UNSCO 2017

Question 1

Lithium reacts with water to form lithium hydroxide. What mass of lithium is required to produce 12 g of lithium hydroxide?

2.0 g
3.5 g
7.0 g
12 g

Answer 1

3.5 g

Question 2

Complete combustion of 1.00 g of the hydrocarbon pagodane gives 3.38 g carbon dioxide. What is the empirical formula of pagodane?

CH
CH2
C2H5
C3H8

Answer 2

CH

Question 3

Electrolysis of 10.00 g of a binary metal chloride deposits 6.207 g of the pure metal. What is the metal?

Cu (Z = 29)
Cd (Z = 48)
Ce (Z = 58)
Th (Z = 90)

Answer 3

Th (Z = 90)

Question 4

Decomposition of 1.0 g of which of the following compounds into its constituent elements gives the greatest amount of N2 gas?

NO
NO2
N2O4
NH3

Answer 4

NH3

Question 5

Permanganate ion oxidizes hydrogen peroxide in acidic solution according to the following equation: 2 MnO4–(aq) + 5 H2O2(aq) + 6 H+(aq)  2 Mn2+(aq) + 5 O2(g) + 8 H2O(l) If 35.0 mL of an acidic 0.150 M KMnO4 solution is required to consume all the H2O2 in 50.0 mL of a disinfectant solution, what is the concentration of H2O2 in the disinfectant?

0. 0420 M
1. 105 M
2. 263 M
3. 368 M

Answer 5

0.263 M

Question 6

A solution containing 10 g of which substance dissolved in 100 g of water will show the greatest freezing point depression compared to pure water?

Anhydrous magnesium sulfate, MgSO4
Magnesium sulfate heptahydrate, MgSO4•7 H2O
Anhydrous sodium thiosulfate, Na2S2O3
Sodium thiosulfate pentahydrate, Na2S2O3•5 H2O

Answer 6

Anhydrous sodium thiosulfate, Na2S2O3

Question 7

When a solution of barium hydroxide is mixed with a solution of iron(III) chloride, what is observed?

Precipitation of a colored solid
Precipitation of a colorless solid
Evolution of a colorless gas
Neither precipitation nor gas evolution

Answer 7

Precipitation of a colored solid

Question 8

Which element is a liquid at 25 ºC and 1 atm pressure?

Fluorine
Chlorine
Bromine
Iodine

Answer 8

Bromine

Question 9

The concentration of which approximately 0.01 M solution could be most accurately determined by a visible spectrophotometer (or colorimeter)?

Mn(NO3)2
Co(NO3)2
Zn(NO3)2
Pb(NO3)2

Answer 9

Co(NO3)2

Question 10

A beaker containing 25 mL of liquid 1-aminopentane, CH3(CH2)4NH2, is placed on a hotplate and brought to a boil. As the 1-aminopentane boils,

the total energy of the system stays constant.

the hydrogen bonding between the 1-aminopentane molecules is disrupted.

the ion-dipole forces between the 1-aminopentane molecules are disrupted.

pentane and ammonia gas are formed.

Answer 10

the hydrogen bonding between the 1-aminopentane molecules is disrupted.

Question 11

Into both ends of a meter-long glass tube samples of gases are introduced simultaneously. One end receives hydrogen chloride gas (HCl) while the other end receives ammonia gas (NH3). When the gases meet in the tube, they react to form solid ammonium chloride (NH4Cl). Where in the tube does the NH4Cl form?

At the center of the tube

Closer to the end where the hydrogen chloride is inserted

Closer to the end where the ammonia is inserted

Uniformly at all positions in the tube

Answer 11

Closer to the end where the hydrogen chloride is inserted

Question 12

At its normal boiling point of –1.0 ºC, a sample of gaseous butane at 1.0 atm occupies a volume of 1.0 L. What is the pressure if the volume is decreased to 0.70 L while maintaining the temperature at –1.0 ºC?

0. 70 atm
1. 0 atm
2. 4 atm
3. 0 atm

Answer 12

1.0 atm

Question 13

What is the principal energetic factor in the lack of miscibility between C6H14(l) and H2O(l)?

The strength of intermolecular forces of attraction between C6H14(l) molecules

The strength of intermolecular forces of attraction between H2O(l) molecules

The difference between the molecular weights of the molecules

The difference in electronegativity between carbon and hydrogen

Answer 13

The strength of intermolecular forces of attraction between H2O(l) molecules

Question 14

The melting point of silicon dioxide (1713 ºC) is higher than the melting point of silicon (1414 ºC). What is the best explanation for this difference?

