Unit 3 Practice: Hard

Answer: Pentane generally has the higher boiling point.

Explanation: Pentane is less compact and has greater surface contact between neighboring molecules.

That allows stronger London dispersion interactions.

Neopentane is more compact, reducing effective surface contact.

Answer: HCl is polar, while Cl₂ is nonpolar.

Explanation: H and Cl have different electronegativities, so the H—Cl bond has an unequal electron distribution.

Cl₂ contains identical atoms, so electrons are shared equally and no permanent dipole exists.

Answer: Very large, highly polarizable molecules can have extremely strong London dispersion forces.

Explanation: Hydrogen bonding is strong for many small molecules, but intermolecular force comparisons depend on the entire molecule.

A very large nonpolar molecule may have a huge electron cloud and strong dispersion forces that exceed the total attractions in a smaller hydrogen-bonding substance.

Answer: Charge carriers are fixed in the solid but mobile in the liquid.

Explanation: Electrical conduction requires charged particles to move.

In solid NaCl, ions occupy fixed lattice positions.

When melted, Na⁺ and Cl⁻ become mobile and can carry current.

Answer: Diamond and SiO₂ are network covalent solids, while graphite is a layered network structure.

Explanation: Diamond has a strong three-dimensional covalent network, making it extremely hard and generally nonconductive.

SiO₂ also forms a strong covalent network and has a high melting point.

Graphite has strong covalent bonds within layers but weaker forces between layers, so it is softer. Delocalized electrons also allow it to conduct.

Answer:

PHe = 2.00 atm

PNe = 8.00 atm

Work:

Total moles:

5.00 mol

He mole fraction:

1/5 = 0.200

PHe = (0.200)(10.0)=2.00 atm

Ne mole fraction:

4/5 = 0.800

PNe=(0.800)(10.0)=8.00 atm

Answer: Attractions pull particles away from the container walls and toward each other.

Explanation: Ideal gases are assumed to have no intermolecular attractions.

In a real gas, attractive forces can reduce the force of particle-wall collisions.

Lower collision force results in lower measured pressure.

Answer: The actual volume occupied by gas particles is no longer negligible compared with the container volume.

Explanation: The ideal gas model treats particles as having no volume.

At high pressure, particles are packed closely together, so their own physical size matters.

This can cause deviations from ideal behavior that differ from the effect of attractions.

Answer: The curve becomes broader, lower, and shifts toward higher speeds or energies.

Explanation: Higher temperature increases average kinetic energy.

A larger fraction of particles now have high energies, while the distribution becomes more spread out.

The total area under the curve stays the same because the number of particles has not changed.

Answer: He has a distribution shifted toward higher speeds.

Explanation: Both gases have the same average kinetic energy at the same temperature.

Because He has a much smaller mass, its particles must move faster on average.

Answer: 20

Explanation: Na₂SO₄ dissociates according to:

Na₂SO₄ → 2Na⁺ + SO₄²⁻

The ratio is:

2 Na⁺ : 1 SO₄²⁻

So:

40/2 = 20

Answer: Solubility depends on the overall balance of particle-particle interactions, not simply labels like polar or nonpolar.

Explanation: A solute dissolves when new solute-solvent interactions are favorable enough compared with the attractions that must be disrupted.

Molecular size, lattice strength, hydrogen bonding, entropy, and other factors can all matter.

Answer: Dissolution depends on competition between ionic lattice attractions and ion-water attractions.

Explanation: Water stabilizes ions through ion-dipole attractions.

However, an ionic crystal may have a very strong lattice.

If breaking the lattice is difficult enough, the compound may have low solubility even though water is polar.

Answer: Distillation separates using differences in volatility, while chromatography separates using different interactions with stationary and mobile phases.

Explanation: In distillation, components vaporize and condense at different temperatures.

In chromatography, components move different distances or speeds because they interact differently with the two phases.

Answer: Compound A

Explanation: A smaller Rf means the compound moved a shorter distance relative to the solvent front.

That usually indicates stronger attraction to the stationary phase under those specific conditions.

Answer: Rf depends on experimental conditions.

Explanation: Changing the solvent, stationary phase, temperature, or other conditions can alter how strongly a compound interacts with each phase.

Rf is most meaningful when comparing substances under the same experimental setup.

Answer: 3.98 × 10⁻¹⁹ J

Work:

Use:

E = hc/λ

Convert wavelength:

500 nm = 5.00 × 10⁻⁷ m

Then:

E = [(6.626 × 10⁻³⁴)(3.00 × 10⁸)] / (5.00 × 10⁻⁷)

E ≈ 3.98 × 10⁻¹⁹ J

Answer: 300 nm

Explanation: Photon energy is:

E = hc/λ

Energy and wavelength are inversely related.

Therefore, the shorter wavelength has greater energy.

Answer: 0.300 M

Work:

0.750 = 2.50c

c = 0.750/2.50

c = 0.300 M

Answer:

Wavelength ↑ → frequency ↓ → photon energy ↓

and

Wavelength ↓ → frequency ↑ → photon energy ↑

Explanation: Light follows:

c = λν

Since c is constant, wavelength and frequency are inversely related.

Photon energy is:

E = hν

So higher frequency means higher energy.

Combining them gives:

E = hc/λ

This shows directly that shorter wavelengths have greater photon energy.