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Quiz Session - Attempts With Random Questions

College Chemistry Exam Questions

Page 3 of 25

41.

What is the fundamental difference between a molecular formula and an empirical formula?

  • A molecular formula represents the actual number of atoms in a molecule, while an empirical formula represents the most simplified ratio of atoms in the molecule

  • A molecular formula represents only covalent compounds, while an empirical formula can represent ionic or covalent compounds

  • A molecular formula represents any chemical compound, while an empirical formula represents only a compound that is demonstrably reactive in a lab

  • A molecular formula is used to discuss the general properties of a compound, while an empirical formula is used to discuss specific reactivities

  • There is no difference between a molecular formula and an empirical formula (the terms are interchangeable)

Correct answer: A molecular formula represents the actual number of atoms in a molecule, while an empirical formula represents the most simplified ratio of atoms in the molecule

For example, the molecular formula of glucose is C6H12O6 while the empirical formula is CH2O. The latter version is called the "empirical formula" because it is easier in the lab to determine empirically the ratio of atoms in a compound. It is more difficult to arrive at the molecular formula.

42.

All of the following are considered strong acids, except:

  • HF

  • HCl

  • HBr

  • HI

  • HCIO4

Correct answer: HF

It is important to memorize the strong acids: HCl, HBr, HI, HCIO4, and HNO3. Notably, hydrofluoric acid is the only halogen-containing acid that is not a strong acid.

43.

On a periodic table, the atomic mass of boron is listed as 10.8 amu (atomic mass units). What fraction of boron atoms have a mass of 10.8 amu?

  • None of them

  • All of them

  • 4/5

  • The vast majority

  • Can't tell with the information given

Correct answer: None of them

The atomic mass listed on the periodic table is a weighted average of the masses of the isotopes found in nature. Boron naturally occurs as approximately 20% Boron-10 and 80% Boron-11, so on average boron has a mass of 10.8. However, no boron atom actually has a mass of 10.8! Similarly, carbon's mass is listed as 12.01 on most periodic tables because 99% of carbon occurs as Carbon-12, but 1% occurs as Carbon-13 (and a tiny proportion occurs as Carbon-14).

44.

In which of the following scenarios is it most likely that the entropy of a system increases?

  • One large molecule splits into three smaller ones

  • Two small molecules join to create a larger one

  • The system absorbs heat

  • A gaseous element transitions to a liquid

  • A liquid element transitions to a solid

Correct answer: One large molecule splits into three smaller ones

In general, more molecules in a system translate into greater entropy because these molecules can all vibrate, rotate, and move independently of each other. Similarly, gas tends to have the greatest entropy, followed by liquid and then solid.

45.

Fe2O3 (s)  + CO (g) → Fe (s) + CO2 (g)

How much iron (Fe) is produced from 20g iron oxide (Fe2O3), assuming that carbon monoxide (CO) is in excess?

Molar masses: Fe=55.86, O=16.00, C=12.01

  • 13.99g

  • 6.99g

  • 10g

  • 8.74g

  • 11.82g

Correct answer: 13.99g

First, balance the equation to determine the stoichiometric ratios: Fe2O3 (s)  + 3CO (g) → 2Fe (s) + 3CO2 (g)

Then, use dimensional analysis:

(20g Fe2O3) * (1mol Fe2O3 / 159.72g Fe2O3) * (2mol Fe / 1mol Fe2O3) * (55.86g Fe / 1mol Fe) = 13.99g Fe

46.

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). When glutamic acid is in your blood (pH = 7.4), what are the states of the amino and carboxylic acid groups?

  • NH3+ and COO-

  • NH3+ and COOH

  • NH2 and COO-

  • NH2 and COOH

  • Both NH3+ and NH2, and COO-

Correct answer: NH3+ and COO-

The amino group has pKa > pH, so it is protonated. The carboxylic acid group has pKa < pH, so it is deprotonated. This leads to the unusual state called a "zwitterion": a molecule that has both a positive and negative charge.

47.

On a phase diagram, what phase(s) do you expect to be present along the x-axis (temperature)?

  • Gas

  • Liquid

  • Solid

  • Solid and liquid

  • Liquid and gas

Correct answer: Gas

Along the temperature axis, there is almost zero pressure. At very low pressures, there is not enough atmospheric force to keep solids or liquids together, and all substances become gaseous. This is true even at very low temperatures, so the gas phase occupies the entirety of the x-axis.

48.

What are the products of this reaction?

 NaOH +  HCl   ⇔  ?

Note the following Ka values: 2.5x10-20 for NaOH, 6.8x10-7 for HCl 

  • H2O + NaCl

  • H3O+ + NaOH + Cl-

  • H3O+ + NaCl

  • OH- + H3O+ + NaCl

  • OH- + H3O+ + Na+ + Cl-

Correct answer: H2O + NaCl

Based on the Ka values, we know that NaOH is a stronger base than H2O, and HCl is a stronger acid than H2O. Therefore, NaOH will lose the hydroxide group and HCl will lose its proton, forming water and a neutral salt.

49.

Mg2+ has 12 protons and how many electrons?

  • 10

  • 12

  • 14

  • 8

  • 16

Correct answer: 10

All ions are written with their net charge as a superscript. If magnesium has 12 protons and a net charge of 2+, then it must have 12-2=10 electrons. Magnesium tends to form the Mg2+ ion because when it loses two electrons, it is left with a full valence shell.

50.