Silicon-oxygen bonds are stronger than siliconsilicon bonds.

Silicon dioxide is an ionic solid while silicon is a metallic solid.

Silicon dioxide is polar while silicon is nonpolar.

Silicon dioxide forms tetragonal crystals while silicon forms cubic crystals.
18

Answer 14

Silicon-oxygen bonds are stronger than siliconsilicon bonds

Question 15

Upon which factors can the Gibbs free energy change for a reaction (∆Grxn) depend?

I. Temperature
II. Concentration of species in solution

I only
II only
Both I and II
Neither I nor II

Answer 15

Both I and II

Question 16

Which has the greatest entropy at 0 ºC?

1. 0 mol H2O(s)
2. 0 mol H2O(l)
3. 0 mol H2O(g)
4. 0 mol H2(g) + 0.5 mol O2(g)

Answer 16

1.0 mol H2(g) + 0.5 mol O2(g)

Question 17

For the reaction A + B  products, the rate law is rate = k[A]2[B]. Which change will cause the greatest decrease in reaction rate?

Decreasing [A] by a factor of 2
Decreasing [B] by a factor of 2
Decreasing both [A] and [B] by a factor of 2
Decreasing [B] by a factor of 4

Answer 17

Decreasing both [A] and [B] by a factor of 2

Question 18

For a reaction with an activation energy of 65 kJ mol–1, by what percentage is the rate constant decreased if the temperature is decreased from 37 °C to 22 °C?

13%
27%
51%
72%

Answer 18

72%

Question 19

A plot of ln[A] as a function of time in an irreversible reaction A → products is linear, with a slope of –0.0175 s–1. What conclusions may be drawn from these observations? I. The reaction is first-order in A. II. The rate constant for the reaction is 0.0175 s–1.

I only
II only
Both I and II
Neither I nor II

Answer 19

Both I and II

Question 20

Hydrogen peroxide disproportionates to water and molecular oxygen in the presence of iodide in neutral solution according to a mechanism consisting of two elementary steps:
H2O2(aq) + I –(aq)  IO–(aq) + H2O(l)
reaction 1 H2O2(aq) + IO–(aq)  I –(aq) + O2(g) + H2O(l) reaction 2 The rate constant for reaction 1 is much larger than the rate constant for reaction 2.

Which statement is correct?

As the reaction proceeds, the predominant form of iodine in solution is IO–(aq).

Adding more iodide to the reaction will not increase the rate of production of O2.

The reaction is zeroth-order in H2O2.

The reaction will go more slowly at higher O2 pressures.
30

Answer 20

As the reaction proceeds, the predominant form of iodine in solution is IO–(aq).

Question 21

What is the pH of a 0.25 M solution of NaCN? (The pKa of HCN is 9.21.)

4.91

8. 61
9. 30
10. 40

Answer 21

11.30

Question 22

The autoionization constant of water at 60 ºC is Kw = 1.0  10–13. Which of the following statements are correct?

I. Autoionization of water is exothermic.
II. A sample of pure water at 60 ºC is slightly acidic.

I only
I only
Both I and II
Neither I nor II

Answer 22

Neither I nor II

Question 23

Barium carbonate, BaCO3, is stable at ambient temperatures, but decomposes to barium oxide and carbon dioxide at higher temperatures.

BaCO3(s) BaO(s) + CO2(g) At a certain temperature, this system is in equilibrium in a closed system and contains appreciable amounts of all three compounds. Which changes will lead to an increase in the pressure of CO2 present at equilibrium?

I. Adding more BaCO3(s)
II. Increasing the volume of the container

I only
II only
Both I and II
Neither I nor II

Answer 23

Neither I nor II

Question 24

If 0.10 mol solid NaOH is added to 1.00 L of a saturated solution of Ca(OH)2 (Ksp = 8.0  10–6),
what percentage of the calcium hydroxide will precipitate at equilibrium?

Roughly 50%
Roughly 75%
Roughly 95%
Over 99%

Answer 24

Roughly 95%

Question 25

The concentration of formic acid (pKa = 3.75) is being determined by titration with sodium hydroxide solution. Which indicators are suitable for this titration? I. Bromophenol blue (pH transition range 3.0 – 4.6) II. Neutral red (pH transition range 6.8 – 8.0) I only II only Both I and II Neither I nor II

Answer 25

II only

Question 26

The ionization of ammonium ion is endothermic: NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq) ∆Hº > 0 Which changes will result in the increase in [H3O+] of a 0.100 M solution of NH4Cl? I. Diluting the solution from 1.00 L to 2.00 L II. Raising the temperature from 25 ºC to 35 ºC I only II only Both I and II Neither I nor II

