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Quiz Session - Attempts With Random Questions

College Chemistry Exam Questions

Page 10 of 25

181.

The heat of vaporization is defined as:

  • The energy required to change a liquid into a gas

  • The energy required to change a solid into a gas

  • The energy required to change a solid into a liquid

  • The energy required to increase the temperature of 1g of a gas by 1ºC

  • The energy required to increase the temperature of 1L of a gas by 1ºC

Correct answer: The energy required to change a liquid into a gas

For example, water’s heat of vaporization is 540 calories/g. This means that when liquid water reaches its boiling point (100ºC), it will absorb 540 calories/g to change from liquid water to gaseous water while remaining at 100ºC.

182.

Why do ionic solids have, on average, high melting points?

  • The strong electrostatic attractions between molecules require a lot of energy to break

  • The strong intramolecular forces require a lot of energy to disrupt

  • Ionic solids form lattice structures that are not conducive to absorbing heat

  • The charges on the ionic solids repel heat

  • They do not—ionic solids do not melt

Correct answer: The strong electrostatic attractions between molecules require a lot of energy to break

Ionic solids contain arrays of charged particles. For example, solid NaCl (table salt) is an array of Na+ and Cl- ions. The arrangement of alternatively charged particles creates a strong attraction that stabilizes the structure. It, therefore, takes a lot of energy to overcome that stability and break apart the molecule.

183.

Electron orbitals are described with shorthand notation, for example, 1s22s22p63s1 for Na (sodium). Which numbers are the quantum number, and what information does that tell us?

  • The coefficients; average distance from the nucleus

  • The coefficients; electron capacity of that subshell

  • The exponents; the number of orbitals in that subshell

  • The exponents; the number of dimensions occupied by the orbital

  • The letters; the first initial of the chemist who discovered each orbital

Correct answer: The coefficients; average distance from the nucleus

For example, the 1s2 orbital has a quantum number 1 (closest to the nucleus) and contains two electrons; the 2p6 orbital has quantum number 2 (second-closest to the nucleus) and contains six electrons. The letters s, p, d, and f are arbitrary convention.

184.

Water has capillary action while mercury has the opposite (sometimes called "capillary repulsion"). All of the following statements must be true, except:

  • Surface tension of mercury < surface tension of water

  • Water has adhesion > cohesion

  • Mercury has cohesion > adhesion

  • Water has cohesion > 0

  • Mercury has adhesion > 0

Correct answer: Surface tension of mercury < surface tension of water

Both mercury and water have adhesive forces (attraction to unlike molecules) and cohesive forces (attraction to like molecules). However, mercury is highly attracted to other mercury molecules because of intermetallic bonding, which is stronger than water's hydrogen bonding. Therefore, mercury has stronger cohesion while water has stronger adhesion. Surface tension is a consequence of cohesion, so mercury has greater surface tension than water does.

185.

What would be a reasonable threshold for a "strong acid"?

  • pKa < 0

  • Ka < 0

  • Ka > 100

  • pKa < 7

  • pKa < 14

Correct answer: pKa < 0

Since the hydronium ion is defined to have pKa = 0, it makes sense to identify anything more acidic as hydronium as a "strong acid;" in other words, an acid that will dissociate completely in aqueous solution.

186.

Glutamic acid is one of the twenty amino acids our bodies use. The molecule contains an amino group (NH3+/NH2, pKa = 9.7) and a carboxylic acid group (COOH/COO-, pKa = 2.2). In what pH range do you expect glutamic acid to have a negative net charge?

  • pH > 9.7

  • pH > 7

  • 2.2 < pH < 9.7

  • pH < 2.2

  • pH < 2.2 , pH > 9.7

Correct answer: pH > 9.7

Glutamic acid has a positive net charge when both the carboxylic acid and amino groups are protonated (pH < 2.2), a neutral net charge when the carboxylic acid is deprotonated but the amino group is protonated (2.2-9.7), and a negative net charge when both groups are deprotonated (pH > 9.7).

187.

What is the unit of kb, the Boltzmann Constant?

