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CHEMISTRY 150

December 11th, 2013 B. MacLean NAME: (please print) _____________________________ ID #: _____________________________

This is a two-and-a-half (2.5) hour exam.

For multiple choice questions, use the data card provided and hand this card in with your exam when you are finished. Try to budget your time according to the value of each question.

There should be 13 pages in this exam (including this page). If your exam copy does not have 13 pages, notify me immediately.

Necessary data, including equations, constants, an electronegativity table, and a Periodic Table may be found at the end of the exam questions.

Be sure to indicate your name and student ID number on this card and the exam.

READ ALL QUESTIONS CAREFULLY

Questions Out of

Sec A 30

B1 16

B2 12

B3 16

B4 12

B5 14

TOTAL ____________ 100

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Section A; Multiple choice: 30 points (1 each): Answer each of the following multiple choice questions on the computer card provided by shading (be sure this is dark enough) the letter (a, b, c, or d) that corresponds to the best response in each case.

A1) Mercury has a density of 13.6 g/mL. What volume of mercury has a mass of 0.35 kg?

a) 0.0388 mL b) 0.039 mL c) 26 mL d) 2.6 mL

(

) (

)

A2) A doctor’s order is 0.125 g ampicillin. The liquid suspension on hand contains 250 mg/5.0 mL. How many mL of the suspension are required?

a) 0.0063 mL b) 2.5 mL c) 2.0 mL d) 6.3 mL

(

) (

)

A3) For the following calculation:

4.392g + 102.40g + 2.51 g =

the correct answer is

a) 109.302 g b) 109.30 g c) 109.3 g d) 109 g

I indicate the uncertain figure in each number above in red. The answer must have its uncertainty in the second decimal place.

A4) A sample of chlorine has two naturally occurring isotopes. The isotope Cl-35 makes up 75.8% of the sample and the isotope Cl-37 makes up 24.3% of the sample. Which of the following statements is true?

a) the atomic mass of chlorine will be less than 35 b) the atomic mass of chlorine will be greater than 37 c) the atomic mass will be closer to 37 than 35 d) the atomic mass of chlorine will be between 35 and 37

This problem is the reason why the atomic mass of Cl was omitted from the periodic table.

A5) An atom of iron has ___ valence electrons

a) 1 b) 2

3

c) 6 d) 8

Fe: 1s22s22p63s23p64s23d6 – valence electrons indicated in red.

A6) The correct name for the compound Al2(SO4)3 is

a) aluminum(II) sulfate b) dialuminum trisulfate c) dialuminum sulfate d) aluminum sulfate

Chapter 4, sides 27, 30.

A7) Which of the following compounds would you expect to be H2O-soluble?

a) KOH(s) b) CF4(g) c) Cl2(g) d) Fe(s)

Water is a polar molecule. CF4(g) and Cl2(g) are non-polar. Fe(s) is a metal (steel wool is Fe(s)). Also, KOH(s) is a strong

base – must be water-soluble.

A8) When the chemical equation: 4NH3 + 6NO 5N2 + 6H2O is balanced, the coefficient for the NH3 is

a) 6 b) 5 c) 4 d) 3

A9) When the temperature of 5.0 L a gas is changed from 20 oC to 10 oC, the volume of the gas becomes

a) 2.5 L

b) 4.8 L

c) 10 L

d) 5.2 L

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A10) The following diagram describes

a) an endothermic reaction that has an activation energy of 85 J

b) an exothermic reaction that has an activation energy of 25 J

c) an exothermic reaction that has an activation energy of 85 J

d) an exothermic reaction that has an activation energy of 60 J

Endothermic because E is positive and Ea = 85 J (25 J + 60 J).

A11) In response to Boyle’s Law, the pressure of a gas increases as the volume decreases because

a) the gas molecules become bigger

b) kinetic energy of the gas particles increases

c) the gas molecules strike the container walls with more force

d) the gas molecules strike the container walls more frequently

Molecules’ sizes are constant, and the kinetic energy of the molecules is only affected by the temperature. Making the

volume smaller means the gas molecules travel a shorter distance before colliding with the container walls.

A12) Which of the following can form H-bonds?

a) NaH

b) BH3

c) HCl

d) H2O

The only compound that has a H-O, H-N, or H-F bond is H2O.

