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Praxis Chemistry 5245 study materials
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Praxis Chemistry 5245 practice questions

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Question 1Matter and Its Structure

Which subatomic particle determines the identity of an element?

  1. The total number of nucleons in the nucleus
  2. The number of neutrons in the nucleus
  3. The number of protons in the nucleus Correct answer.
  4. The number of electrons in the outer shell

Explanation

An element is defined by its atomic number, which is the count of protons in the nucleus. Changing the proton count changes the element, while variations in neutrons or electrons produce isotopes or ions of the same element.

Question 2Matter and Its Structure

An atom of an isotope has 17 protons, 18 neutrons, and 17 electrons. What is its mass number?

  1. 18
  2. 35 Correct answer.
  3. 34
  4. 17

Explanation

Mass number equals the sum of protons and neutrons in the nucleus. Here 17 + 18 = 35. Electrons are negligible in mass and are not counted in the mass number.

Question 3Matter and Its Structure

How many electrons can the n = 3 principal shell hold at maximum?

  1. 8
  2. 18 Correct answer.
  3. 6
  4. 32

Explanation

The maximum number of electrons in a shell is 2n². For n = 3, this gives 2(9) = 18, distributed over the 3s, 3p, and 3d subshells.

Question 4Matter and Its Structure

Which type of bonding is best described as the electrostatic attraction between a lattice of cations and a sea of delocalized electrons?

  1. Covalent bonding
  2. Hydrogen bonding
  3. Ionic bonding
  4. Metallic bonding Correct answer.

Explanation

Metallic bonding arises when metal atoms release valence electrons into a shared delocalized pool that holds the positive ion cores together. Ionic and covalent bonds involve electron transfer or sharing between specific atoms.

Question 5Matter and Its Structure

Which of the following molecules has a linear molecular geometry?

  1. H2O
  2. NH3
  3. CO2 Correct answer.
  4. CH4

Explanation

CO2 has two bonding regions and no lone pairs on the central carbon, so its electron domains arrange at 180° to give a linear shape. The other molecules have lone pairs or more bonding regions that produce bent, trigonal pyramidal, or tetrahedral shapes.

Question 6Matter and Its Structure

Which intermolecular force is primarily responsible for the unusually high boiling point of water?

  1. Ion–dipole interactions
  2. London dispersion forces
  3. Hydrogen bonding Correct answer.
  4. Dipole–dipole interactions

Explanation

Water molecules form extensive hydrogen bonds through O–H···O interactions, which require substantial energy to overcome. These bonds are stronger than ordinary dipole–dipole or dispersion forces.

Question 7Matter and Its Structure

Which of the following is a homogeneous mixture?

  1. A saline solution Correct answer.
  2. A mixture of sand and iron filings
  3. A sealed container of ice and liquid water
  4. A suspension of clay in water

Explanation

A homogeneous mixture has uniform composition throughout, as in dissolved salt in water. Sand with iron filings and clay suspensions are heterogeneous because distinct phases are visible.

Question 8Matter and Its Structure

Which statement correctly distinguishes a physical change from a chemical change?

  1. A chemical change is always easily reversed.
  2. A physical change alters the identity of a substance.
  3. A chemical change produces new substances with different compositions. Correct answer.
  4. A physical change always releases energy as heat.

Explanation

A chemical change produces one or more new substances with different chemical compositions, while a physical change alters only state or form without changing identity. Many chemical changes are difficult to reverse.

Question 9Matter and Its Structure

What is the electron configuration of a neutral chlorine atom (Z = 17)?

  1. 1s² 2s² 2p⁶ 3s² 3p⁶
  2. 1s² 2s² 2p⁶ 3s² 3p⁵ Correct answer.
  3. 1s² 2s² 2p⁵ 3s² 3p⁶
  4. 1s² 2s² 2p⁶ 3s² 3p⁴

Explanation

Filling orbitals in order by energy gives 1s² 2s² 2p⁶ 3s² 3p⁵, which totals 17 electrons. Chlorine is one electron short of the noble-gas configuration of argon.

Question 10Matter and Its Structure

Which element has the largest atomic radius?

  1. Chlorine
  2. Fluorine
  3. Sodium
  4. Potassium Correct answer.

Explanation

Atomic radius increases down a group as electrons occupy higher principal shells. Potassium lies below sodium in group 1, so it has the largest radius among these elements.

Question 11Matter and Its Structure

Which of the following represents a valid set of quantum numbers for an electron in a 3d orbital?

  1. n = 3, l = 2, ml = −2 Correct answer.
  2. n = 3, l = 3, ml = 0
  3. n = 2, l = 2, ml = 0
  4. n = 3, l = 1, ml = 2

Explanation

A 3d orbital has n = 3 and l = 2, and ml may range from −2 to +2, so ml = −2 is allowed. The other options violate the rule that l < n and that |ml| ≤ l.

Question 12Matter and Its Structure

Which species has the ground-state electron configuration 1s² 2s² 2p⁶?

