Chapter 4 — Stoichiometry
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STOICHIOMETRY
John C. Kotz
Paul M. Treichel
John Townsend
http://academic.cengage.com/kotz
- the study of the
quantitative aspects of
chemical reactions.
Chapter 4
Chemical Equations and Stoichiometry
John C. Kotz • State University of New York, College at Oneonta
© 2009 Brooks/Cole - Cengage
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Gravimetric Analysis
• 
• 
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Determining the Formula of an Organic
Compound by Combustion
A 0.123 g impure sample of the mineral thenardite contains Na2SO4
(142.0 g/1 mol). What is the mass percent of Na2SO4 in the mineral?
Na2SO4 + BaCl2(aq) → 2 NaCl(aq) + BaSO4(s) Collect and dry BaSO4 (233.4 g/mol); mass of BaSO4 = 0.177 g
0.177g BaSO 4 !
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1molBaSO 4 1molNa 2SO 4 142.0g Na 2SO 4
!
!
233.4g BaSO 4
molBaSO 4
molNa 2SO 4
! 100% = 87.5%
0.123g mineral
PLAY MOVIE
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See Active Figure
4.7
Chapter 4 — Stoichiometry
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Using Stoichiometry to
Determine a Formula
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What’s wrong with this technique?
Caproic acid, which is responsible for the foul odor of dirty
socks, is composed of C, H, and O atoms (CxHyOz).
Combustion of 0.225 g produces 0.512 g CO2 and 0.209 g
H2O. What is the empirical formula?
1.00 L of water was
added to 0.10 mol
of CuSO4⋅5H2O to
make a 0.10M
solution.
CxHyOz + excess O2 → x CO2 + y/2 H2O
• Determine mass of C in CO2, mass of H in H2O
• Calculate mass of O by difference
• Calculate moles of C, H, O
1.00 L of 0.100 M
CuSO4
All the
water
didn’t fit!
Ans:
C3H6O
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25.0 g or 0.100 mol
of CuSO4⋅5H2O
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Preparing a Solution
PROBLEM: Dissolve 5.00 g of NiCl2•6 H2O in
enough water to make 250. mL of solution.
Calculate molarity.
Step 1: Calculate moles of
NiCl2• 6H2O
Molar mass of NiCl2 •6H2O includes 6 H2O!
! 1 mol $
5.00 g #
& = 0.0210 mol
" 237.7 g %
Distilled water is added
to fill the flask with
solution just
to the mark on
the flask.
Step 2: Calculate molarity
250 mL
volumetric flask
0.0210 mol
= 0.0841 M
0.250 L
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© 2009 Brooks/Cole - Cengage
© 2009 Brooks/Cole - Cengage
0.435 g KMnO4
The KMnO4 is first dissolved in
a small amount of water.
A mark on the neck of a volumetric
flask indicates the volume of exactly
250 mL at 25°C.
Chapter 4 — Stoichiometry
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The Nature of a CuCl2 Solution:
Ion Concentrations
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PROBLEM: You have 50.0 mL of 3.0 M NaOH and
you want 0.50 M NaOH. What do you do?
CuCl2(aq) → Cu2+(aq) + 2 Cl-(aq)
How much water do we add?
If [CuCl2] = 0.30 M, then [Cu2+] = 0.30 M
[Cl-] = 2 x 0.30 M
0.050L of
moles of NaOH in ORIGINAL solution =
moles of NaOH in FINAL solution
© 2009 Brooks/Cole - Cengage
© 2009 Brooks/Cole - Cengage
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Preparing Solutions by
Dilution
You have 50.0 mL of 3.0 M NaOH and you want 0.50
M NaOH. What do you do?
Cinitial • Vinitial = Cfinal • Vfinal 0.050L of
Where C = concentration in M or wt %
M1V1 = M2V2
3.0 mol/L × 0.050L = 0.50 mol/L × V2 L where V2 = FINAL volume
V2 = 0.30L
ANS: add 250 mL of water to 50.0 mL of 3.0 M NaOH; dilute to
final volume of 300 mL
Note: V2 = 300 mL; volume of water added ≠ 300 mL
© 2009 Brooks/Cole - Cengage
© 2009 Brooks/Cole - Cengage
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Chapter 4 — Stoichiometry
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Preparing a Solution by Dilution
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SOLUTION STOICHIOMETRY
•  Zinc reacts with acids to produce
H2 gas. •  Have 10.0 g of Zn
•  What volume of 2.50 M HCl is
needed to convert the Zn
completely?