  • (m2*kg)/(s2*K)

  • (m*kg)/(s*K)

  • (m2*s2)/(K*kg)

  • (kg*s2)/(K*m)

  • (K*kg*s2)/m

Correct answer: (m2*kg)/(s2*K)

From the Kinetic Molecular Theory of gases, we know that kinetic energy KE = (1/2)mv2 = (3/2)kbT. Using the second set of equivalencies and plugging in units (ignore the coefficients):

(1/2)mv2 = (3/2)kbT

kg * (m2/s2) = kb K

kb = (m2*kg)/(s2*K)

188.

When is an atom's molecular geometry the same as its electron geometry?

  • When the atom has no lone pairs

  • When the atom fulfills the octet rule

  • When the atom is a metal

  • When the atom has only covalent bonds

  • When the atom is part of a non-polar molecule

Correct answer: When the atom has no lone pairs

The electron geometry model considers all electrons, both those in bonds and in lone pairs. The molecular geometry model considers only atoms. Therefore, the only way the models will match is if the central atom is surrounded only by other atoms but not by lone pairs.

189.

Most cars are relatively inefficient at using energy. What happens to the majority (~80%) of the chemical energy in gasoline when used by a car?

  • It is converted into heat energy and released into the environment

  • It is converted into the mechanical energy that moves the car

  • It is converted into heat energy that is used to power the engine

  • It is recycled into a different type of chemical energy

  • It disappears

Correct answer: It is converted into heat energy and released into the environment

Modern human societies use so much fuel because energy conversion is inherently inefficient (Second Law of Thermodynamics). The majority of energy used by cars is released as heat through exhaust and the friction of tires with the road. The minority is actually converted into the mechanical energy that moves the car. Importantly, all the energy is accounted for (according to the First Law of Thermodynamics), no energy can be created or destroyed.

190.

What are the oxidation states in the molecule Fe3O4?

  • Fe = +8/3, O = -2

  • Fe = +3, O = -2

  • Fe = +4/3, O = -1

  • Fe = +1, O = -1

  • Fe = +2, O = -3/2

Correct answer: Fe = +8/3, O = -2

Though it seems strange to have a fractionated oxidation state, this is correct because O always has a state of -2 when paired with non-oxygen atoms.

191.

What is found at each of the "points" of a crystal lattice? (A "point" is where three faces of the lattice come together, e.g., the corner of a cube.)

  • An atom's nucleus

  • An atom's innermost electron shell

  • An atom's valence electron shell

  • The triangular space between two atoms

  • The margin between the electrons of two different atoms

Correct answer: An atom's nucleus

When you see sketches of lattice structures, remember that the lattices are really composed of atoms, centered around each point of the lattice, and those atoms take up most of the space. (Most drawings of lattices look like balls-and-sticks models, with lots of space between atoms. This is not the case.)

192.

How would you write the solubility product constant for the following equilibrium?

MgF2(s)⇌Mg2+(aq)+2F-(aq)

  • Ksp=[Mg2+][F−]2

  • Ksp=[Mg2+][F−]

  • Ksp=2[Mg2+][F−]2

  • Ksp=([Mg2+][F−]2)/[MgF2]

  • Ksp=([Mg2+][F−])/[MgF2]

Correct answer: Ksp=[Mg2+][F−]2

As with all types of equilibrium constants, only solutes and gases should be included. Pure liquids (like the water that is implied as the solution) and pure solids (like magnesium difluoride) are excluded.

193.

What makes water a good solvent for many substances?

  • Polarity

  • Hydrogen bonding

  • High boiling point

  • Small mass

  • Moderate viscosity

Correct answer: Polarity

Water is a very polar molecule with increased electron density around the oxygen and reduced electron density around the hydrogen (this accounts for its ability to hydrogen-bond). Polarity gives a pseudo-charge to the two ends of the water molecule, and this is attractive to other charged or pseudo-charged molecules. For example, NaCl is an ionic compound; Na+ is attracted to the electron-rich oxygen, while Cl- is attracted to the electron-poor hydrogens. Water molecules can, therefore, pull apart the ionic compound.

194.

Which of the following is always soluble in aqueous solution?

