Q: A sample of a solid has a cubic unit cell with an edge length of $$400\ \mathrm{pm}$$ and contains 2 atoms per unit cell. What is the density of the solid if the molar mass is $$60\ \mathrm{g/mol}$$? ($$1\ \mathrm{pm} = 1 \times 10$$^{-10}$$\ \mathrm{cm}$$, $$N_A = 6.022 \times 10$$^{23}$$\ \mathrm{mol}^{-1})$$

$$\displaystyle \frac{2 \times 60\ \mathrm{g/mol}}{(4.00 \times 10$$^{-8}$$\ \mathrm{cm})^3$ \times 6.022 \times 10$$^{23}$$} = 1.87\ \mathrm{g/cm^3$}$$

Question 1

Which of the following substances would you expect to have the highest boiling point, and why?

A. Methane ($$\mathrm{CH_4$})
B. $$Ammonia ($$\mathrm{NH_3$})
C. $$Carbon dioxide ($$\mathrm{CO_2$})
D. $$Oxygen ($$\mathrm{O_2$})

$$Consider the types of intermolecular forces present in each.

Accepted Answer

B. Ammonia, because it exhibits hydrogen bonding in addition to dipole-dipole and London dispersion forces.

Question 2

Which equation is used to calculate the vapor pressure of a liquid at a different temperature, given the enthalpy of vaporization ($$\Delta H_{\mathrm{vap}}$$)?

Accepted Answer

$$\ln \left( \frac{P_2$}{P_1$} \right) = -\frac{\Delta H_{\mathrm{vap}}}{R} \left( \frac{1}{T_2$} - \frac{1}{T_1$} \right)$$

Question 3

A sample of water at 100°C is boiling at 1 atm. Which of the following best explains why water boils at a lower temperature at higher altitudes?

Accepted Answer

The atmospheric pressure is lower, so the vapor pressure required for boiling is reached at a lower temperature.

Question 4

Which of the following best describes the relationship between surface tension and intermolecular forces?

Accepted Answer

Surface tension increases as intermolecular forces increase.

Question 5

A solution is prepared by mixing 50 g of NaCl in 500 mL of water. Which of the following best describes the process at the molecular level?

Accepted Answer

NaCl dissociates into $$\mathrm{Na^+}$$ and $$\mathrm{Cl^-}$$ ions, which are surrounded by water molecules due to ion-dipole interactions.

Question 6

Which of the following solids is held together primarily by London dispersion forces?

Accepted Answer

Solid argon ($$\mathrm{Ar})$$

Question 7

A phase diagram for carbon dioxide shows that at 1 atm, solid $$\mathrm{CO_2$}$$ sublimes directly to gas at $$-78.5^\circC. $$What does this indicate about the triple point of $$\mathrm{CO_2$}$$?

Accepted Answer

The triple point of $$\mathrm{CO_2$}$$ occurs at a pressure higher than 1 atm.

Question 8

Which of the following statements about viscosity is correct?

Accepted Answer

Viscosity increases as intermolecular forces increase.

Question 9

Which of the following best explains why ionic solids like $$\mathrm{NaCl}$$ have high melting points?

Accepted Answer

They are held together by strong electrostatic forces between oppositely charged ions.

Question 10

A sample of a solid has a cubic unit cell with an edge length of $$400\ \mathrm{pm}$$ and contains 2 atoms per unit cell. What is the density of the solid if the molar mass is $$60\ \mathrm{g/mol}$$? ($$1\ \mathrm{pm} = 1 	imes 10$$^{-10}$$\ \mathrm{cm}$$, $$N_A = 6.022 	imes 10$$^{23}$$\ \mathrm{mol}^{-1})$$

Accepted Answer

$$\displaystyle \frac{2 	imes 60\ \mathrm{g/mol}}{(4.00 	imes 10$$^{-8}$$\ \mathrm{cm})^3$ 	imes 6.022 	imes 10$$^{23}$$} = 1.87\ \mathrm{g/cm^3$}$$

Question 11

Which of the following best describes what happens when a nonpolar compound is mixed with a polar compound?

Accepted Answer

They are generally immiscible because the intermolecular forces between like molecules are stronger than those between unlike molecules.

Question 12

Which of the following is true about the effect of temperature on solubility of most solid solutes in water?

Accepted Answer

Solubility increases as temperature increases.

Question 13

Which of the following best explains why water has a much higher surface tension than hexane ($$\mathrm{C_6H_{14}}$$)?

Accepted Answer

Water molecules form hydrogen bonds, which are much stronger than the London dispersion forces present in hexane.

Question 14

Which of the following is the correct order of strength for intermolecular forces from weakest to strongest?

Accepted Answer

London dispersion \u003c dipole-dipole \u003c hydrogen bonding \u003c ion-dipole

Question 15

A heating curve for a pure substance shows a plateau at 0°C and another at 100°C. What is happening at these plateaus?

Accepted Answer

At 0°C, the substance is melting; at 100°C, it is boiling. The temperature remains constant as energy is used to overcome intermolecular forces.

Question 16

Which of the following best describes the structure of a metallic solid?

Accepted Answer

A lattice of positive metal ions surrounded by a sea of delocalized electrons.

Question 17

Which of the following statements about phase diagrams is correct?

Accepted Answer

The triple point is the unique set of conditions where solid, liquid, and gas phases coexist in equilibrium.

Question 18

Which of the following best explains why increasing the strength of intermolecular forces in a liquid decreases its vapor pressure?

Accepted Answer

Stronger intermolecular forces make it harder for molecules to escape from the liquid to the gas phase.

Question 19

A solution is saturated at 25°C. What will happen if more solute is added?

Accepted Answer

The added solute will not dissolve and will remain as a solid.

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