Download Exam 2, Fall 2001

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Exam 3—Page 1
AQUEOUS SOLUTIONS
1. (8 points) Names, formulas, and water-solubility of compounds.
Name
Formula
Soluble in Water?
Ammonium chloride
NH4Cl
yes
Cu(OH) 2
Ni(NO3)2
Iron(III) carbonate
Sodium phosphate
2. (6 points) Ions and ion concentrations
Name
Cation
Cation
Anion
Conc (M)
Conc (M)
Cl–
0.12 M iron(II) chloride
3.6 M Na2SO4
Anion
Na+
OXIDATION-REDUCTION REACTIONS
1. (4 points) Specify the oxidation number for each of the underlined atoms.
(a) HNO3
N = _________________
(b) Na2SO4
Na = _________________ and S = _________________
(c) S2O72–
S = _________________
2. (8 points) The following is a balanced, net ionic equation for the reaction of dichromate ion with
tin(II) ion.
Cr2O72-(aq) + 3 Sn2+(aq) + 14 H+(aq) → 2 Cr3+(aq) + 3 Sn4+ (aq) + 7 H2O(l)
(a) The substance oxidized is __________ and the substance reduced is __________
(b) The reducing agent is __________
Name
Exam 3—Page 2
BALANCING EQUATIONS & BASIC STOICHIOMETRY
1. (8 points) Balance equations for the following reactions:
(a) Decomposition of ammonium nitrate
______ NH4NO3(s) → ______ N2(g) + ______ H2O(g) + ______ O2(g)
(b) Combustion of octane.
______ C8H18(liq) + ______ O2(g) → ______ H2O(g) + ______ CO2(g)
2. (8 points) Iron ore is reduced to iron metal with carbon.
2 Fe2O3(s) + 3 C(s) → 4 Fe(s) + 3 CO2 (g)
(a) If 6.2 moles of Fe2O3(s) are reduced, _____________ moles of C(s) are needed, and
____________________ moles of Fe are produced.
(b) If you obtain 579 g of Fe from this reaction, but you should have obtained 1026 g, the percent
yield is ____________________ %
(c) If you begin this process with 6.2 mol Fe2O3 and 10.0 mol of C, the limiting reactant is _______.
3. (6 points) Ethanol (C2H6O, molar mass = 46 g/mol) burns in air according to the balanced equation.
C2H6O(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)
If you burn 23 g of ethanol, what mass of water is formed?
(a) 27 g H2 O
(b) 90 g H2O
(c) 108 g H2O
(d) 57000 g H2O
If 130 g of CO2(g) are formed, what mass of C2H6O(g) was burned?
(a) 36 g C2H6O(g)
(b) 68 g C2H6O(g)
(c) 78 g C2H6O(g)
(d) 136 g C2H6O(g)
Name
Exam 3—Page 3
SOLUTION CONCENTRATIONS AND SOLUTION STOICHIOMETRY
1. (3 points) You need to make 300 mL of an aqueous solution of 0.235 M ammonium chloride (molar
mass = 53.5 g/mol). How much solid ammonium chloride should you add ?
(a) 3.77 g
(b) 41.9 g
(c) 53.5 g
(d) 68.3 g
2. (3 points) You wish to make a 0.125 M hydrochloric acid solution from 12.0 M hydrochloric acid (concentrated HCl). How much concentrated acid must you add to obtain a total volume of 100 mL of the
dilute solution ?
(a) 0.00104 mL
(b) 1.04 mL
(c) 120 mL
(d) 150 mL
3. (4 points) You have a 0.207 M solution of magnesium acetate (molar mass = 142.4 g/mol).
The concentration of the acetate ion in the solution is __________________ M
What volume of the 0.207 M magnesium acetate solution contains 17.3 grams of the salt?
(a) 25.1 mL
(b) 39.8 mL
(c) 83.6
(d) 587 mL
4.
(3 points) A 100.-mL sample of a soft drink containing citric acid requires 33.51 mL of 0.0102 M
NaOH to neutralize the acid completely. What mass of citric acid (molar mass = 192.1 g/mol) does
the soft drink sample contain? The reaction of citric acid and NaOH is
H3C6H5O7(aq) + 3 NaOH(aq) → Na3C6H5O7(aq) + 3 H2O(l)
(a) 0.0219 g citric acid
(b) 0.197 g citric acid
(c) 19.5 g citric acid
(d) 210 g citric acid
Name
Exam 3—Page 4
4. (8 points) You react zinc chloride with ammonia and isolate a solid with the formula Zn(NH3)xCl2 .
