PowerPoint Presentation - Empirical Formula

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Determining the Empirical Formula of Copper
Chloride
Purpose of the Experiment Determine the
empirical formula of a compound containing only
copper and chlorine
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Molar Mass or Molecular Weight
The molecular weight is the mass, in grams, of
one mole of a compound.
H2O MWt 18.0148 g/mol AlCl3 MWt 133.341
g/mol
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Percentage Composition
The percentage composition is the percent of a
compounds mass that results from each of its
constituent elements.
For ethanol, C2H5OH,
2 ? 12.011 g/mol C 24.022 g/mol of C2H5OH 6 ?
1.008 g/mol H 6.048 g/mol of C2H5OH 1 ?
15.999 g/mol O 15.999 g/mol of
C2H5OH Molecular Weight the sum 46.069 g/mol
of C2H5OH C (24.022 g/mol 46.069 g/mol)
??102 52.14 H ( 6.048 g/mol 46.069
g/mol) ??102 13.13 ? O (15.999 g/mol
46.069 g/mol) ??102 34.73 Sum 100
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Empirical Formula
The empirical formula is the simplest
whole- number ratio of the atoms in a compound.
Examples benzene, CH phosphorus pentoxide,
P2O5
Molecular Formula
The molecular formula is the simplest
whole- number ratio of the atoms in a single
molecule of a compound.
Examples benzene, C6H6 phosphorus pentoxide,
P4O10
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Schematic diagram of the combustion device used
to analyze substances for carbon and hydrogen
CxHy O2 ----gt y/2H2O xCO2 excess
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Determination of the Empirical Formula
A white compound has been analyzed and found to
contain 43.6 wt. phosphorous and 56.4 wt.
oxygen. In a separate study it molecular weight
has been found to be 284 g/mol. What is the
empirical formula of this compound? Assume
exactly 100 g of the compound, then one has 43.6
g P 30.974 g/mol P 1.408 mol of P 56.4 g O
15.999 g/mol O 3.525 mol of O then the molar
ratios are 1.408 mol 1.408 mol 1.000 mol
P 3.525 mol 1.408 mol 2.503 mol O and the
empirical formula is P2O5.
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Determination of the Molecular Formula
A white compound has been analyzed and found to
contain 43.6 wt. phosphorous and 56.4 wt.
oxygen. In a separate study it molecular weight
has been found to be 284 g/mol. What is the
molecular formula of this compound? First
determine the empirical weight of P2O5 2
??30.974 g/mol P 61.948 g/mol P 5 ??15.999
g/mol O 79.997 g/mol O The empirical weight of
P2O5 the sum 141.945 g/mol Molecular weight
Empirical weight 284 g/mol 141.945 g/mol
2.001 and the molecular
formula is twice the empirical formula and the
molecular formula is P4O10.
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Determining the Empirical Formula of Magnesium
Oxide The goal is to determine x and y in the
unknown magnesium oxide, MgxOy, by burning a
known mass of Mg in an excess of oxygen.
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Determining the Empirical Formula of Magnesium
Oxide Note mass of O total mass mass Mg
0.585 g 0.353 g 0.232 g then Mg (mass Mg
total mass MgxOy) ??102 (0.353 g 0.585 g)
??102 60.3 O (mass O total mass MgxOy)
??102 (0.232 g 0.585 g) ??102 39.7
Mg 60.3 and O 39.7
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Mg 60.3 and O 39.7
Stoichiometry and the percentage composition of
three possible magnesium oxides
Possible Oxides MgxOy Mg
O
60.3
39.7
MgO
43.2
56.8
MgO2
Mg2O
75.2
24.8
and the unknown oxide is MgO
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Todays Experiment
Original experiment
Zn(s, silvery white) CuxCly(aq, blue)
ZnCl2(aq) Cu(s, reddish)
Modified experiment
Al(s, silvery white) CuxCly(aq, blue)
AlCl3(aq) Cu(s, reddish)
25 mL Limiting reagent
0.3 g ea. strip (excess)
known mass
known mass
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From http//wine1.sb.fsu.edu/chm1045/tables/peri
od/PT_large.jpg
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Copper is a transition metal with partially
filled d orbitals. A transition metal may
exhibit multiple oxidation states, such as 1,
2, or 3, oxidation states which are not easily
predicted by its position in the periodic
table. Because of the partially filled d
orbitals, a transition metal ion in aqueous
solution frequently is brightly colored, e.g.,
copper ions are blue in aqueous solution. Zinc
and aluminum are both stronger reducing agents
than copper, see their redox potentials, so
either will yield metallic copper from a solution
of a copper salt.
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These potentials indicate the relative
thermodynamic tendency for the indicated
half-reaction to occur.
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Other Reactions in the Procedure Removal of
Excess Reducing Agent
Zn(s) 2 HCl(aq) ----gt ZnCl2(aq) H2(g) 2
Al(s) 6 HCl(aq) ----gt 2 AlCl3(aq) 3
H2(g) Cu(s) n HCl(aq) --x--gt no
reaction Note Cu is below H (0.00 V ref) in
table, so It will not react with acids H(aq) to
form H2(g) Metals above H in the table will react
with acids.
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Checkout
2 piece Al foil, 0.3 g ea 1 pr Beaker Tongs
Reagents in Lab
CuClx solution in 4 L spigot jugs, use 25 ml for
each analysis Record data 0.08067 g CuClx/ml, d
1.074 g/ml 10 HCl in 1 L wash bottles, use
5 ml Use solid NaHCO3 on acid spills
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Flow Chart for Procedure
waste
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Flow Chart for Procedure
Cu
Wash with distilled water to remove aluminum
chloride
Transfer Cu residue to a pre-heated and
pre-weighed casserole
waste
CAUTION Do not overheat to avoid oxidation
heat
Determine the mass of Cu
waste
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Procedure Notes

• Record all weights to 0.001g
• Weigh 25 ml of CuClx solution, use exact density
  to calculate exact
• volume, then calculate the mass of CuClx
• Do not use metal forceps or spatulas
• Add Al foil until blue color is gone, allow
  excess foil to dissolve
• Allow container to cool before weighing
• Speed up cooling by placing flask in front of
  hood sash raised 4-6
• The second beaker does not have to be 150 mL
• A casserole will also work as an evaporating dish

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Hazards

• 10 HCl is a corrosive strong acid
• CuClx solution-heavy metal, irritant
• Hot surfaces - hotplates, glassware

Waste

• Liquid waste Al3 solution and HCl
• Cu solids

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Summary of Data Calculations
Results (calculations)
Collected data
Mass percent of Cu Mass of Cl Mass percent of
Cl Empirical formula
Mass of CuClx Mass of Cu
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Next Weeks Experiment Antacid Analysis (packet)
Additional background reading for Antacid
Analysis/Titrations Atkins, Chemical
Principles, 3rd ed, pp. F67-F72, F85-F88,
415-425