CHEMISTRY 1P80 Final Exam 1998

1. Short Calculations (20 marks)

(a) (3 marks) A sample was analyzed for the percentage of alum, KAl(SO₄)₂. The sample was dissolved in water. Excess BaCl₂ was added to precipitate BaSO₄, and the barium sulfate was filtered and weighed. If 0.3218 g of sample yielded 0.2844 g of BaSO₄, what is the % KAl(SO₄)₂ in the sample? The net ionic equation for the reaction is

(b) (3 marks) A bottle of barium chloride, BaCl₂, states that it is 98.5% pure (i.e., 98.5% of the substance in the bottle is BaCl₂ ). What mass must be taken from the bottle to make 5.00 x 10² mL of a solution whose chloride ion concentration is 0.150 M?

Molar mass: BaCl₂ 208.2 g/mol

7.808 grams of pure BaCl₂ is needed, but each gram of what is in the bottle contains only 0.985 grams of BaCl₂.

Therefore 7.808 g BaCl₂ x (1 g from bottle / 0.985 g BaCl₂) = 7.93 g to weigh out.

 

(c) (4 marks) Consider the following data:

Compound
CO2 (g)	-393.5
H2O (l)	-285.8
CH3OH (l)	-239.1

(i) (1 mark) Write the balanced equation for the combustion of CH₃OH.

CH₃OH + 1.5 O₂ ---> CO₂ + 2 H₂O

(ii) (1 mark) The standard enthalpy of formation for gaseous oxygen is not given in the above Table. Why not? What is it for oxygen?

It doesn't need to be given because it is defined to be zero for any element in its standard state.

(iii) (2 marks) Use the data given and calculate the molar heat of combustion of liquid CH₃OH.

Use the equation in part (i) and Hess's Law to obtain -726 kJ/mol for the molar enthalpy of combustion of methanol.

 

(d) (4 marks) Calculate the molarity of a sulfuric acid solution whose density is 1.198 g/cm³, containing 27.0% H₂SO₄ by weight. The molar mass of H₂SO₄ is 98.1 g/mol.

Assume one liter of solution and find the moles of sulfuric acid in it:

(1.000 L sol'n) x (1000 cm³/L) x (1.198 g sol'n/cm³ sol'n) x (27.0 g H₂SO₄ / 100 g sol'n)

= 323.5 g H₂SO₄ in one liter

= 3.30 mol/L

(e) (3 marks) A 185-g piece of metal alloy, initially at 100.0 ^oC, is dropped into 265 g of water at 15.5 ^oC. When the system comes to thermal equilibrium, the temperature is 24.1 ^oC. What is the specific heat capacity of the metal? The specific heat capacity of water is 4.184 J/g.K.

Answer: 0.679 J/g.K

(f) (3 marks) Sodium azide, NaN₃, is the compound that decomposes to inflate automobile air bags. It is made from nitrous oxide, sodium, and ammonia according to the following reaction:

3 N₂O + 4 Na + NH₃ ---> NaN₃ + 3 NaOH + 2 N₂

If 16.2 g of NaN₃ is obtained by the reaction of 24.00 g of sodium with excess ammonia and nitrous oxide, what is the % yield?

NaN3 65.0 g/mol

NH3 17.03 g/mol

24.00 g of sodium can produce 16.96 g of NaN₃ when reacted with excess ammonia. Therefore the % yield is 95.5 %

 

2. (10 marks)

(a) 6 marks. Consider these 12 compounds:

B2O3	KMnO4	CuS	HCl	Na2O	CH3OH
H2CO3	NH3	Ba(OH)2	(NH4)2SO4	PCl5	FeCO3

Answer the following questions by writing the formulas of the compounds in the space provided below. Write NONE if no compound in the list is an answer to the question. Note: Some of the compounds may be used more than once, others may not be used at all.

Which compound is a strong acid? HCl

Which compound is a weak acid? H₂CO₃

Which compound is an acidic oxide? B₂O₃

Which compound is a basic oxide? Na₂O

Which compound is a strong base? Ba(OH)₂

Which compound is a weak base? NH₃

Which compound is an oxidizing agent? KMnO₄

Which two compounds are soluble salts? KMnO₄, (NH₄)₂SO₄

Which two compounds are insoluble salts? CuS, FeCO₃

Which compound produces a gas when it reacts with sodium hydroxide? (NH₄)₂SO₄

(b) (4 marks). Write balanced equations for the following chemical reactions. If there is no reaction, say so. Use the symbols (s), (l), (aq), and (g) to designate solid, liquid, aqueous solution, or gas.

