HESI A2 CHEMISTRY
QUESTIONS WITH ANSWERS AND EXPLANATIONS
1. Which of the following substances allows for the fastest diffusion?
A. gas
B. solid
C.
... [Show More] liquid
D. plasma
Explanation: Diffusion is fastest through gases. The next fastest medium for diffusion is
liquid, followed by plasma, and then solids. In chemistry, diffusion is defined as the movement
of matter by the random motions of molecules. In a gas or a liquid, the molecules are in
perpetual motion. For instance, in a quantity of seemingly immobile air, molecules of nitrogen
and oxygen are constantly bouncing off each other. There is even some miniscule degree of
diffusion in solids, which rises in proportion to the temperature of the substance.
2. What is the oxidation number of hydrogen in CaH2?
A. +1
B. –1
C. 0
D. +2
Explanation: The oxidation number of the hydrogen in CaH2 is –1. The oxidation number is
the positive or negative charge of a monoatomic ion. In other words, the oxidation number
is the numerical charge on an ion. An ion is a charged version of an element. Oxidation
number is often referred to as oxidation state. Oxidation number is sometimes used to
describe the number of electrons that must be added or removed from an atom in order to
convert the atom to its elemental form.
3. Which of the following does not exist as a diatomic molecule?
A. boron
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B. fluorine
C. oxygen
D. nitrogen
Explanation: Boron does not exist as a diatomic molecule. The other possible answer choices,
fluorine, oxygen, and nitrogen, all exist as diatomic molecules. A diatomic molecule always
appears in nature as a pair: The word diatomic means “having two atoms.” With the exception
of astatine, all of the halogens are diatomic. Chemistry students often use the mnemonic
BrINClHOF (pronounced “brinkelhoff”) to remember all of the diatomic elements: bromine,
iodine, nitrogen, chlorine, hydrogen, oxygen, and fluorine. Note that not all of these diatomic
elements are halogens.
4. What is another name for aqueous HI?
A. hydroiodate acid
B. hydrogen monoiodide
C. hydrogen iodide
D. hydriodic acid
Explanation: Hydriodic acid is another name for aqueous HI. In an aqueous solution, the
solvent is water. Hydriodic acid is a polyatomic ion, meaning that it is composed of two or
more elements. When this solution has an increased amount of oxygen, the -ate suffix on the
first word is converted to -ic. The HESI exam will require you to know the fundamentals of
naming chemicals. This process can be quite complex, so you should carefully review this
material before your exam.
5. Which of the following could be an empirical formula?
A. C4H8
B. C2H6
C. CH
D. C3H6
Explanation: CH could be an empirical formula. An empirical formula is the smallest
expression of a chemical formula. To be empirical, a formula must be incapable of being
reduced. For this reason, answer choices A, B, and D are incorrect, as they could all be reduced
to a simpler form. Note that empirical formulas are not the same as compounds, which do not
have to be irreducible. Two compounds can have the same empirical formula but different
molecular formulas. The molecular formula is the actual number of atoms in the molecule.
6. What is the name for the reactant that is entirely consumed by the reaction?
A. limiting reactant
B. reducing agent
C. reaction intermediate
D. reagent
Explanation: A limiting reactant is entirely used up by the chemical reaction. Limiting
reactants control the extent of the reaction and determine the quantity of the product. A
reducing agent is a substance that reduces the amount of another substance by losing
electrons. A reagent is any substance used in a chemical reaction. Some of the most common
reagents in the laboratory are sodium hydroxide and hydrochloric acid. The behavior and
properties of these substances are known, so they can be effectively used to produce
predictable reactions in an experiment.
7. What is the name for the horizontal rows of the periodic table?
A. groups
B. periods
C. families
D. sets
Explanation: The horizontal rows of the periodic table are called periods. The vertical
columns of the periodic table are known as groups or families. All of the elements in a group
have similar properties. The relationships between the elements in each period are similar as
you move from left to right. The periodic table was developed by Dmitri Mendeleev to
organize the known elements according to their similarities. New elements can be added to
the periodic table without necessitating a redesign.
8. What is the mass (in grams) of 7.35 mol water?
A. 10.7 g
B. 18 g
C. 132 g
D. 180.6 g
Explanation: The mass of 7.35 mol water is 132 grams. You should be able to find the mass of
various chemical compounds when you are given the number of mols. The information
required to perform this function is included on the periodic table. To solve this problem, find
the molecular mass of water by finding the respective weights of hydrogen and oxygen.