A13) Which of the following is a homogeneous mixture that does not settle upon standing?

a) a compound

b) a suspension

c) a colloid

d) an element

Suspensions settle with time and compounds and elements are pure substances.

A14) A 10% starch solution is separated from a 2% starch solution by a semipermeable membrane. Which of the

following is correct?

a) The 10% solution has a higher osmotic pressure

b) The 2% solution has a higher osmotic pressure

c) Water will initially flow from the 10% solution, across the membrane, into the 2% solution

d) Water will initially flow across the membrane, into the 2% solution, from the 10% solution

Water flows into the solution that has the greater osmolarity (and greater osmotic pressure).

A15) Which of the following observations is a result of a change in the colligative properties of some substance?

a) Burning a match gives off heat.

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b) Adding Kool-Aid to water causes the water to become colored, letting less light pass through it.

c) Adding salt to water before making the Kool-Aid (see above) creates an interesting, but very bad, flavor.

d) Putting vinegar on your windshield in the winter gets rid of the frost on it.

The only colligative properties we looked at in the course were vapor pressure lowering, b.p. elevation, m.p. lowering,

and osmotic pressure. In d) the vinegar lowers the freezing point of water on the windshield.

A16) Which of the following compounds would be expected to have the smallest value for Ka?

a) H2SO4

b) H3PO4

c) HClO4

d) HNO3

a, c, and d are all strong acids. H3PO4 is the only weak acid. It has the smallest Ka.

A17) Which of the following is a true statement?

a) In acidic solutions, pH > 7

b) In basic solutions, [OH-] = [H3O+]

c) In neutral solutions, [H2O] > [H3O+]

d) In basic solutions, [OH-] > [H3O+]

In a basic solution, there is more OH- present than H3O+.

A18) Rubbing alcohol is 70.% isopropanol by volume. How many mL of isopropanol are present in a 473 mL container?

a) 370 mL

b) 0.15 mL

c) 470 mL

d) 330 mL

mLmLmLsoluteofvolume

mL

mLsoluteofvolumevv

solutionofvolume

soluteofvolumevv

330331)(__

%100*_473

)(__)/.%(70

%100*__

__)/%(

A19) What type of reaction is the following? 2HCl + Zn ZnCl2 + H2

a) Combination

b) Decomposition

c) Single replacement

d) Combustion

A20) How many protons are in the isotope of sodium with mass number = 25?

a) 11

6

b) 14

c) 25

d) 36

Sodium is element # 11 (atomic # = 11). The atomic number is the number of protons.

A21) Which of the following is a true statement regarding density?

a) The density of a gas is greater than the density of a liquid

b) Density is calculated as force divided by area

c) The units of density are g/mol

d) The density of ice is lower than the density of water

Gases have low densities (lots of empty space – Ch – 7, slide 5). Pressure is force/area. Density is g/mL units. Ice floats

on water.

A22) There are ______ different subshells present in the fourth energy shell.

a) 4

b) 8

c) 16

d) 32

# subshells = shell number. In 4th shell, have s, p, d, and f subshells.

A23) Which of the following molecules/ions is flat?

a) NH3

b) NH4+

c) SO42-

d) CO32-

Planar means flat. You’re looking for a geometry in the above examples that has a trigonal planar geometry. CO32- is

trigonal planar. A linear molecule would also be flat, but none of the above are linear.

A24) In a sample of pure CO2, the strongest intermolecular force of attraction that exists would be

a) London forces

b) dipole-dipole forces

c) H-bonding

d) ion-dipole forces

CO2 is a non-polar molecule. Only London forces exist between CO2 molecules.

A25) The measure of an atom’s ability to attract electrons in bonds toward itself is known as

a) electrostatic attraction

b) electronegativity

c) dipole moment

d) polarity

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A26) Which of the following molecules/ions would be expected to have the lowest pKa value?

a) H3PO4

b) H2PO4-

c) HPO42-

d) H2O

The weaker the acid the bigger pKa is. The weakest acid in the list above is actually H2O (pKa for water is pKw = 10-14). I

meant to make d) C6H12O6 or something that has no acidic hydrogens, which would then make c) the weakest acid, but I

didn’t get to it before printing. Probably not many would have gotten this, so I’ll give this point automatically.

