HESI A2 Chemistry Sample Questions
- What is the atomic number of an element?
a) The number of neutrons in the nucleus
b) The number of protons in the nucleus
c) The number of electrons in a neutral atom
d) The total number of protons and neutrons
Explanation: The atomic number defines an element and represents the number of protons in the nucleus, which also equals the number of electrons in a neutral atom. - Which subatomic particle has a negative charge?
a) Proton
b) Neutron
c) Electron
d) Nucleus
Explanation: Electrons carry a negative charge, while protons are positively charged, and neutrons are neutral. - What type of bond forms between a metal and a nonmetal?
a) Covalent bond
b) Ionic bond
c) Metallic bond
d) Hydrogen bond
Explanation: Ionic bonds form between metals and nonmetals due to the transfer of electrons, creating oppositely charged ions that attract each other. - What is the chemical formula for water?
a) H₂O
b) CO₂
c) H₂SO₄
d) NH₃
Explanation: Water is composed of two hydrogen atoms and one oxygen atom, represented by the formula H₂O. - What is the pH of a neutral solution at 25°C?
a) 0
b) 7
c) 14
d) 10
Explanation: At 25°C, a neutral solution has a pH of 7, where the concentrations of H⁺ and OH⁻ ions are equal. - Which gas is diatomic in its elemental form?
a) Helium
b) Nitrogen
c) Argon
d) Neon
Explanation: Nitrogen exists as N₂ in its elemental form, making it diatomic, unlike the noble gases helium, argon, and neon. - What is the molar mass of carbon dioxide (CO₂)?
a) 32 g/mol
b) 44 g/mol
c) 28 g/mol
d) 56 g/mol
Explanation: CO₂ has one carbon (12 g/mol) and two oxygens (16 g/mol each), so 12 + (16 × 2) = 44 g/mol. - Which of the following is a strong acid?
a) Acetic acid
b) Hydrochloric acid
c) Carbonic acid
d) Citric acid
Explanation: Hydrochloric acid (HCl) completely ionizes in water, making it a strong acid, unlike the others, which are weak acids. - What type of reaction is 2H₂ + O₂ → 2H₂O?
a) Decomposition
b) Combination
c) Single replacement
d) Double replacement
Explanation: This is a combination reaction, where two reactants (H₂ and O₂) combine to form a single product (H₂O). - What is the charge of a sulfate ion?
a) +1
b) -2
c) +2
d) -1
Explanation: The sulfate ion (SO₄²⁻) has a charge of -2 due to its polyatomic structure. - Which element is in Group 1 of the periodic table?
a) Sodium
b) Calcium
c) Sulfur
d) Chlorine
Explanation: Sodium is in Group 1 (alkali metals), which are highly reactive elements with one valence electron. - What is the oxidation number of oxygen in H₂O?
a) +2
b) -2
c) 0
d) +1
Explanation: In most compounds, oxygen has an oxidation number of -2, including in H₂O, where hydrogen is +1. - What is Avogadro’s number?
a) 6.022 × 10²³ particles/mol
b) 1.602 × 10⁻¹⁹ particles/mol
c) 3.141 × 10² particles/mol
d) 8.314 particles/mol
Explanation: Avogadro’s number represents the number of particles (atoms, molecules, etc.) in one mole of a substance. - Which of the following is a noble gas?
a) Nitrogen
b) Helium
c) Oxygen
d) Fluorine
Explanation: Helium is a noble gas (Group 18), characterized by its full valence electron shell and low reactivity. - What is the product when sodium reacts with water?
a) Sodium chloride
b) Sodium hydroxide and hydrogen gas
c) Sodium oxide
d) Sodium sulfate
Explanation: Sodium reacts with water to produce sodium hydroxide (NaOH) and hydrogen gas (H₂): 2Na + 2H₂O → 2NaOH + H₂. - What is the molarity of a solution containing 0.5 moles of NaCl in 2 liters of solution?
a) 0.25 M
b) 0.5 M
c) 1.0 M
d) 2.0 M
Explanation: Molarity (M) = moles of solute / liters of solution = 0.5 mol / 2 L = 0.25 M. - Which of the following is a balanced chemical equation?
a) 2Mg + O₂ → 2MgO
b) Mg + O₂ → MgO
c) 2Mg + O₂ → MgO
d) Mg + 2O₂ → 2MgO
Explanation: The equation 2Mg + O₂ → 2MgO is balanced, with 2 Mg and 2 O atoms on both sides. - What is the primary source of energy for Earth’s climate system?
a) Geothermal energy
b) Solar radiation
c) Nuclear energy
d) Tidal energy
Explanation: Solar radiation drives Earth’s climate system by providing energy for atmospheric and oceanic processes. - What type of bond involves the sharing of electrons?
a) Ionic bond
b) Covalent bond
c) Metallic bond
d) Van der Waals bond
Explanation: Covalent bonds involve the sharing of electrons between nonmetal atoms to achieve stability. - What is the percent composition of oxygen in H₂O?
a) 11.1%
b) 88.9%
c) 50%
d) 75%
Explanation: Molar mass of H₂O = 18 g/mol (2 H at 1 g/mol each + 1 O at 16 g/mol). Oxygen’s contribution = (16 / 18) × 100 = 88.9%. - Which of the following is an endothermic process?
a) Combustion
b) Melting ice
c) Condensation
d) Freezing water
Explanation: Melting ice requires energy input to break intermolecular forces, making it endothermic. - What is the name of the compound Fe₂O₃?
a) Iron(II) oxide
b) Iron(III) oxide
c) Iron(I) oxide
d) Iron(IV) oxide
Explanation: Iron in Fe₂O₃ has a +3 oxidation state (since O is -2, total charge of -6 balanced by two Fe³⁺ ions). - What gas, discovered on the sun before Earth, is the second most abundant element in the universe?
a) Hydrogen
b) Helium
c) Oxygen
d) Nitrogen
Explanation: Helium was first detected in the sun’s spectrum and is the second most abundant element after hydrogen. - What is the volume of 1 mole of an ideal gas at STP (0°C, 1 atm)?
a) 22.4 L
b) 11.2 L
c) 44.8 L
d) 1.0 L
Explanation: At standard temperature and pressure (STP), 1 mole of an ideal gas occupies 22.4 liters. - Which of the following is a characteristic of a base?
a) Sour taste
b) Slippery feel
c) Produces H⁺ ions in solution
d) Turns litmus paper red
Explanation: Bases have a slippery feel, produce OH⁻ ions, and turn litmus paper blue, unlike acids.
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