Remember that water contains two hydrogen molecules and one oxygen molecule. The
molecular mass of hydrogen is roughly 1, and the molecular mass of oxygen is roughly 16. A
molecule of water, then, has approximately 18 grams of mass. Multiply this by 7.35 mol, and
you will obtain the answer 132.3, which is closest to answer choice C.
9. Which of the following orbitals is the last to fill?
A. 1s
B. 3s
C. 4p
D. 6s
Explanation: Of these orbitals, the last to fill is 6s. Orbitals fill in the following order: 1s, 2s,
2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and 7p. The number is the orbital
number, and the letter is the sublevel identification. Sublevel s has one orbital and can hold a
maximum of two electrons. Sublevel p has three orbitals and can hold a maximum of six
electrons. Sublevel d has five orbitals and can hold a maximum of 10 electrons. Sublevel f has
seven orbitals and can hold a maximum of 14 electrons.
10. What is the name of the binary molecular compound NO5?
A. nitro pentoxide
B. ammonium pentoxide
C. nitrogen pentoxide
D. pentnitrogen oxide
Explanation: Nitrogen pentoxide is the name of the binary molecular compound NO5. The
format given in answer choice C is appropriate when dealing with two nonmetals. A prefix is
used to denote the number of atoms of each element. Note that when there are seven atoms of
a given element, the prefix hepta- is used instead of the usual septa-. Also, when the first atom
in this kind of binary molecular compound is single, it does not need to be given the prefix
mono-.
11. What is the mass (in grams) of 1.0 mol oxygen gas?
A. 12 g
B. 16 g
C. 28 g
D. 32 g
Explanation: The mass of 1.0 mol oxygen gas is 32 grams. The molar mass of oxygen can be
obtained from the periodic table. In most versions of the table, the molar mass of the element
is directly beneath the full name of the element. There is a little trick to this question. Oxygen
is a diatomic molecule, which means that it always appears in pairs. In order to determine the
mass in grams of 1.0 mol of oxygen gas, then, you must double the molar mass. The listed mass
is 16, so the correct answer to the problem is 32.
12. Which kind of radiation has no charge?
A. beta
B. alpha
C. delta
D. gamma
Explanation: Gamma radiation has no charge. This form of electromagnetic radiation can
travel a long distance and can penetrate the human body. Sunlight and radio waves are both
examples of gamma radiation. Alpha radiation has a 2+ charge. It only travels short distances
and cannot penetrate clothing or skin. Radium and uranium both emit alpha radiation. Beta
radiation has a 1– charge. It can travel several feet through the air and is capable of
penetrating the skin. This kind of radiation can be damaging to health over a long period of
exposure. There is no such thing as delta radiation.
13. What is the name of the state in which forward and reverse chemical reactions
are occurring at the same rate?
A. equilibrium
B. constancy
C. stability
D. toxicity
Explanation: When forward and reverse chemical reactions are taking place at the same
rate, a chemical reaction has achieved equilibrium. This means that the respective
concentrations of reactants and products do not change over time. In theory, a chemical
reaction will remain in equilibrium indefinitely. One of the common tasks in the chemistry lab
is to find the equilibrium constant (or set of relative concentrations that result in equilibrium)
for a given reaction. In thermal equilibrium, there is no net heat exchange between a body
and its surroundings. In dynamic equilibrium, any motion in one direction is offset by an equal
motion in the other direction.
14. What is 119°K in degrees Celsius?
A. 32°C
B. –154°C
C. 154°C
D. –32°C
Explanation: 119°K is equivalent to –154 degrees Celsius. It is likely that you will have to
perform at least one temperature conversion on the HESI exam. To convert degrees Kelvin to
degrees Celsius, simply subtract 273. To convert degrees Celsius to degrees Kelvin, simply add
273. To convert degrees Kelvin into degrees Fahrenheit, multiply by 9/5 and subtract 460. To
convert degrees Fahrenheit to degrees Kelvin, add 460 and then multiply by 5/9. To convert
degrees Celsius to degrees Fahrenheit, multiply by 9/5 and then add 32. To convert degrees
Fahrenheit to degrees Celsius, subtract 32 and then multiply by 5/9.
