Chapter 4 — Stoichiometry
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STOICHIOMETRY
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PROBLEM: Mix 5.40 g of Al with 8.10 g of Cl2. What
mass of Al2Cl6 can form?
- the study of the
quantitative aspects of
chemical reactions.
Ans: 10.1 g Al2Cl6 can form
How much mass of which reactant will remain when
reaction is complete?
Ans:
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= 0.124 mol Al = 3.35 g in excess
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Chemical Analysis
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determine sulfate content
Na2SO4(aq),
clear soln
BaCl2(aq),
clear soln
BaSO4,
NaCl(aq), NaCl(aq),
white solid clear soln clear soln
BaSO4,
Filter paper weighed
white solid
caught in filter
Gravimetric Analysis
• 
A 0.123 g impure sample of the mineral thenardite contains
Na2SO4 (142.0 g/1 mol).
• 
The Na2SO4 in the sample is converted to insoluble BaSO4 (233.4
g/mol).
• 
The mass of BaSO4 is 0.177 g
• 
What is the mass percent of Na2SO4 in the mineral?
0.177g BaSO4 ×
= 87.5%
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© 2009 Brooks/Cole - Cengage
1 mol BaSO4 1 mol Na 2SO4 142.0 g Na 2SO4
×
×
233.4 g BaSO4
mol BaSO4
mol Na 2SO4
= 0.875
0.123 g mineral
Chapter 4 — Stoichiometry
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Determining the Formula of an Organic
Compound by Combustion
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Using Stoichiometry to
Determine a Formula
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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?
All the C in CO2 and all the H in H2O is from CxHyOz
• Calculate mass of C, H
• Calculate mass of O by difference
• Calculate moles of C, H, O
PLAY MOVIE
See Active Figure
4.7
Ans:
C3H6O
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© 2009 Brooks/Cole - Cengage
Quantitative Aspects of
Reactions in Solution
5 Fe2+ + MnO4- + 8 H+ " 5Fe3+ + Mn2+ + 4 H2O
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1.00 L of water
was used to make
1.00 L of 0.100M
solution. Notice
the water left
over.
Volume of water
remaining when
1.00 L of water
was used to make
1.00 L of a solution
PLAY MOVIE
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© 2009 Brooks/Cole - Cengage
1.00 L of 0.100 M
CuSO4
25.0 g or 0.100 mol
of CuSO4⋅5H2O
Chapter 4 — Stoichiometry
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PROBLEM: Dissolve 5.00 g of NiCl2•6 H2O in
enough water to make 250. mL of solution.
Calculate molarity.
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Preparing a Solution
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Step 1: Calculate moles
of NiCl2• 6H2O
! 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
0.0210 mol
= 0.0841 M
0.250 L
250 mL
volumetric flask
0.435 g KMnO4
The KMnO4 is first dissolved in
a small amount of water.
NiCl2 = 0.0841 M
A mark on the neck of a volumetric
flask indicates the volume of exactly
250 mL at 25°C.
PLAY MOVIE
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© 2009 Brooks/Cole - Cengage
The Nature of a CuCl2 Solution:
Ion Concentrations
CuCl2(aq) ➜ Cu2+(aq) + 2 Cl-(aq)
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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?
How much water do we add?
moles of NaOH in ORIGINAL solution =
moles of NaOH in FINAL solution
H2 O
If [CuCl2] = 0.30 M,
then [Cu2+] = 0.30 M
[Cl-] = 2 x 0.30 M
0.050L of 3.0 M NaOH
Concentrated
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© 2009 Brooks/Cole - Cengage
0.50 M NaOH
Dilute
Chapter 4 — Stoichiometry
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Preparing Solutions by
Dilution
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You have 50.0 mL of 3.0 M NaOH and you want 0.50
M NaOH. What do you do?
H2 O
Cinitial • Vinitial = Cfinal • Vfinal Where C = concentration in M or wt %
0.050L of 3.0 M NaOH
Concentrated
0.50 M NaOH
Dilute
M1V1 = M2V2
3.0 mol/L × 0.050L = 0.50 mol/L × Y L where Y = FINAL volume
Y = 0.30L
ANS: add 250 mL of water to 50.0 mL of 3.0 M NaOH; dilute to
final volume of 300 mL
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© 2009 Brooks/Cole - Cengage
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Preparing a Solution by Dilution
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
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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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Chapter 4 — Stoichiometry
ACID-BASE REACTIONS
Titrations
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Setup for titrating an acid with a base
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H2C2O4(aq) + 2 NaOH(aq) ➜ Na2C2O4(aq) + 2 H2O(liq)
acid
base
Carry out this reaction using a TITRATION.
Flask containing
aqueous sample of
sample being
analyzed
Oxalic acid,
H2C2O4
(a)
Buret containing aqueous NaOH of accurately
known concentration.
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Use a “primary standard” e.g. KHC8H4O4, potassium hydrogen
phthalate (KHP), monoprotic acid
–  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?
Balanced equation: KHP + NaOH → H2O + KNaP
1 mol KHP 1 mol NaOH
×
= 0.02365 mol NaOH
204.2 g
mol KHP
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Use standardized NaOH to determine the amount of
an acid in an unknown.
Apples contain malic acid, C4H6O5, a diprotic acid
C4H6O5 + 2 NaOH ➜ Na2C4H4O5 + 2 H2O
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?
0.03456 L ×
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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(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.
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Lab Technique: Standardize a solution of NaOH — i.e.,
accurately determine its concentration.
4.83g KHP ×
(b)
Solution of NaOH is
added slowly to the
sample being analyzed.
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Chapter 4 — Stoichiometry
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Spectrophotometry
An Absorption Spectrophotometer
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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.
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© 2009 Brooks/Cole - Cengage
Spectrophotometry
•  Amount of light absorbed by a sample depends on path length
and solute concentration.
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Spectrophotometry
•  BEER-LAMBERT LAW relates amount of light absorbed and the 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.
•  This means you can find unknown solution
concentration if A is measured.
Different concs of Cu2+
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Same concs but different
path lengths
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Chapter 4 — Stoichiometry
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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.
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