Q: Which of the following statements about molecular orbital theory is correct?

Bond order is calculated as $$\frac{\text{number of bonding electrons} - \text{number of antibonding electrons}}{2}.$$

Question 1

Which of the following best describes the difference between intermolecular and intramolecular forces?

Accepted Answer

Intermolecular forces are the forces between molecules, while intramolecular forces are the forces that hold atoms together within a molecule.

Question 2

Given the reaction $$2\mathrm{C}_2\mathrm{H}_6 + 7\mathrm{O}_2 \rightarrow 4\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O}$$, and the following bond enthalpies: $$\mathrm{C-H} = 414\ \mathrm{kJ/mol}$$, $$\mathrm{O=O} = 498\ \mathrm{kJ/mol}$$, $$\mathrm{C=O} = 799\ \mathrm{kJ/mol}$$, $$\mathrm{O-H} = 463\ \mathrm{kJ/mol}$$, calculate the enthalpy change $$\Delta H$$ for the reaction.

Accepted Answer

$$-2748\ \mathrm{kJ/mol}$$

Question 3

Which of the following statements about exothermic and endothermic reactions is correct?

Accepted Answer

Exothermic reactions release heat to the surroundings, while endothermic reactions absorb heat.

Question 4

Which of the following molecules is polar?

Accepted Answer

$$\mathrm{NH}_3$$

Question 5

Which of the following best describes a chiral molecule?

Accepted Answer

A molecule that cannot be superimposed on its mirror image.

Question 6

What is the electron-pair geometry and molecular shape of $$\mathrm{NH}_3$$?

Accepted Answer

Tetrahedral electron-pair geometry, trigonal pyramidal molecular shape

Question 7

Given the following atomic weights: $$\mathrm{Br}-79 = 78.9183$$, $$\mathrm{Br}-81 = 80.9163$$, and the average atomic mass of bromine is $$79.904$$ amu, what is the percent abundance of $$^{79}\mathrm{Br}$$ and $$^{81}\mathrm{Br}$$, respectively?

Accepted Answer

$$^{79}\mathrm{Br} 49.3$$\%, $$^{81}\mathrm{Br} 50.7$$\%

Question 8

How many grams of barium chloride ($$\mathrm{BaCl}_2) $$are needed to make $$25.0\ \mathrm{mL} of a$$ $$3.5\ \mathrm{M}$$ stock solution? (Molar mass of $$\mathrm{BaCl}_2 = 208\ \mathrm{g/mol})$$

Accepted Answer

$$18.2\ \mathrm{g}$$

Question 9

Which of the following best describes the relationship between actual nuclear charge ($$Z) $$and effective nuclear charge ($$Z_{\mathrm{eff}}) $$experienced by an electron in a many-electron atom?

Accepted Answer

The actual nuclear charge is always greater than the effective nuclear charge.

Question 10

A sample of an element has atoms with an average mass of $$1.9934 	imes 10$$^{-23}$$\ \mathrm{g}. $$What is the identity of the element? ($$1\ \mathrm{mol} = 6.022 	imes 10$$^{23}$$ atoms)

Accepted Answer

Carbon

Question 11

Which of the following best describes the trend in effective nuclear charge ($$Z_{\mathrm{eff}}) $$across a period in the periodic table, as shown in the provided bar graph?

Accepted Answer

$$Z_{\mathrm{eff}}$$ increases from left to right across a period.

Question 12

Given the molecule in the second image, what is the hybridization of the central carbon atom labeled 3, and what is its molecular geometry?

Accepted Answer

sp$$^2$$ hybridization, trigonal planar geometry

Question 13

For the molecule in the second image, how many sigma ($$\sigma) $$and pi ($$\pi) $$bonds are present in total?

Accepted Answer

16 $$\sigma$$ bonds, 4 $$\pi$$ bonds

Question 14

Which of the following statements about molecular orbital theory is correct?

Accepted Answer

Bond order is calculated as $$\frac{	ext{number of bonding electrons} - 	ext{number of antibonding electrons}}{2}.$$

Question 15

Which of the following molecules is chiral?

Accepted Answer

A molecule with four different groups attached to a central carbon atom.

Question 16

Which of the following is the correct electron configuration for a neutral sodium atom ($$Z=11$$)?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^6$$\ $$3s^1$$

Question 17

Which of the following best explains why lattice energy is important in the formation of solid ionic compounds?

Accepted Answer

Lattice energy is the energy released when gaseous ions form an ionic solid, making the process exothermic and stabilizing the solid.

Question 18

A solution is prepared by dissolving $$5.00\ \mathrm{g} of $$NaCl in enough water to make $$250.0\ \mathrm{mL} of $$solution. What is the molarity of the solution? (Molar mass of NaCl = $$58.44\ \mathrm{g/mol})$$

Accepted Answer

$$0.342\ \mathrm{M}$$

Question 19

Which of the following best describes the Heisenberg uncertainty principle?

Accepted Answer

It is impossible to simultaneously know both the exact position and exact momentum of an electron.

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