Zn + 2 HCl → ZnCl2 + H2
0.100 M K2Cr2O7
Use a 5.00-mL pipet to
withdraw 5.00 mL of a
0.100 M K2Cr2O7
solution.
Add the 5.0- mL sample
of 0.100 M K2Cr2O7
solution to a 500-mL
volumetric flask.
Fill the flask to the mark
with distilled water to give
0.00100 M K2Cr2O7
solution.
10.0 g Zn ×
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1 mol Zn 2 mol HCl
1 L HCl
×
×
= 0.122 L HCl
65.4 g
mol Zn 2.50 mol HCl
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ACID-BASE REACTIONS
Titrations
How much acetic acid is contained in a sample of vinegar?
Titrate the acid with a known concentration of base:
HC2H3O2(aq) + NaOH(aq) → NaC2H3O2 (aq) + H2O
acid
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base
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Setup for titrating an acid with a base
Flask containing
aqueous sample of
sample being
analyzed
(a)
Buret containing aqueous NaOH of accurately
known concentration.
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(b)
Solution of NaOH is
added slowly to the
sample being analyzed.
(c)
When the amount of NaOH
added from the buret exactly
equals the amount of H+
supplied by the acid being
analyzed, the dye (indicator)
changes color.
Chapter 4 — Stoichiometry
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Must standardize the solution of NaOH — i.e., accurately
determine its concentration.
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Use standardized NaOH to determine the amount of
an acid in an unknown.
Use a primary standard which is a monoprotic acid, e.g. KHC8H4O4,
potassium hydrogen phthalate (KHP)
Apples contain malic acid, C4H6O5, a diprotic acid
C4H6O5 + 2 NaOH → Na2C4H4O5 + 2 H2O
–  Solid, pure, non-hydroscopic
4.83 g of KHP (204.2 g/mol) requires 35.62 mL of NaOH for
titration to an equivalence point. What is NaOH concentration?
76.80 g of apple requires 34.56 mL of 0.664 M NaOH for
titration. What is weight % of malic acid in the apple?
Balanced equation: KHP + NaOH → H2O + KNaP
4.83g KHP ×
0.03456 L ×
1 mol KHP 1 mol NaOH
×
= 0.02365 mol NaOH
204.2 g
mol KHP
0.664 mol NaOH 1 mol malic acid 134.1 g malic acid
×
×
L
2 mol NaOH
mol malic acid
= 0.0200
76.80 g apple
= 2.00% malic acid in apple
0.02365 mol NaOH
= 0.664 M NaOH
0.03562 L
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© 2009 Brooks/Cole - Cengage
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Spectrophotometry
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An Absorption Spectrophotometer
Sample absorbs light in green-blue
part of spectrum and transmits light
in the remaining wavelengths. The
sample would appear red to orange
to your eye.
© 2009 Brooks/Cole - Cengage
© 2009 Brooks/Cole - Cengage
Chapter 4 — Stoichiometry
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Spectrophotometry
Spectrophotometry
•  BEER-LAMBERT LAW relates amount of light absorbed and the path
length and solute concentration.
•  Amount of light absorbed by a sample depends on path length
and solute concentration.
Absorbance ∝ path length × concentration
A=abc
A = absorbance
a = molar absorptivity
b = path length
c = concentration
•  There is a linear relation between A and c for a given
path length and compound.
Different concs of Cu2+
•  This means you can find unknown solution
concentration if A is measured.
Same concs but different
path lengths
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© 2009 Brooks/Cole - Cengage
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Spectrophotometry
•  To use the Beer-Lambert law you must first calibrate the
instrument at a specific wavelength, using a cell with a
specific path length.
Use the calibration curve to
determine the unknown
conc of a solution using the
same cell at the same
wavelength.
© 2009 Brooks/Cole - Cengage