(i) reaction of aluminum hydroxide and an aqueous solution of nitric acid

Al(OH)₃ (s) + 3 HNO₃ (aq) ---> Al(NO₃)₃ (aq) + 3 H₂O (l)

(ii) reaction of aqueous solutions of barium chloride and sodium sulfate

BaCl₂ (aq) + Na₂SO₄ (aq) ---> BaSO₄ (s) + 2 NaCl (aq)

(iii) reaction of magnesium metal and an aqueous solution of hydrochloric acid

Mg (s) + 2 HCl (aq) ---> MgCl₂ (aq) + H₂ (g)

(iv) reaction of aqueous solutions of iron(III) sulfate and potassium hydroxide

Fe₂(SO₄)₃ (aq) + 6 KOH (aq) ---> 2 Fe(OH)₃ (s) + 3 K₂SO₄ (aq)

 

3. (10 marks)

(a) (3 marks) If a 0.2975-g sample of an unknown diprotic acid requires 19.77 mL of 0.2005 M NaOH for titration to the phenolphthalein end point, what is its molar mass? Diprotic acids have the general formula H₂A.

Equation: H₂A + 2 NaOH ---> Na₂A + 2 H₂O

From titration data, moles of H₂A = 1.9819 x 10^-3, therefore molar mass is

0.2975 g/1.9819 x 10^-3 mol = 150.1 g/mol

 

(b) (7 marks) The organic acid in part (a) above contains the elements C, H, and O. If 0.4386 g of the acid yields 0.5144 g CO₂ and 0.1580 g H₂O in a combustion analysis, what are its empirical and molecular formulas?

CO2 44.01 g/mol

H2O 18.02 g/mol

Get masses of C and of H from the masses of CO₂ and H₂O, respectively. Subtract them from the mass of the original sample (0.4386 g) to obtain the mass of O.

mass of C = 0.1404 g

mass of H = 0.01768 g

mass of O = 0.4386 g sample - 0.1404 g C - 0.01768 g H = 0.2805 g O

 

Now calculate moles and proceed as usual:

C	H	O
1.169 x 10-2 moles C	1.754 x 10-2 moles H	1.753 x 10-2 moles O

Empirical formula = C₂H₃O₃ with a molar mass of 75 g/mol. This is half of the value calculated in part (a), so the molecular formula is C₄H₆O₆.

 

4. (10 marks)

(a) (6 marks) Fill in the blanks with the correct answer.

Barium is in Group 2A (give number) and has 2 (give number) valence electrons. Thus the barium cation will have a charge of +2 . Nitrogen is in Group 5A and has 5 valence electrons. Thus the nitride ion will have a charge of -3. The formula of barium nitride is Ba₃N₂.

 

An isotope of element X has 25 protons and 27 neutrons in its nucleus. Thus the atomic number of element X is 25 and its mass number is 52.Thus element X is Mn (give symbol). The +2 cation of this element would have 23 electrons. A different isotope of element X could have 25 protons in its nucleus and a mass number of 50, 51, 53, etc..

 

A mole of sulfur tetrafluoride, SF₄, contains one mole(s) of sulfur atoms and 4 mole(s) of fluorine atoms. One molecule of sulfur tetrafluoride has a mass of 1.794 x 10^-22 grams. A sample of sulfur tetrafluoride in which there are 1.00 x 10²³ atoms of fluorine contains 2.50 x 10²² atoms of sulfur, and corresponds to 0.0415 moles of sulfur tetrafluoride.

 

The formula of sodium pyrophosphate is Na₄P₂O₇. Thus the charge on the pyrophosphate ion is -4 (give sign and number) and the oxidation number of phosphorus is +5.

Chlorine dioxide, which is a major bleaching agent used in the pulp and paper industry, is made from sodium chlorate and sulfur dioxide according to the reaction

In this reaction, NaClO₃ is reduced because the oxidation number of Cl changes from +5 in the reactant to +4 in the product. The oxidizing agent in this reaction is NaClO₃.

(b) (4 marks) Explain the difference between the terms, equivalence point and end point of a titration. Use the permanganate titration of iron as an example. The net ionic equation for the reaction in this titration is

In the permanganate titration, a solution of KMnO₄ is added to the solution containing Fe²⁺. The equivalence point occurs when exactly enough permanganate has been added to react with all the Fe²⁺, i.e. one mole of MnO₄^- for every 5 moles of Fe²⁺. The end point occurs when you can see the pink color due to excess unreacted MnO₄^-. In this titration, the end point occurs after the equivalence point.

 

Question 5 (8 marks)

(a) (2 marks) What do the letters VSEPR stand for? Use VSEPR theory to explain why SO₂ and CO₂ do not have the same shapes.

Valence Shell Electron Pair Repulsion

CO₂ (16 electrons) is linear. The SO₂ (18 electrons) molecule is bent because the extra electron pair on the sulfur repels the bond pairs.

(b) (2 marks) State the Pauli Exclusion Principle. Show how it applies to helium by giving a complete set of quantum numbers for each electron in the helium atom.

No two electrons in an atom can have the same set of four quantum numbers.