A27) Which of the following solutions would be expected to have the highest osmotic pressure?

a) 2 M K3PO4

b) 4 M NaCl

c) 3 M Na2CO3

d) 3 M C6H12O6

The osmolarities of the above solutions are a) 8M; b) 8M; c) 9M; d) 3M. The highest osmolarity solution will have the

highest osmotic pressure.

A28) Which of the following is amphiprotic?

a) H3O+

b) HCl

c) NH3

d) H2O

Water can act as an acid or a base.

A29) Which one of the following quantities are not affected by temperature? (hint: look at the units)

a) density

b) reaction rate

c) %(m/v)

d) %(m/m)

Mass isn’t affected by temperature, but volume is. So is reaction rate. %(m/m) only involves masses, so is not affected

by temperature.

A30) For the following reaction, [N2O4] = [O2] = 1.5 M; [N2] = 4 M at equilibrium. The equilibrium constant would have a

value of

N2O4(g) N2(g) + 2O2(g)

a) 16

b) 4

c) 9

d) 6

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Section B; Problems: 70 points (5 questions): Answer each of the following questions in the space provided.

B1a) The following is a skeletal structure for, cysteine, an amino acid that serves a role as an antioxidant for the body:

Complete the Lewis structure and answer the following questions: [5]

This was similar to Assignment #2, question 5. Count up all of the valence electrons for C3H7O2NS:

3C: 3 x 4; 7H: 7 x 1; 2O: 2 x 6; N: 5; S: 6

Total = 42 electrons. Already used 26 (13 bonds) so use the remainder to complete the octets on the “outside” atoms.

Leaves 16 electrons for completing octets:

Carbon still does not have an octet, so it gets one by forming a double bond between itself and the top O atom (using a

pair of electrons from that O). Forming a double bond to this O gives it two bonds, as expected for a group 6 element.

Then answer the following questions:

What is the local (molecular) geometry around the sulfur atom in cysteine? _bent (angular)____ [2]

C1

6]5.1[

]5.1][4[

][

]][[ 2

42

2

22 ON

ONKeq

9

What is the local (molecular) geometry around the nitrogen atom in cysteine? _trigonal pyramid____ [2]

What is the local (molecular) geometry around the atom in cysteine labeled “C1”? _trigonal planar___ [2]

Circle the acidic hydrogen atom in this molecule [2]

Bonus [1]: To how many positions on a cysteine molecule can water molecules hydrogen bond? _8 positions______

H2O can H-bond though its H-atoms to the non-bonding electron pairs on each O (total 4 positions), the non-bonding

pair on N (1 position), and through its O atom’s non-bonding electrons to any of the H-atoms in cysteine’s O-H or N-H

bonds (3 of these). This was a tough question, but also only a 1 point and a bonus question.

b) A piece of Ca metal is placed in a 1.0-L container with pure N2. The N2 is at a pressure of 850 mm Hg and a

temperature of 26 oC. One hour later, the pressure has dropped to 24 oC and the pressure is now 702 mm Hg. How

many moles of N2 have reacted with the Ca? [3]

The only gas equation that has moles units in it is PV = nRT. This was a blue problem straight out of the text (#7.53).

Calculate the moles of N2 initially present and subtract the moles of N2 left in the container when the reaction is

finished.

)_(_...0378787.0

2974.62

0.1702

)_(_...045557.0

2994.62

0.1850

2

.

.

2

.

.

endNmol

KKmol

mmHgL

LmmHg

RT

PVn

startNmol

KKmol

mmHgL

LmmHg

RT

PVn

nRTPV

The difference in the number of moles of N2 at the start and finish is the number of moles of N2 that have reacted:

0.04577777… mol N2 – 0.0378787… mol N2 = 0.0076… mol N2 = 0.008 mol N2.

