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Quiz Session - Attempts With Random Questions

College Chemistry Exam Questions

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1.

You have a radioactive substance that decays according to a first-order reaction, [A] = [A]0 – kt, where [A]0 is the substance's initial concentration and k is the rate constant. How would you calculate the substance's half-life? Use t1/2 to indicate the half-life or the time at which half of the substance has decayed.

  • t1/2 = [A]0/(2k)

  • t1/2 = [A]0/k

  • t1/2 = (2[A]0)/k

  • t1/2 = [A]/(2k)

  • t1/2 = [A]2/k

Correct answer: t1/2 = [A]0/(2k)

The rate law can be solved by substituting ½[A]0 for [A], since by definition there is half of the initial concentration remaining at time t1/2:

[A] = [A]0 – kt

½[A]0 = [A]o – kt1/2

kt1/2 = ½[A]0

t1/2 = [A]0/(2k)

2.

Which of the following is an acid according to the Lewis definition but not according to the Arrhenius definition?

  • BF3

  • H3O+

  • NH4+

  • H2O

  • HCl

Correct answer: BF3

A Lewis acid is defined by its ability to accept an electron pair, rather than its ability to donate a proton in solution. Boron has an empty orbital and can, therefore, accept ammonia's lone pair, making it a Lewis acid.

3.

What is the basic concept underlying all types of chromatography?

  • Separating the components of a mixture into stationary and mobile phases

  • Separating species based on mass:charge ratios

  • Separating liquid species based on different boiling points

  • Determining molecular structure based on proton position

  • Determining pH with an indicator

Correct answer: Separating the components of a mixture into stationary and mobile phases

Both gas and liquid chromatography rely on the concept that different species will move through the same medium at different speeds. There are many options for the stationary and mobile phases, which must be selected with consideration of how they will interact with components in the mixture you are trying to separate.

4.

In the ionic compound magnesium oxide (MgO), which atoms have the noble gas electron configuration?

  • Both Mg and O

  • Mg only

  • O only

  • Neither Mg nor O

  • Can't say without knowing their respective electronegativities

Correct answer: Both Mg and O

Ionic compounds generally form to redistribute valence electrons in a way that leaves all atoms with the noble gas configuration (a full valence shell). In this case, magnesium loses two electrons to oxygen, leaving both with the configuration 1s22s22p6.

5.

What allotropes of oxygen exist in nature?

  • Dioxygen and trioxygen

  • Dioxygen only

  • Monoxide and dioxygen

  • Monoxide, dioxygen, and tetraoxygen

  • Dioxygen and tetraoxygen

Correct answer: Dioxygen and trioxygen

Dioxygen (O2) is what we breathe and require to live. Trioxygen (O3) is also known as ozone and is found in the atmosphere.

6.

Fluorine has the electron configuration 1s22s22p5. What configuration would you expect to find in the fluoride ion?

  • 1s22s22p6

  • 1s22s22p5

  • 1s22s22p53s2

  • 1s22s22p63s2

  • 1s22s2

Correct answer: 1s22s22p6

Ions usually form through the loss or gain of one to three electrons in a way that leaves the ion with a filled valence shell. Since fluorine is only one electron away from having eight valence electrons, it makes the most sense to gain one electron, thereby becoming the fluoride (F- ion).

7.

What is the fundamental principle behind dimensional analysis?

  • A unit can be mathematically canceled by multiplying by the inverse of that unit

  • Any unit can be converted into any other unit with the right mathematically manipulations

  • Any unit can be decomposed into other, simpler units

  • A unit has a unique set of dimensional qualities that set it apart from all other types of units

  • There is a fundamental level of unit that cannot be analyzed any further

Correct answer: A unit can be mathematically canceled by multiplying by the inverse of that unit

Dimensional analysis is a technique to help you perform calculations while taking care of the units involved. For example, if you want to determine how many grams are in 0.214 mL benzene, and you know the density of benzene is 0.8765 g/mL, you can calculate:

(0.214 mL) * (0.8765 g/mL) = 0.1878 g

By placing mL in inverse positions, you cancel them out to get a final answer in terms of grams.

8.

What type of symmetry occurs in a crystal lattice?

  • Translational

  • Rotational

  • Point

  • Reflectional

  • Helical

Correct answer: Translational

A lattice is composed of identical structures repeating over and over again. In other words, you could pick up a piece of the lattice and move it ("translate it") so that it fits perfectly over a different piece of the lattice.

9.

In a Voltaic cell, where do electrons flow?

  • Anode to cathode

  • Cathode to anode

  • Across the salt bridge

  • Depends on the contents of the cell

  • Electrons do not flow in Voltaic cells

Correct answer: Anode to cathode

Voltaic cells contain oxidation and reduction reactions separated into half-cells, between which electrons flow. Though many different molecules can participate in the redox reactions, the cells are always constructed the same way: oxidation (loss of electrons) at the anode, reduction (gain of electrons) at the cathode.

10.

A solid is hard and does not conduct electricity well. You place it in water, the solid dissolves, and it can now conduct an electric current. What type of solid is it?

  • Ionic solid

  • Covalent solid

  • Metallic solid

  • Either a molecular or ionic solid

  • Either a covalent or metallic solid

Correct answer: Ionic solid

Ionic solids are characteristically hard and soluble in water. Unlike metallic solids, ionic solids do not readily conduct electricity because their charges are “locked” in place by the lattice structure and the arrangement of positive and negative charges. Once they are dissolved in water, however, the charged particles are free to transfer electrons through the solution.

11.

How does the atomic radius of a neutral atom compare to the radius of its cation? Its anion?