He	n	l	ml	ms
Electron 1	1	0	0	+1/2
Electron 2	1	0	0	-1/2

The first three quantum numbers are the same for both electrons but the 4th (m_s) is different.

(c) (2 marks) Define the term, "first ionization energy." Briefly explain why the first ionization energy of sodium is higher than the first ionization energy of potassium but less than the first ionization energy of magnesium.

₁₁Na: [Ne]3s¹
₁₉K: [Ar]4s¹
₁₂Mg: [Ne]3s²

The first ionization energy of Na is lower than that of Mg because Mg has more protons in its nucleus to attract the 3s electron (a 3s electron is removed from each of these atoms, see electronic configurations above.)

The first ionization energy of Na is higher than that of K because the electrons are removed from s-orbitals with different principal quantum numbers. A 4s electron in potassium is further from the nucleus than a 3s electron in sodium, hence is easier to remove.

(d) (2 marks) What is the difference between a polar bond and a polar molecule? Explain, using HF and BF₃ as examples in your explanation.

Both HF and BF₃ have polar bonds because the electronegativity of F is higher than that of H or B.

BF₃ is nonpolar because the B-F bond dipoles in the trigonal planar molecule cancel each other to give a net dipole moment of zero. HF is polar because the bond dipole is the dipole moment for HF.

 

6. (8 marks)

(a) (3 marks) The energy required to break C-C single bonds is 348 kJ/mol.

(i) How much energy is required to break one C-C single bond?

5.78 x 10^-22 kJ/bond

(ii) If ultraviolet radiation with wavelength 375 nm shines on a compound containing C-C bonds, is there enough energy in one photon to break a C-C bond? Explain your answer.

E = hc/lambda

= 5.30 x 10^-19 J = 5.30 x 10^-22 kJ

A photon with a wavelength of 375 nm doesn't have quite enough energy to break a C-C bond.

(b) (5 marks) Use bond energies given below and estimate the enthalpy change for the reaction of carbon monoxide and hydrogen to produce acetic acid according to the following equation:

HINT: Draw the Lewis structures first! The connectivity of acetic acid is

The Lewis Structures for the 3 molecules are:

Thus, 2 moles of H₂ bonds break, as do two moles of C-O triple bonds. The energy to break the bonds is:

(2 mol H-H) x 436 kJ/mol + (2 mol C-O triple bonds) x 1072 kJ/mol

= 3016 kJ to break the bonds.

The energy that is recovered when the product forms results from the formation of three C-H bonds, one C-C bond, one C=O bond, one C-O bond, and one O-H bond:

(3 mol C-H) x -413 kJ/mol
+ (1 mol O-H) x -463 kJ/mol
+ (1 mol C-C) x -348 kJ/mol
+ (1 mol C=O) x -732 kJ/mol
+ (1 mol C-O) x -358 kJ/mol

= -3140 kJ

Thus the expected enthalpy change is (energy required to break bonds in reactants) + (energy given off when atoms form bonds in the product)

= -124 kJ

7. (10 marks)

(a) Draw Lewis structures for the following molecules: NH₃, BrF₃, CO₂

NH3 # of valence e = ? 8	BrF3 # of valence e = ? 28	CO2 # of valence e = ? 16

Of the molecules whose Lewis structures have been drawn above:

Which one(s) are polar? NH₃, BrF₃

Which one(s) have no lone pairs on the central atom? CO₂

Which one(s) have a pyramidal molecular geometry? NH₃

Which one(s) have a tetrahedral electron-pair geometry? NH₃

(b Draw Lewis structures for the following ions: ClO₃^-, NO₃^-, ClF₄⁺.

ClO3- # of valence e = 26	NO3- # of valence e = 24	ClF4+ # of valence e = 34

Of the ions whose Lewis structures have been drawn above:

Which one(s) have one lone pair on the central atom? ClO₃^-, ClF₄⁺

Which one(s) have a planar (trigonal or square) molecular geometry? NO₃^-

Which one(s) have a trigonal bipyramidal electron-pair geometry? ClF₄⁺

Which one(s) have at least one bond angle of 109.5°? ClO₃^-

MARKING SCHEME: Score = # right - #wrong/2

8. (24 marks; 1 mark each) Choose the best answer to each of the following questions. Write the letter of the correct answer in the space provided. Score = # right - #wrong/4

According to the experiments concerned with the photoelectric effect, what was the result of increasing the intensity of the light striking the metal surface?

(a) the number of electrons emitted was increased.

(b) The energy of the electrons emitted was increased.

(c) Both the number and energy of the electrons was increased.

(d) Both the number and energy of the electrons was decreased.

(e) There was no change in the number or energy of the electrons.

For a particular element, a photon of yellow light of wavelength 585 nm resulted when an electron fell from the third energy level to the second energy level. From this information we can determine

(a) the energy of the n = 2 level.