  1. F⁻
  2. Cl⁻
  3. Na⁺
  4. Mg²⁺ Correct answer.

Explanation

Mg²⁺ has lost two electrons from neutral magnesium (12 electrons), leaving 10 electrons with the neon configuration. Na⁺ and F⁻ also have 10 electrons, but the question asks which listed species matches after accounting for each ion's electron count.

Question 13Matter and Its Structure

According to VSEPR theory, what is the shape of the SF4 molecule?

  1. Square planar
  2. Trigonal bipyramidal
  3. Seesaw Correct answer.
  4. Tetrahedral

Explanation

Sulfur in SF4 has five electron domains (four bonds and one lone pair). The lone pair occupies an equatorial position, giving a seesaw molecular geometry derived from trigonal bipyramidal electron geometry.

Question 14Matter and Its Structure

Which bond is the most polar?

  1. H–F Correct answer.
  2. H–Cl
  3. H–Br
  4. H–I

Explanation

Bond polarity reflects the electronegativity difference between the bonded atoms. Fluorine is the most electronegative halogen, so the H–F bond has the greatest electronegativity difference and the largest dipole.

Question 15Matter and Its Structure

A sample of a compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is its empirical formula?

  1. CHO
  2. C2H6O
  3. C2H4O
  4. CH2O Correct answer.

Explanation

Dividing each percentage by its atomic mass gives moles of C, H, and O in a 1:2:1 ratio (3.33 : 6.7 : 3.33). Dividing by the smallest value gives CH2O.

Question 16Matter and Its Structure

Which statement best explains why ionic compounds conduct electricity when molten but not when solid?

  1. Covalent bonds form during melting.
  2. Ions become free to move in the liquid state. Correct answer.
  3. The lattice becomes more tightly packed on melting.
  4. Melting removes electrons from the compound.

Explanation

In the solid state, ions are locked in a rigid lattice and cannot migrate. Melting frees the ions to move, allowing them to carry current.

Question 17Matter and Its Structure

Which molecule is nonpolar despite containing polar bonds?

  1. NH3
  2. H2O
  3. CCl4 Correct answer.
  4. HCl

Explanation

CCl4 is tetrahedral and symmetric, so its four bond dipoles cancel to give a net dipole moment of zero. The other molecules have asymmetric geometries that produce a net dipole.

Question 18Matter and Its Structure

What is the oxidation state of manganese in the permanganate ion, MnO4⁻?

  1. +4
  2. +7 Correct answer.
  3. +8
  4. +2

Explanation

Each oxygen is assigned −2, giving a total of −8 for four oxygens. For the overall charge to be −1, manganese must be +7 because +7 + (−8) = −1.

Question 19Matter and Its Structure

Which of the following best describes a coordinate covalent bond?

  1. Each atom contributes one electron to the shared pair.
  2. Both shared electrons come from the same atom. Correct answer.
  3. Electrons are transferred completely from one atom to another.
  4. The bond is formed by electrostatic attraction between ions.

Explanation

In a coordinate covalent bond, one atom donates both electrons of the shared pair, as in the formation of the ammonium ion. Once formed, the bond is indistinguishable from an ordinary covalent bond.

Question 20Matter and Its Structure

Which element would be expected to have the highest first ionization energy?

  1. Calcium
  2. Argon
  3. Neon Correct answer.
  4. Potassium

Explanation

First ionization energy generally increases across a period and decreases down a group. Neon, a noble gas at the end of period 2, holds its electrons most tightly among these choices, giving it the highest first ionization energy.

Question 21Matter and Its Structure

Which set of quantum numbers correctly describes the last electron added in the ground-state configuration of iron (Z = 26)?

  1. n = 4, l = 1, ml = −1
  2. n = 3, l = 2, ml = −2 Correct answer.
  3. n = 3, l = 2, ml = +2
  4. n = 4, l = 0, ml = 0

Explanation

Iron's configuration ends in 3d⁶, so the last electron enters a 3d orbital with n = 3 and l = 2. Following Hund's rule, the sixth d electron pairs in the first orbital, corresponding to ml = −2.

Question 22Matter and Its Structure

A sample contains 0.250 mol of a compound with a molar mass of 180.0 g/mol. How many molecules are present?

  1. 4.52 × 10²³
  2. 2.71 × 10²⁵
  3. 1.51 × 10²³ Correct answer.
  4. 1.51 × 10²⁴

Explanation

Molecules equal moles times Avogadro's number: 0.250 × 6.022 × 10²³ = 1.51 × 10²³. The molar mass is not needed for this count.

Question 23Matter and Its Structure

Which pair of elements would most likely form an ionic compound with a 1:1 stoichiometry?

  1. Nitrogen and hydrogen
  2. Carbon and oxygen
  3. Sulfur and oxygen
  4. Sodium and chlorine Correct answer.

Explanation

Ionic compounds form between a metal with low ionization energy and a nonmetal with high electron affinity. Sodium (group 1) and chlorine (group 17) form NaCl in a 1:1 ratio by transferring one electron.

Question 24Matter and Its Structure

Which observation provides the strongest evidence that a sample is a compound rather than a mixture?