  • Neutral atom > cation; neutral atom < anion

  • Neutral atom = cation; neutral atom > anion

  • Neutral atom > cation; neutral atom > anion

  • Neutral atom = cation; neutral atom < anion

  • Neutral atom > cation; neutral atom = anion

Correct answer: Neutral atom > cation; neutral atom < anion

If an atom loses an electron to become a cation, its radius shrinks because the atom has lost its outer-most electron and has a newly-vacant outer orbital. If an atom gains an electron to become an anion, its radius grows, not only because of the space the electron occupies, but also because of the electron's added repulsive effects.

12.

Which of the following properties would you expect to change with temperature?

i. Mass

ii. Weight

iii. Density

iv. Volume

  • iii. and iv.

  • iv. only

  • ii. and iii.

  • i. and ii.

  • i., ii., iii., and iv.

Correct answer: iii. and iv.

Mass is an intrinsic property of matter that is determined by how much matter is present; it does not fluctuate with temperature. Weight accounts for the effects of gravity on mass, and since gravity does not change with temperature, neither does weight. However, the volume that a particular mass occupies is temperature-dependent. At higher temperatures, atoms vibrate/move more and therefore, occupy more space. Density = mass/volume, so if volume fluctuates, then density must as well.

13.

When does saturation occur?

  • When there is so much solute present in a solution that it precipitates rather than dissolves

  • When there is so much liquid present at high temperature that it evaporates

  • When there are so many free electrons present in solution that an electric current begins to flow

  • When there are so many free protons in solution that hydrogen gas begins to form

  • When there is so much liquid present in a porous solid that it seeps out

Correct answer: When there is so much solute present in a solution that it precipitates rather than dissolves

Solubility depends on the solute, the solvent, and other factors such as temperature. A solid will often initially dissolve when you add a little bit into a solvent, but as you continue to add more, you will eventually reach the point of saturation and begin to see precipitate form.

14.

When balancing an ionic equation, which chemical species should be included in the equation?

  • Only the species that participate in oxidation/reduction

  • Only the solutes, not the solvents

  • Only the ions, not the neutral compounds

  • All solutes and gases, not pure solids and liquids

  • All species that are present should be included

Correct answer: Only the species that participate in oxidation/reduction

Since you start the balancing process by writing half-reactions, it should be easy to figure out which species do and do not have changes in oxidation states. You may additionally add water and protons to the equations as sources of oxygen and hydrogen.

15.

How many significant figures are in 3035400?

  • Ambiguous, could be 5-7

  • Ambiguous, could be 4 or 5

  • 4

  • 5

  • 7

Correct answer: Ambiguous, could be 5-7

Non-zero numbers are always significant. Zeros are significant unless they are 1) before the decimal point, or 2) after the decimal point but before the first non-zero number. Terminal zeros, on the other hand, are ambiguous.

It would be better, for the sake of clarity, to write this number in scientific notation: 3.0354 x 10^6 (has five sig figs) or 3.03540 x 10^6 (has six sig figs) or 3.035400 x 10^6 (has seven sig figs).

16.

In organic chemistry, what range of pKa values can you expect to work with most of the time?

  • 4 - 50

  • -10 - 0

  • -10 - 10

  • 0 - 14

  • 7 - 14

Correct answer: 4 - 50

Many organic molecules have very high pKa values because they contain primarily C, H, O, and N; none of which are especially good at stabilizing negative charge. Most organic molecules, therefore, do not readily lose protons.

17.

What is the defining characteristic of a peroxide?

  • Oxygen has an oxidation state of -1

  • Oxygen forms an ionic bond with itself

  • Oxygen is bonded only to other oxygens

  • Oxygen has a free radical

  • Oxygen is bonded to hydrogen

Correct answer: Oxygen has an oxidation state of -1

Though oxygen almost always has a -2 oxidation state, this trend is disrupted in peroxides such as H2O2 and Na2O2. Peroxides do not contain free radicals themselves, but they are involved in various free radical pathways.

18.

The following statements about mass and weight are all true, except:

  • Mass and weight are essentially interchangeable

  • Mass remains constant no matter which planet you are on

  • An object will have more weight on the earth than on the moon

  • Mass is an intrinsic property, while weight is the effect of gravity on the mass

  • Because mass is not subject to the effect of gravity, it is the preferred measurement for atoms, substances, and compounds

Correct answer: Mass and weight are essentially interchangeable

Though many people use "mass" and "weight" to refer to the same property, it is important to understand the distinction: mass is an intrinsic property of an object while weight is the pull of gravity on that object. Because weight varies with gravitational pull, it is not the preferred unit of measurement in chemistry.

19.

Among the following gases, which has the slowest rate of effusion? Assume all are at the same temperature and pressure.

H2, Ne, O2, CO2

  • CO2

  • H2

  • Ne

  • O2

  • Impossible to tell with the information provided

Correct answer: CO2

Graham’s Law states that a gas’s rate of effusion is inversely proportional to the square root of its mass. In other words, larger gases move slower. The molecular mass of CO2 is the greatest, so it will effuse slower than the other gases.

20.

You ran an acid-base equilibrium reaction and now have a mixture of soluble products. What procedure would be most useful to determine the endpoint of the reaction?

  • Titration

  • Mass spec

  • Proton NMR

  • Filtration

  • Distillation

Correct answer: Titration

Titration allows you to determine the final pKa of a solution, from which you can calculate the endpoint of the reaction.

While a diagnostic tool like mass spec or NMR would tell you something about the structures of molecules present, it would not clarify the final acid-base endpoint.

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