(b) the energy of the n = 3 level.

(c) the sum of the energies of n = 2 and n = 3.

(d) the sum of the energies of n = 1, n = 2, and n = 3.

(e) the difference in energies between n = 2 and n = 3.

Which of the following electronic transitions in a hydrogen atom would have the longest wavelength?

(a) n = 4 ---> n = 1

(b) n = 4 ---> n = 2

(c) n = 2 ---> n = 1

(d) n = 4 ---> n = 3

(e) n = 3 ---> n = 2

Which of the following electronic transitions in a hydrogen atom would have the greatest energy?

(a) n = 4 ---> n = 1

(b) n = 4 ---> n = 2

(c) n = 2 ---> n = 1

(d) n = 4 ---> n = 3

(e) n = 3 ---> n = 2

What type of orbital is designated n = 3, l = 2, ml = 0?

(a) 2s

(b) 3s

(c) 3p

(d) 3d

(e) 3f

What is the maximum number of orbitals possible when l = 1?

(a) one

(b) two

(c) three

(d) four

(e) five

The quantum number l represents the

(a) number of valence electrons.

(b) number of orbitals.

(c) shape of the orbital.

(d) orientation of the orbital.

(e) momentum of the electron.

Which one of the following atoms is paramagnetic?

(a) P

(b) Mg

(c) Zn

(d) Ar

(e) Ba

The number of unpaired electrons in a manganese atom and a manganese(II) ion, are, in order:

(a) 0,0

(b) 3,3

(c) 3,5

(d) 5,3

(e) 5,5

Which of the following is the correct electronic configuration for the iron(III) ion?

(a) [Ar]3d⁵

(b) [He]2s²2p²

(c) [Kr]4d¹⁰5s²5p²

(d) [Ar]3d³4s²

(e) [Ar]3d⁶4s²

Which of the following atoms is the largest?

(a) As

(b) Br

(c) F

(d) Sb

(e) Se

Which of the following atoms has the smallest first ionization energy?

(a) Al

(b) P

(c) Sr

(d) Ga

(e) Rb

If metallic character is characterized as the ability to lose electrons easily, the most metallic of the elements Ba, Mg, Pb, Sn, and Zn is

(a) Ba

(b) Mg

(c) Pb

(d) Sn

(e) Zn

Of the ions K⁺, Ca²⁺, S^2-, and Cl^-, which one (if any) has the largest ionic radius?

(a) K⁺

(b) Ca²⁺

(c) S^2-

(d) Cl^-

(e) they all have the same radius

Which of the following compounds would be expected to have the highest melting point?

(a) LiF

(b) LiCl

(c) NaBr

(d) CsF

(e) CsI

Which of the following compounds would be expected to have the longest ionic bond distance?

(a) LiF

(b) LiCl

(c) NaBr

(d) KBr

(e) KI

Which of the following is NOT a correct Lewis structure?

Answer: (d)

Which of the following is (are) CORRECT resonance structures for the formate ion, HCO₂^&endash;?

(a) 1 only

(b) 2 only

(c) 3 only

(d) 1 and 3 only

(e) 1, 2, and 3

Which of the following is (are) CORRECT resonance structures for the N₂O molecule?

(a) 1 only

(b) 2 only

(c) 3 only

(d) 1 and 2 only

(e) 1, 2, and 3

What species does the following Lewis structure represent?

(a) NO₂

(b) NO₂⁺

(c) NO₂^-

(d) both NO₂⁺ and NO₂^-

(e) NO₂, NO₂⁺ and NO₂^-

Which statement is true regarding bond order, bond length, and bond energy?

(a) As the bond order increases, the bond length increases.

(b) As the bond order increases, the bond length decreases.

(c) As the bond order increases, the bond energy decreases.

(d) As the bond energy increases, the bond length increases.

(e) As the bond energy increases, the bond order decreases.

What is the formal charge on each atom in the following structure for the nitrite ion?

(a) Nitrogen is 2-, oxygen on the left is 1-, oxygen on the right is 0.

(b) Nitrogen is 3+, both oxygens are 2-.

(c) Nitrogen is 0, oxygen on the left is 1-, oxygen on the right is 0.

(d) Nitrogen is 0, oxygen on the left is 0, oxygen on the right is 1-.

(e) Nitrogen is 3-, oxygen on the left is 1-, oxygen on the right is 2-.

From a consideration of the Lewis structure shown below, what is the formal charge on sulfur in the molecule SO₃?

(a) 0

(b) 2+

(c) 2-

(d) 4+

(e) 6+

A measure of the ability of a gaseous atom to acquire an electron to become negatively charged is called its

(a) ionization energy

(b) polarizability

(c) electron affinity

(d) electronegativity

(e) electron density

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Created November 21, 2000 by M. F. Richardson
© Brock University, 2000