B2a) Propane (C3H8) is a hydrocarbon that is used in home heating. The pilot light in my fireplace uses about 1 liter of

propane a day, which amounts to 504g C3H8.

i) Write the combustion reaction for C3H8. [2]

All hydrocarbons combust to produce CO2 and H2O:

C3H8 + 5O2 3CO2 + 4H2O

ii) How many kilograms of CO2 does the pilot light produce each day? [4]

(

) (

)(

) (

)

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b) For the following redox reaction, indicate the oxidizing agent and reducing agent: [2]

3 Cu + 2 HNO3 + 6 H+ → 3 Cu2+ + 2 NO + 4 H2O

Cu: oxidation number changes from 0 +2 (oxidized)

N: oxidation number changes from +5 +2 (reduced)

Oxidizing agent: __HNO3_

Reducing agent: __Cu__

Remember, OA and RA are always on the left side.

c) Indicate whether each of the following changes would increase, decrease, or have no effect on the rate of the

chemical reaction shown:

N2 + 3 H2 2 NH3

i. Adding some H2 to the reaction mixture __increases___ [1]

ii. Raising the temperature of the reaction __increases___ [1]

iii. Removing a catalyst that is present in the reaction mixture __decreases___ [1]

iv. Adding some NH3 to the reaction container __no effect____ [1]

Rate of a reaction is proportional to the amount of reactant present. More reactant means more collisions, which

increases reaction rate. (Ch – 9, slide 20). Increasing the temperature enables more molecules to get past the activation

energy barrier, increasing the rate (Ch – 9, slide 22). Catalysts lower the activation energy barrier, so removing a catalyst

would increase the activation energy barrier, decreasing the reaction rate. NH3 is not involved in collisions between H2

and N2, so changing its concentration will not affect the rate of the reaction shown.

B3a) A 0.100 M solution of an acid, HA, is found to be 7.3% ionized. Write the equation showing the ionization of this

acid in water and calculate Ka for this acid. [5]

HA(aq) + H2O(l) A-(aq) + H3O+(aq)

If 7.3% (7.3%/100% = 0.073) of the acid ionizes, then (0.073)(0.100 M) = 0.0073 M HA will have reacted to produce

0.0073 M A- and 0.0073 M H3O+. This will leave behind 0.100 M – 0.0073 M = 0.0927 M HA at equilibrium.

Ka would be calculated as:

443 107.510...7486.5]0927.0[

]0073.0][0073.0[

][

]][[

xxHA

OHAKa

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b) Milk of magnesia is a suspension of Mg(OH)2 in water. When it is ingested, it ionizes according to the following

equation:

Mg(OH)2(s) Mg2+(aq) + 2 OH-(aq)

Mg(OH)2 is considered to be a slightly soluble ionic compound in water (equilibrium far to the left); however, Mg(OH)2

tablets quickly and completely ionize in the stomach. If this is so, the equilibrium shown above must shift fully to the

right side. Why does this happen and what is affected in the above reaction that causes this shift? [2]

Stomach acids would consume the OH- ions produced by the above reaction, removing them from the equilibrium

reaction (and convert them into water by H+ + OH- H2O). Removing OH- ions would pull the reaction to the right,

causing more Mg(OH)2 to dissolve. This would happen until all of the Mg(OH)2 dissolved.

c) List an example of each of the following (use a chemical formula in each case)

a weak base __NH3_________________ [2]

a strong acid __HCl/HBr/HI/HNO3/H2SO4/HClO3/HClO4___ [2]

a conjugate base of a weak acid __examples: C2H3O2-, NH3, and conj. Base of a weak acid [2]

d) What is the pH of a 0.0015 M solution of NaOH? [3]

NaOH is a strong base: NaOH Na+ + OH-

0.0015M NaOH is thus 0.0015 M Na+ and 0.0015 M OH-. If [OH-] is 0.0015 M, then

pOH = -log[OH-] = -log(0.0015) = 2.82

pH + pOH = 14.00. So pH = 14.00 – 2.82 = 11.18.

B4a) What mass (g) of water is present in exactly one kilogram of a 9.20 %(m/m) solution of NaCl? [2]

gsoluteofmass

solutiong

soluteofmass

solutionofmass

soluteofmassmm

92__

__1000

__

%100

%20.9

%100*__

__)/%(

The mass of the solute (NaCl) is 92g. The mass of the solution is 1000 g. The mass of the H2O in the solution is thus

1000g - 92 g = 908 g H2O.

b) If 250 mL of H2O is added to 750 mL of the above solution, what will the %(m/m) concentration of the resulting

solution be? [2]

If you have 750 mL of 9.20%(m/m) NaCl solution, and add 250 mL of water to it, you will have diluted the solution to a

final volume of (about) 1000 mL (“about” because volumes aren’t additive, but this is water being added to a water-

based solution, so the final volume should be very close to 1000 mL). The dilution equation is:

