Practice Problems
Chang, Chapter 4
Reactions in Aqueous Solutions
How to keep things straight when solving quantitative problems:
• First identify what you are being asked to find
– If it is not clear to you what the problem asks, list
down the data or information you are provided with
• Connect this with a formula, a definition or a
topic you have learned about
• Find out if you have all the information you need
to use the formula. If not, find other information
or other formulas that can be used to supply the
missing information
• Check out the units. Find conversion factors or
equivalences if necessary
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Typical quantitative problems
• Questions based on defined terms
– Atomic mass, molar mass
– Average atomic mass of an element
– Avogadro’s number
– Molar volume
• Questions based on formulas
– Density, volume, mass, moles
– % composition
– Molarity
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• Questions based on the Chemical formula
• Given the formula, find the amount of one of its
components
• Given a component in a formula, find the amount
• Questions based on the balanced chemical
equation (Stoichiometry)
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Terms that can be used as factors
for dimensional analysis
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Unit conversion equivalences
Percentages (%)
Density
Molar mass
Molar volume
Avogadro’s number
Concentration expressions
Ratios, mole ratios
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• The following exercises are a review of basic
calculations done in Chapter 3 as a foundation
for dong problems in Chapter 4
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3.2 Avogadro’s Number & the Molar Mass of an Element
• How many moles of magnesium (Mg) are
there in 87.3 g of Mg?
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3.2 Avogadro’s Number & the Molar Mass of an Element
• Calculate the number of grams of lead (Pb) in
12.4 moles of lead.
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3.2 Avogadro’s Number & the Molar Mass of an Element
• Calculate the number of atoms in 0.511 g of
potassium (K).
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3.3 Molecular Mass
• What is the molecular mass of methanol
(CH4O)?
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3.3 Molecular Mass
• Calculate the number of moles of chloroform
(CHCl3) in 198 g of chloroform.
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3.3 Molecular Mass
• How many H atoms are in 72.5 g of
isopropanol (rubbing alcohol), C3H8O?
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3.5 Percent Composition of Compounds
• Calculate the percent composition by mass of
each of the elements in sulfuric acid (H2SO4).
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3.5 Percent Composition of Compounds
• Determine the empirical formula of a
compound having the following percent
composition by mass: K: 24.75%; Mn: 34.77%;
O: 40.51%.
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3.5 Percent Composition of Compounds
• Calculate the number of grams of Al in 371 g
of Al2O3.
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3.6 Experimental Determination of Empirical Formulas
• A sample of a compound containing boron (B)
and hydrogen (H) contains 6.444 g of B &
1.803 g of H. The molar mass of the
compound is about 30 g. What is its
molecular formula?
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3.7 Chemical Reactions & Chemical Equations
• Balance the equation representing the
reaction between iron(III) oxide, Fe2O3, &
carbon monoxide (CO) to yield iron (Fe) &
carbon dioxide (CO2).
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3.8 Amounts of Reactants & Products
Methanol (CH3OH) burns in air according to the
equation
2CH3OH + 3O2 → 2CO2 + 4H2O
If 209 g of methanol are used up in a combustion
process, what is the mass of H2O produced?
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3.8 Amounts of Reactants & Products
The reaction between nitric oxide (NO) and
oxygen to form nitrogen dioxide (NO2) is a key
step in photochemical smog formation:
2NO(g) + O2(g) → 2NO2(g)
How many grams of O2 are needed to produce
2.21 g of NO2?
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3.9 Limiting Reagents
The reaction between aluminum and iron(III) oxide
can generate temperatures approaching 3000°C and
is used in welding metals:
2Al + Fe2O3 → Al2O3 + 2Fe
In one process, 124 g of Al are reacted with 601 g of
Fe2O3. (a) Calculate the mass (in grams) of Al2O3
formed. (b) How much of the excess reagent is left
at the end of the reaction?
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3.10 Reaction Yield
Industrially, vanadium metal, which is used in steel
alloys, can be obtained by reacting vanadium(V) oxide
with calcium at high temperatures:
5Ca + V2O5 → 5CaO + 2V
In one process, 1.54 x 103 g of V2O5 react with 1.96 x 103
g of Ca. (a) Calculate the theoretical yield of V.
(b) Calculate the percent yield if 803 g of V are obtained.
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Solutions and Reactions
• The important thing to remember is that
substances exist in different forms in solution
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4.2 Precipitation Reactions
• Classify the following ionic compounds as
soluble or insoluble:
a) CuS
b) Ca(OH)2
c) Zn(NO3)2
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4.2 Precipitation Reactions
• Predict the precipitate produced by mixing an
Al(NO3)3 solution with a NaOH solution. Write
the net ionic equation for the reaction.
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4.3 Acid-Base Reactions
• Classify each of the following species as a
Bronsted acid or base:
a) SO42b) HI
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4.4 Oxidation-Reduction Reactions
• Assign oxidation numbers to all of the
elements in the following compound and ion:
a) PF3
b) MnO4-
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4.4 Oxidation-Reduction Reactions
• Identify the following redox reactions by type:
a) Fe + H2SO4 → FeSO4 + H2
b) S + 3F2 → SF6
c) 2CuCl → Cu + CuCl2
d) 2Ag + PtCl2 → 2AgCl + Pt
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4.5 Concentration of Solutions
• What is the molarity of an 85.0 mL ethanol
(C2H5OH) solution containing 1.77 g of
ethanol?
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4.5 Concentration of Solutions
• What volume (in milliliters) of a 0.315 M
NaOH solution contains 6.22 g of NaOH?
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4.5 Concentration of Solutions
• How would you prepare 2.00 x 102 mL of a
0.866 M NaOH solution, starting with a 5.07 M
stock solution?
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Solution Stoichiometry
• Stoichiometry of solutions relies on the same
pattern of using the mole ratios from balanced
chemical equations
• When given grams moles can be calculated as
usual from molar masses
• When given solutions, moles can be calculated
from M and V
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4.6 Gravimetric Analysis
• A sample of 0.3220 g of an ionic compound
containing the bromide ion (Br-) is dissolved in
water and treated with an excess of AgNO3. If
the mass of the AgBr precipitate that forms is
0.6964 g, what is the percent by mass of Br in
the original compound?
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4.7 Acid-Base Titrations
• How many grams of KHP (KHC8H4O4 ) are
needed to neutralize 18.64 mL of a 0.1004 M
NaOH solution?
KHC8H4O4(aq) + NaOH(aq) → KNaC8H4O4(aq) + H2O(l)
KHC8H4O4 molar mass = 204.2 g
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4.7 Acid-Base Titrations
• How many milliliters of a 1.28 M H2SO4 solution are
needed to neutralize 60.2 mL of a 0.427 M KOH
solution?
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4.8 Redox Titrations
• How many milliliters of a 0.206 M HI solution are
needed to reduce 22.5 mL of a 0.374 M KMnO4
solution according to the following equation:
10HI + 2KMnO4 + 3H2SO4 → 5I2 + 2MnSO4 + K2SO4 + 8H2O
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• According to the following balanced reaction,
how many moles of NO are formed from 8.44
moles of NO2 if there is plenty of water
present?
3 NO2(g) + H2O(l) → 2 HNO3(aq) + NO(g)
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• A 14.01 g sample of N2 reacts with 3.02 g of H2
to form ammonia (NH3). If ammonia is the
only product, what mass of ammonia is
formed?
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