In each of the following groups of substances, pick the one that has the given property. Justify your answer. a. highest boiling point: \(\mathrm{HBr}, \mathrm{Kr}\), or \(\mathrm{Cl}_{2}\) b. highest freezing point: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{NaCl}\), or \(\mathrm{HF}\) c. lowest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{Cl}_{2}, \mathrm{Br}_{2}\), or \(\mathrm{I}_{2}\) d. lowest freezing point: \(\mathrm{N}_{2}, \mathrm{CO}\), or \(\mathrm{CO}_{2}\) e. lowest boiling point: \(\mathrm{CH}_{4}, \mathrm{CH}_{3} \mathrm{CH}_{3}\), or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) f. highest boiling point: \(\mathrm{HF}, \mathrm{HCl}\), or \(\mathrm{HBr}\) g. lowest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}, \mathrm{CH}_{3} \mathrm{CCH}_{3}\), or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Step by step solution

01

Consider Intermolecular Forces

HBr has dipole-dipole forces due to its polarity while Kr and Cl2 are nonpolar with London dispersion forces. Dipole-dipole forces are stronger than London dispersion forces.

02

Compare Molecular Weights

HBr has a molar mass of 80 g/mol, Kr has a molar mass of 83.8 g/mol, and Cl2 has a molar mass of 70.9 g/mol. In general, higher molar mass correlates with stronger London dispersion forces.

03

Solution

HBr has the highest boiling point because it has dipole-dipole forces, which are stronger than the London dispersion forces present in Kr and Cl2. b. highest freezing point: H2O, NaCl, or HF

04

Consider Intermolecular Forces

H2O has hydrogen bonding, ionic bonding is present in NaCl, and HF has hydrogen bonding. Hydrogen bonding is a special type of dipole-dipole interaction, and ionic forces are even stronger than hydrogen bonds.

05

Solution

NaCl has the highest freezing point, due to strong ionic bonding between its ions. c. lowest vapor pressure at 25°C: Cl2, Br2, or I2

06

Consider Intermolecular Forces

All three molecules are non-polar, so they have London dispersion forces.

07

Compare Molecular Weights

Cl2 has a molar mass of 70.9 g/mol, Br2 has a molar mass of 159.8 g/mol, and I2 has a molar mass of 253.8 g/mol. Higher molar mass correlates with stronger London dispersion forces.

08

Solution

I2 has the lowest vapor pressure at 25°C because its stronger London dispersion forces require more energy to be exerted before it vaporizes. d. lowest freezing point: N2, CO, or CO2

09

Consider Intermolecular Forces

All of these molecules are nonpolar and have the London Dispersion forces.

10

Compare Molecular Weights

N2 has a molar mass of 28 g/mol, CO has a molar mass of 28 g/mol, and CO2 has a molar mass of 44 g/mol.

11

Solution

N2 and CO have the lowest freezing points due to their lower molar masses and therefore weaker London dispersion forces. e. lowest boiling point: CH4, CH3CH3, or CH3CH2CH3

12

Consider Intermolecular Forces

All of these molecules are nonpolar and have the London Dispersion forces.

13

Compare Molecular Weights

CH4 has a molar mass of 16 g/mol, CH3CH3 has a molar mass of 30 g/mol, and CH3CH2CH3 has a molar mass of 44 g/mol.

14

Solution

CH4 has the lowest boiling point due to its lower molar mass and therefore weaker London dispersion forces. f. highest boiling point: HF, HCl, or HBr

15

Consider Intermolecular Forces

All three molecules are polar and form hydrogen bonds.

16

Compare Molecular Weights

HF has a molar mass of 20 g/mol, HCl has a molar mass of 36.5 g/mol, and HBr has a molar mass of 80 g/mol.

17

Solution

HBr has the highest boiling point due to its greater molar mass and thus stronger hydrogen bonding. g. lowest vapor pressure at 25°C: CH3CH2CH3, CH3CCH3, or CH3CH2CH2OH

18

Consider Intermolecular Forces

CH3CH2CH3 and CH3CCH3 have London dispersion forces, while CH3CH2CH2OH has hydrogen bonding due to the -OH group.

19

Solution

CH3CH2CH2OH has the lowest vapor pressure at 25°C due to the stronger hydrogen bonding compared to the London dispersion forces in other molecules.

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