To find out the value of x, first dissolve the sample in water. This releases the ammonia into the solution where it can be titrated with HCl.
Zn(NH3)xCl2(s) → ZnCl2(aq) + x NH3(aq)
In this experiment you titrate a 0.563-g sample of Zn(NH 3)xCl2 with 20.29 mL of 0.543 M HCl. The
reaction occurring is
NH3 (aq) + HCl(aq) → NH4Cl(aq)
Is the value of x equal to 2 or 4 in Zn(NH3)xCl2?
You must show your work completely
and clearly to obtain full credit.
Name
Exam 3—Page 5
WRITING CHEMICAL EQUATIONS
1. (19 points) Writing equations
(a) Complete and balance
, by giving correct formulas for the product, the equation for the exchange
reaction involving sodium carbonate and iron(II) acetate.
_____ Na2CO3 + _____ Fe(CH3CO2)2
→ _______________________________________
products
Is this reaction an
(a) acid-base reaction
(b) a precipitation reaction
(c) a gas-forming reaction?
(b) Complete and balance
the following equation. Give formulas for the reactants that would have produced MgBr2, CO2, and H2O.
_____________________________________ → ______ MgBr2 + _____ CO2 + _____ H2O
reactants
(c) Write a balanced, net ionic equation
for the reaction of potassium hydroxide (KOH) and copper(II)
sulfate (CuSO4)
(d) Describe what you would observe if you mix aqueous solutions of lithium nitrate and sodium
hydroxide. Include in your answer whether or not you would see a gas and if the product or products precipitate.
(e) Describe what you would observe if you mix aqueous solutions of manganese(II) carbonate and
phosphoric acid? Include in your answer whether or not you would see a gas and if the product
or products precipitate.
(f) You are asked to prepare a sample of aluminum nitrate. Circle the pair of reactants in the list below that
you would mix to eprpare aluminum nitrate.
Possible Reactants
Al(OH) 3
KOH
HNO 3
NaCl
AlCl 3
KCl
C a ( N O 3) 2
HCl
Al2(CO3)3
K A l ( S O 4 )2
KNO 3
NH 3
Describe what you would see when the reactants are mixed. Include in your answer whether or not
you would see a gas and if the product or products precipitate.
Name
Exam 3—Page 6
THERMOCHEMISTRY
NOTE: specific heat capacity values are found in Table 6.1 in Chemistry & Chemical Reactivity. Other required data is
found in Chapter 6.
1. (3 points) A sample of solid tin (specific heat capacity = 0.213 J/g-K) is heated with an electrical coil.
If 54.3 Joules of energy are added to a 14.0 gram sample initially at 21.1˚C, what is the final temperature of the tin ?
(a) 18.2 ˚C
(b) 29.4 ˚C
(c) 39.3 ˚C
(d) 42.2 ˚C
2. (3 points) You mix 100 g of liquid A at 100 ˚C with 100 g of another liquid, B, at 50 ˚C. The final temperature of the mixture is 80 ˚C. Which has the larger heat capacity, A or B?
(a) Liquid A
(b) Liquid B
3. (6 points) (Study Question 6.65) A 9.36-g piece of platinum is heated to 98.6 ˚C in a boiling water bath
and then dropped onto ice. When the metal’s temperature has dropped to 0.0 ˚C, it is found that 0.37 g
of ice had melted.
Part 1: What quantity of heat was transferred to the ice from the platinum?
(a) 1.11 J
(b) 90.0 J
(c) 123 J
(d) 333 J
Part 2: What is the specific heat capacity of platinum?
(If you did not get an answer to Part 1of the question, use 1000 J for Part 2. Check here ______ if you use 1000 J
for part 2.)
(a) 0.13 J/g-K
(b) 1.08 J/g-K
(c) 11.7 J/g-K
(d) 114 J/g-K
(e) none of the above
Name
Exam 3—Page 7
Substitute Question for Page 4:
Cobalt(III) ion forms many compounds with ammonia. To find the formula of one of these compounds,
you titrate the NH3 in the compound with standardized acid.
Co(NH3)xCl3(aq) + x HCl(aq) → x NH4+(aq) + Co3+(aq) + (x + 3) Cl-(aq)
Assume that 23.63 mL of 1.500 M HCl is used to titrate 1.580 g of Co(NH3)xCl3. Is the value of x equal
to 4 or 6?