15. What is the SI unit of energy?
A. ohm
B. joule
C. henry
D. newton
Explanation: The joule is the SI unit of energy. Energy is the ability to do work or generate
heat. In regard to electrical energy, a joule is the amount of electrical energy required to pass
a current of one ampere through a resistance of one ohm for one second. In physical or
mechanical terms, the joule is the amount of energy required for a force of one newton to act
over a distance of one meter. The ohm is a unit of electrical resistance. The henry is a unit of
inductance. The newton is a unit of force.
16. What is the name of the device that separates gaseous ions by their mass-tocharge ratio?
A. mass spectrometer
B. interferometer
C. magnetometer
D. capacitance meter
Explanation: A mass spectrometer separates gaseous ions according to their mass-to-charge
ratio. This machine is used to distinguish the various elements in a piece of matter. An
interferometer measures the wavelength of light by comparing the interference phenomena of
two waves: an experimental wave and a reference wave. A magnetometer measures the
direction and magnitude of a magnetic field. Finally, a capacitance meter measures the
capacitance of a capacitor. Some sophisticated capacitance meters may also measure
inductance, leakage, and equivalent series resistance.
17. Which material has the smallest specific heat?
A. water
B. wood
C. aluminum
D. glass
Explanation: Of the given materials, aluminum has the smallest specific heat. The specific
heat of a substance is the amount of heat required to raise the temperature of one gram of the
substance by one degree Celsius. In some cases, specific heat is expressed as a ratio of the heat
required to raise the temperature of one gram of a substance by one degree Celsius to the heat
required to raise the temperature of one gram of water by one degree Celsius.
18. What is the name for a reaction in which electrons are transferred from one
atom to another?
A. combustion reaction
B. synthesis reaction
C. redox reaction
D. double-displacement reaction
Explanation: In a redox reaction, also known as an oxidation-reduction reaction, electrons
are transferred from one atom to another. A redox reaction changes the oxidation numbers of
the atoms. In a combustion reaction, one material combines with an oxidizer to form a
product and generate heat. In a synthesis reaction, multiple chemicals are combined to create
a more complex product. In a double-displacement reaction, two chemical compounds trade
bonds or ions and create two different compounds. Other common chemical reactions you
may need to know for the HESI exam are the acid-base reaction, analysis reaction, singledisplacement reaction, isomerization reaction, and hydrolysis reaction.
19. What are van der Waals forces?
A. the weak forces of attraction between two molecules
B. the strong forces of attraction between two molecules
C. hydrogen bonds
D. conjugal bonds
Explanation: van der Waals forces are the weak forces of attraction between two molecules.
The van der Waals force is considered to be any of the attractive or repulsive forces between
electrons that are not related to electrostatic interaction or covalent bonds. Compared to
other chemical bonds, the strength of van der Waals forces is small. However, these forces have
a great effect on a substance’s solubility and other characteristics. The HESI exam may
require you to demonstrate knowledge of all the major chemical forces.
20. Which of the following gases effuses the fastest?
A. Cl2
B. O2
C. N2
D. H2
Explanation: Of the given gases, H2 effuses the fastest. It has the smallest molecular weight,
and it is therefore capable of moving faster than the molecules represented by the other
answer choices. In chemistry, effusion is defined as the flow of a gas through a small opening.
The rate of effusion of a substance is inversely proportional to the square root of the density of
the substance. This means that the less dense a substance is, the faster it will effuse. This
agrees with the common observation that thick smoke tends to linger in the same form for a
longer period than thin smoke or steam.
21. Which of the following elements is not involved in many hydrogen bonds?
A. fluorine
B. carbon
C. oxygen
D. nitrogen
Explanation: Carbon is not involved in many hydrogen bonds. A hydrogen bond occurs when
an atom of hydrogen that has a covalent bond with an electronegative atom forms a bond
with a third atom. The original covalent bond involving hydrogen gives away protons, and the
third element receives them. One of the reasons that fluorine, oxygen, and nitrogen are
frequently part of a hydrogen bond is that they have a strong electronegativity and are
therefore able to form more durable bonds. Chlorine is another element frequently involved in
hydrogen bonds.
22. What is the mass (in grams) of 0.350 mol copper?
A. 12.5 g
B. 14.6 g
C. 18.5 g
D. 22.2 g... [Show Less]