You have a complex system with solids, liquids, solutes, and gases in some kind of equilibrium. If you write an equilibrium expression, which of the following should be included?

i. Solids

ii. Liquids

iii. Solutes (concentration)

iv. Gases

  • iii. and iv.

  • iii. only

  • ii. and iii.

  • ii., iii., and iv.

  • i., iii., and iv.

Correct answer: iii. and iv.

By definition, pure solids and pure liquids are excluded from equilibrium expressions. This is why water is never included as a reactant or product when working with a reaction in aqueous solution.

51.

What is the fundamental Gas Law?

  • PV=nRT

  • PR=nVT

  • PT=nRV

  • VT = nRP

  • VR = nPT

Correct answer: PV=nRT

This is a combination of relationships derived in Boyle’s Law, Charles’s Law, and Graham’s Law. It is essential to memorize this equation and know how to work with it.

52.

When pH = pKa, how much of the acid is deprotonated?

  • 50%

  • 33.3%

  • 66.7%

  • 99.9%

  • 100%

Correct answer: 50%

pH = pKa at the equivalence point, or the point at which half of the acid is deprotonated and half is protonated. This makes sense if you consider the equation pH = pKa + log([A-]/[HA]): if pH = pKa, then log([A-]/[HA]) must equal zero. log(1) = 0, so [A-]/[HA] must equal 1 and there must be equal concentrations of A- and HA.

53.

You have analyzed a mystery compound in the lab and determined that the empirical formula is CH. (Atomic masses are C=12.01 amu, H=1.01 amu.) You find out that the compound's actual formula mass is 78.11 amu.

What is the compound's molecular formula?

  • C6H6

  • C2H2

  • CH

  • C5H18

  • C5.5H12

Correct answer: C6H6

Since the empirical formula is CH, you know that the molecular formula must preserve the 1:1 ratio of C:H. To determine the actual number of C and H, divide the actual formula weight by the empirical formula weight: 78.11/(12.01+1.01) = 6. Therefore, there must be six carbon atoms and six hydrogen atoms in the molecule.

54.

What is the minimum number of pieces of information you need to calculate the partial pressure of a gas in a mixture?

  • 2: total pressure and the mole fraction of the gas in question

  • 2: total moles of gas and moles of the gas in question

  • 3: total pressure, moles of the gas in question, and total moles of gas

  • 3: total pressure, the molecular mass of the gas in question, and the mole fraction of the gas in question

  • 4: total pressure, the molecular mass of the gas in question, moles of the gas in question, and total moles of gas

Correct answer: 2: total pressure and the mole fraction of the gas in question

No information about actual numbers of moles is needed if the mole fraction is known. Multiplying the mole fraction by the total pressure yields the partial pressure of the gas.

55.

There are versions of liquid and gas chromatography that can separate molecules on the basis of:

i. Polarity

ii. Charge

iii. pH

iv. Size

  • i., ii., iii., and iv.

  • i. only

  • i. and ii.

  • ii. and iv.

  • i., ii., and iv.

Correct answer: i., ii., iii., and iv.

Chromatography is an immensely versatile technique for separating a mixture of particles. As long as there is a way to create stationary and mobile phases with different traits (polarity, charge, etc.), then it is possible to create a chromatography column for that trait.

56.

A combustion reaction is also a type of:

  • Redox reaction

  • Neutralization reaction

  • Double replacement reaction

  • Acid-base reaction

  • Hydrolysis reaction

Correct answer: Redox reaction

Regardless of the specific carbon-source involved, oxygen gas is always an oxidizing agent (so oxygen gets reduced) that oxidizes on a carbon-containing compound.

57.

How many faces do most crystal lattice structures have?

  • 6

  • 8

  • 4

  • 10

  • 5

Correct answer: 6

With the exception of the hexagonal lattice (like solid water) that has eight faces, most lattice structures are essentially variations of a cube.

58.

How does a proton’s charge compare to the charge of other subatomic particles?

  • Compared to the electron’s charge, the proton’s charge has equal magnitude and opposite sign

  • Compared to the electron’s charge, the proton’s charge has greater magnitude and opposite sign

  • Compared to the neutron’s charge, the proton’s charge has equal magnitude and equal sign

  • Compared to the neutron’s charge, the proton’s charge has lesser magnitude and opposite sign

  • Compared to the electron’s charge, the proton's charge has lesser magnitude and equal sign

Correct answer: Compared to the electron’s charge, the proton’s charge has equal magnitude and opposite sign

Protons and electrons balance each other’s charges. If carbon has six protons and six electrons (as well as six neutrons), it has a neutral charge overall.

59.

Group IIA's magnesium forms the ion Mg2+. What ion do you expect calcium to form?

  • Ca2+

  • Ca+

  • Ca2-

  • Ca-

  • Calcium doesn't readily form an ion

Correct answer: Ca2+

Elements in the same group/family tend to form the same ions because they have the same number of valence electrons. Both calcium and magnesium have two valence electrons and, therefore, tend to lose two electrons to reach a full valence shell.

60.

If a reaction's slowest step follows the equation A + B --> products, what is the molecularity of the overall reaction?

  • Bimolecular

  • Unimolecular

  • Termolecular

  • Can't say without knowing the rate law

  • Can't say without knowing all steps involved in the overall reaction

Correct answer: Bimolecular

A reaction's molecularity is equivalent to the reaction order: the number of molecules that must come together at the same time during the reaction's slowest step. Since the rate law is derived from the slowest step alone, you can determine molecularity and reaction order from the equation for the slowest step in the absence of any knowledge of the faster steps.

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