Answer 26

II only

Question 27

Which changes to this galvanic cell increase the measured potential? Cu(s) | Cu2+(aq), 0.10 M || Ag+(aq), 1.0 M | Ag(s) I. Increasing [Cu2+] in the Cu/Cu2+ half-cell to 0.50 M II. Adding Cl– to the Ag+/Ag half-cell until [Cl–] = 0.01 M I only II only Both I and II Neither I nor II

Answer 27

Neither I nor II

Question 28

When the following skeleton equation is balanced with smallest whole number coefficients, what is the coefficient and location of H2O(l)? The reaction takes place in basic solution. Cu(s) + MnO4–(aq)  Cu(OH)2(s) + MnO2(s) 2, on reactant side 4, on reactant side 2, on product side 4, on product side

Answer 28

4, on reactant side

Question 29

Which of the following species contains the element in the highest oxidation state? OsO4 Mn2(CO)10 N5+ XeF82–

Answer 29

OsO4

Question 30

The lead-acid storage battery consists of the following two half-cells: PbO2(s) + 4 H+(aq) + SO42-(aq) + 2 e–  PbSO4(s) + 2 H2O(l) Pb(s) + SO42-(aq)  PbSO4(s) + 2 e– Which of the following concentrations decrease as the battery is discharged? [H+] [Pb2+] I only II only Both I and II Neither I nor II

Answer 30

I only

Question 31

In a ground-state P atom in the gas phase, how many electrons have quantum numbers n = 3, l = 1, ml = –1? 0 1 2 3

Answer 31

1

Question 32

Rank the elements Si, P, Ge, and As in increasing order of their first ionization energies. Si < P < Ge < As As < Ge < P < Si Ge < Si < As < P Ge < As < Si < P

Answer 32

Ge < Si < As < P

Question 33

Which change in principal quantum number for an electron in a hydrogen atom would correspond to emission of the longest-wavelength photon? ``` n = 4  n = 1 n = 5  n = 2 n = 1  n = 5 n = 2  n = 4 ```

Answer 33

n = 5  n = 2

Question 34

Which pair of elements has the most similar electronegativities? B and C B and Al B and Si Al and C

Answer 34

B and Si

Question 35

The permanganate ion, MnO4–, is purple while the pertechnetate ion, TcO4–, is colorless. This difference is most closely related to which difference between manganese and technetium? Manganese(VII) is a stronger oxidant than technetium(VII). Manganese consists of a stable isotope while all isotopes of technetium are radioactive. The Mn–O bond in permanganate is shorter than the Tc–O bond in pertechnetate. Elemental manganese is less dense than elemental technetium.

Answer 35

Manganese(VII) is a stronger oxidant than technetium(VII).

Question 36

Which ion has the longest N–O bond? NO3– NO2– NO2+ NO+

Answer 36

NO3–

Question 37

Which pair of species has the same shape? CO2 and SO2 CCl4 and TiCl4 C2H6 and B2H6 NO3– and PO33–

Answer 37

CCl4 and TiCl4

Question 38

How many distinct C–O bond lengths are present in the oxalate ion, C2O42–? 1 2 3 4

Answer 38

1

Question 39

Which statement about the molecular orbitals in a molecule is correct? No molecular orbital may have a net overlap with any other molecular orbital. Each molecular orbital must have a different number of nodes than every other molecular orbital. The number of molecular orbitals is equal to half the number of atomic orbitals of the atoms that make up the molecule. The lowest-energy molecular orbitals are the most antibonding in character and the highest-energy molecular orbitals are the most bonding in character

Answer 39

No molecular orbital may have a net overlap with any other molecular orbital.

Question 40

NF3 has a bond angle of 102.5º, while PF3 has a bond angle of 96.3º. What is the best explanation for the larger bond angle in NF3? The nitrogen 2s orbital participates more in bonding than does the phosphorus 3s orbital. Nitrogen is more electronegative than phosphorus. NF3 has no unpaired electrons while PF3 has two unpaired electrons. NF3 is an ionic compound while PF3 forms covalent bonds.

Answer 40

The nitrogen 2s orbital participates more in bonding than does the phosphorus 3s orbital.

Question 41

Allene has the structure H2C=C=CH2. What is the best description of the geometry of allene? Geometry at central carbon Positions of hydrogen atoms Linear All in the same plane Linear In two perpendicular planes Bent All in the same plane Bent In two perpendicular planes

Answer 41

Linear In two perpendicular planes

Question 42

END

Answer 42

END