12

)/%(90.6

1000750/%20.9

2

2

2211

mmC

mLCmLmm

VCVC

c) Circle the aqueous solution that should have the highest boiling point [2]

i) 0.05 M NaCl ii) 0.15 M C6H12O6 iii) 0.04 M Mg(C2H3O2)2 iv) 0.04 M K3PO4

Highest boiling point will be the solution with the highest osmolarity (see b.p. elevation slide, Ch-8, slide 45).

d) Circle the aqueous solution that should possess the lowest freezing point [2]

i) 0.05 M NaCl ii) 0.15 M C6H12O6 iii) 0.04 M Mg(C2H3O2)2 iv) 0.02 M K3PO4

Lowest freezing point will be the solution with the highest osmolarity (see m.p. lowering slide, Ch-8, slide 45). e) Circle the aqueous solution that should possess the highest vapor pressure [2]

i) 0.05 M NaCl ii) 0.15 M C6H12O6 iii) 0.04 M Mg(C2H3O2)2 iv) 0.04 M K3PO4

Lowest vapor pressure will be the solution with the highest osmolarity. Highest vapor pressure will be the solution with the lowest osmolarity (see vapor pressure slide, Ch-8, slide 43).

f) Circle the aqueous solution that should possess the highest pH [2]

i) 0.05 M HCl ii) 0.01 M HCl iii) 0.05 M HC2H3O2 iv) 0.05 M H2SO4

i, ii, and iv are strong acids. iii is a weak acid. More acidic the solution, the lower the pH. (ii was supposed to be “0.10 M

HCl” not 0.01 M HCl, which would make this easyier to solve, so I’ll give these 2 points automatically. Merry Christmas.)

B5a) What was the name of the scientific law that I demonstrated earlier this year using marshmallows? [2]

__Boyle’s Law___

b) Write chemical formulas for each of the following compounds/ions:

i) acetate _C2H3O2-_____ [2]

ii) ammonia __NH3_______ [2]

iii) iron(III) sulfide _Fe2S3_______ [2]

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c) Name two elements that are found in nature as liquids (assume T = 25oC) [2]

Br2(l) (bromine) and Hg(l) (mercury)

d) Explain how you could differentiate between a solution, a suspension, and a colloidal dispersion. [4]

To differentiate between a suspension and the other two mixtures, allow sufficient time to pass to permit the solute and

solid to separate (settle under the influence of gravity). To differentiate between a solution and a colloid, pass a beam

of light through them. The light beam will scatter (Tyndall scattering) when passed through a colloidal dispersion, but

not through a solution.

Another bonus question!!!!!!!!!!!!! [2]

Lithium is a reactive alkali metal, and like all alkali metals, it reacts vigorously with water in an exothermic reaction to

produce H2(g):

2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)

But lithium chloride does not explode when put into water. Why?

Lithium chloride involves Li+ ions, not reactive Li metal. Li+ ions possess a stable octet, while Li metal does not. In the

above equation, it can be seen that Li metal gets oxidized by water, losing its valence electron to H2O. This produces the

more stable Li+ ion.

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Formulas, constants

ENJOY YOUR HOLIDAYS!!!!!!!

273)()(

...

7607601

4.620821.0

%100][

][_%

00.14

101]][[

log

][10

]log[

__

__

2

22

1

11

2211

2

2

1

1

.

.

.

.

14

3

3

3

2211

CTKT

PPPP

T

VP

T

VP

VPVP

T

V

T

V

mmHgtorratm

Kmol

mmHgL

Kmol

atmLR

nRTPV

HA

Hionization

pOHpH

xKOHOH

KpK

OH

OHpH

v

md

VCVC

solutionofL

soluteofmolesM

o

CBATot

i

e

w

aa

pH

...___%_

][][

][][

*

%100*__

__)/%(

%100*__

__)/%(

%100*__

__)/%(

__

__

XmassXabundmassatomic

zDyCxBwA

BA

DCK

iMosmolarity

solutionofmL

soluteofgramsvm

solutionofvolume

soluteofvolumevv

solutionofmass

soluteofmassmm

unitsdesiredunitsgiven

unitsdesiredunitsgiven

xw

zy

eq

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