original formula
• Assume 100 g sample, so grams
• Convert grams to moles for each element
• Divide the number of moles for each element by
  the smallest number of moles
• The result for each type of element is its
  subscript in the empirical formula.
• The order of elements is usually
• Organics C,H,O,N
• Inorganics Metal, nonmetal, oxygen
• Keep 4 decimal places when dividing numbers. If
  the result has a decimal between .2 and .8, may
  need to multiply all numbers by the number needed
  to get a whole number.
• Ex 3.5 should be multiplied by 2 to get 7.
  Then multiply all other elements by the same
  number.

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Empirical Formula, cont.

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Example Empirical and Molecular formula

• What is the empirical formula of a compound with
  54.82 C, 5.624 H, 32.45 O, 7.104 N?
• 54.82 g C 1 mole C 4.568 mole C / .5074 9 C
• 12 g C
• 5.624 g H 1 mole H 5.624 mole H / .5074 11 H
• 1 g H C9H11O4N
• 32.45 g O 1 mole O 2.028 mole O / .5074 4 O
• 16 g O
• 7.104 g N 1 mole N 0.5074 mole N / .5074 1 N
• 14 g N
• If a compound has this same composition but its
  molecular weight is 394 g/mol, what is its
  molecular formula?
• MW. C9H11O4N 197 g/mol
• 394/197 2 so molecular formula is C18H22O8N2

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Oxidation Numbers

• Used to indicate the general distribution of
  electrons among the bonded atoms in molecular
  compounds or polyatomic ions.
• Analogous to charges in ionic compounds.
• An oxidation number is assigned to each element.
• Assign the ones you know 1st.
• Find the others based on the numbers it takes to
  make the charge equal to the charge of the ion or
  compound. (A compound has a charge of zero.)

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Oxidation Numbers Rules

1. Pure element 0
2. F -1
3. O -2 (except in peroxides and bonds with
   halogens)
4. H 1 (except in bonds with metals)
5. The more electronegative element same (-)
   charge as its anion
6. The less electonegative element same () charge
   as its cation
7. The sum of a compound or polyatomic ions
   oxidation numbers is equal to its charge.