  • Nitrates (NO3-)

  • Acetates (C2H3O2-)

  • Halides (Cl-, Br-, I-)

  • Sulfates (SO42-)

  • Hydroxides (OH-)

Correct answer: Nitrates (NO3-)

Though all five answer choices are soluble most of the time, all categories except for nitrates are insoluble when paired with a few specific cations. For example, sulfates are soluble unless paired with Pb2+, Ba2+, Ca2+ or Sr2+.

195.

The equation ΔG = -nFE relates free energy to electrode potentials. As an electrochemical cell approaches equilibrium, which values change?

  • ΔG increases and E decreases

  • ΔG decreases and E decreases

  • ΔG increases, n increases, and E decreases

  • ΔG decreases, n decreases, and E increases

  • ΔG decreases, n increases, and F increases

Correct answer: ΔG increases and E decreases

n and F are constants. E represents how far the cell is from equilibrium, so as the difference in electrode potentials decreases, E decreases and goes to zero. When the cell is out of equilibrium and E is large, ΔG is very negative. It becomes less negative (therefore increasing) as E goes to zero. When E reaches zero, so does ΔG, and the cell is in equilibrium.

196.

Which of the following molecules is/are amphoteric?

i. HCO3-

ii. HPO42-

iii. H2O

iv. SO42-

  • i., ii., and iii.

  • i. and ii.

  • iii. and iv.

  • i., ii., iii., and iv.

  • iii. only

Correct answer: i., ii., and iii.

An amphoteric molecule can act as both an acid and a base. Water is the classic example, but there are many others that can both gain or lose a proton. For example, HPO42- can gain a proton to become H2PO4- or lose a proton to become PO43-. However, SO42- is not amphoteric because it is already as reduced as possible and can only gain protons (acting as a base, not an acid).

197.

If you could have 1 mol of a compound in any state, which would you expect to occupy the most space?

  • Gas

  • Solid

  • Liquid

  • Gas and liquid equally

  • Impossible to tell without knowing the specific compound

Correct answer: Gas

In general, a compound becomes less dense (occupies more space) as a gas than as a liquid, and more as a liquid than as a solid. An interesting exception is water, which is actually more dense as a liquid than as a solid (this is why ice floats). However, gases should always be expected to occupy more space because the molecules move faster and are only transiently interacting with each other.

198.

If the enthalpy of a system increases, what happened?

  • Heat was added to the system

  • Heat was released from the system

  • The system's disorder increased

  • The system's disorder decreased

  • A spontaneous reaction occurred

Correct answer: Heat was added to the system

Enthalpy reflects the heat of a system, so an increase in enthalpy indicates that the system absorbed heat. Don't confuse this with entropy, which concerns the "disorder" of a system. Without knowing both enthalpy and entropy, you cannot determine whether a reaction is spontaneous or not.

199.

What is the definition of molality?

  • The number of moles of a substance dissolved in 1kg solvent

  • The number of moles of a substance dissolved in 1L solution

  • The number of moles contained in 1L of a liquid

  • The number of moles of a substance required to fill a 1L volume

  • The number of moles of a substance required to sum to 1kg

Correct answer: The number of moles of a substance dissolved in 1kg solvent

Note that molality is different from molarity (the number of moles of a substance dissolved in 1L solution). Though it is usually more practical to work with molarity, it is important to recognize the difference and be able to work with both.

200.

Water’s heat of fusion is 80 calories/gram. If you have 5g ice (solid water) at 0ºC and you add 240 calories of energy, what state and temperature of water will you have?

  • Liquid, 0ºC

  • Solid, 0ºC

  • Liquid, 5ºC

  • Solid, 4ºC

  • Liquid, 4ºC

Correct answer: Liquid, 0ºC

5g of ice requires 240 calories (80 cal/g * 5g = 240 cal) to melt. In other words, it requires 240 calories to transform from solid water at 0ºC to liquid water at 0ºC. All of the water that is absorbed as the heat of fusion is required to break bonds in the ice, rather than increase the temperature of the water. If you added additional energy (beyond 240 cal), your liquid water would rise above 0ºC.

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