  1. Its components melt over a broad temperature range.
  2. It has a fixed composition by mass and a sharp melting point. Correct answer.
  3. It can be separated by filtration.
  4. It appears uniform to the eye.

Explanation

A compound has a definite composition and a sharp melting point because it is a single substance with a fixed structure. Mixtures have variable composition and melt over a range.

Question 25Matter and Its Structure

Which factor best explains why the ionic radius of Na⁺ is smaller than that of neutral Na?

  1. The nucleus shrinks when an electron is removed.
  2. Loss of the valence electron reduces electron–electron repulsion and the remaining electrons are pulled closer. Correct answer.
  3. Loss of an electron reduces the number of protons.
  4. The ion gains a full d subshell that contracts the atom.

Explanation

Removing the single valence electron leaves 10 electrons held by 11 protons, increasing the effective nuclear charge per electron and reducing electron–electron repulsion. The remaining electron cloud contracts, so Na⁺ is smaller than Na.

Question 26Matter and Its Structure

Which of the following is an example of a heterogeneous mixture?

  1. A sample of pure aluminum
  2. A mixture of oxygen and nitrogen gases
  3. A glass of orange juice with pulp Correct answer.
  4. A solution of salt in water

Explanation

A heterogeneous mixture does not have a uniform composition throughout; the pulp is visibly distinct from the liquid. Solutions and gas mixtures are homogeneous because their components are uniformly distributed.

Question 27Matter and Its Structure

Which change is a chemical change?

  1. Dissolving sugar in water
  2. Sublimation of dry ice
  3. Rusting of iron Correct answer.
  4. Melting of ice

Explanation

A chemical change produces new substances with different compositions and properties. Rusting forms iron oxides, which are chemically different from iron, whereas the other options involve only changes of state or mixing.

Question 28Matter and Its Structure

Which statement describes a compound?

  1. It can be separated into simpler substances by physical means.
  2. It has a variable composition.
  3. It contains only one type of atom.
  4. It consists of two or more elements chemically combined in a fixed ratio. Correct answer.

Explanation

A compound is a pure substance formed when two or more elements bond chemically in a definite, fixed proportion by mass. Its components can be separated only by chemical methods, not physical ones.

Question 29Matter and Its Structure

Which of the following correctly describes the ground-state electron configuration of a neutral oxygen atom (Z = 8)?

  1. 1s² 2s² 2p⁴ Correct answer.
  2. 1s² 2s² 2p⁵
  3. 1s² 2s² 2p⁶
  4. 1s² 2s² 2p³

Explanation

A neutral oxygen atom has eight electrons. Following the Aufbau principle, two fill the 1s orbital, two fill the 2s orbital, and the remaining four occupy the 2p subshell, giving 1s² 2s² 2p⁴.

Question 30Matter and Its Structure

Which of the following sets of quantum numbers (n, l, m_l, m_s) is allowed for an electron in an atom?

  1. (2, 2, 0, +1/2)
  2. (1, 0, 1, -1/2)
  3. (3, 2, -2, +1/2) Correct answer.
  4. (3, 1, -2, +1/2)

Explanation

The rules require n ≥ 1, 0 ≤ l ≤ n−1, −l ≤ m_l ≤ +l, and m_s = ±1/2. For n=3, l=2, m_l=−2, and m_s=+1/2, all conditions are satisfied.

Question 31Matter and Its Structure

Which element has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹?

  1. Potassium (K, Z = 19) Correct answer.
  2. Sodium (Na, Z = 11)
  3. Argon (Ar, Z = 18)
  4. Calcium (Ca, Z = 20)

Explanation

The configuration contains 19 electrons, which identifies the element with atomic number 19. The final electron occupies the 4s orbital, consistent with potassium.

Question 32Matter and Its Structure

Which of the following correctly orders the first ionization energies from lowest to highest?

  1. Na < Mg < Al
  2. Na < Al < Mg Correct answer.
  3. Al < Mg < Na
  4. Mg < Na < Al

Explanation

First ionization energy generally increases across a period, but aluminum is lower than magnesium because removing an electron from Al's 3p orbital is easier than removing one from Mg's filled 3s orbital. Thus the order is Na < Al < Mg.

Question 33Matter and Its Structure

Which of the following correctly describes the trend in atomic radius as one moves down a group in the periodic table?

  1. Atomic radius generally increases. Correct answer.
  2. Atomic radius generally decreases.
  3. Atomic radius remains constant.
  4. Atomic radius first increases, then decreases.

Explanation

Moving down a group adds electrons in higher principal energy levels, increasing the average distance of the outermost electrons from the nucleus. The added inner shells also shield the outer electrons, so the atomic radius generally increases.

Question 34Matter and Its Structure

Which of the following best describes an ionic bond?

  1. A bond formed by the sharing of electron pairs between two nonmetals.
  2. A bond resulting from the electrostatic attraction between oppositely charged ions. Correct answer.
  3. A bond formed by the delocalization of electrons over many atoms.
  4. A bond formed by the overlap of two half-filled orbitals of identical energy.

Explanation

An ionic bond arises when one atom transfers one or more electrons to another, creating cations and anions that attract each other electrostatically. This typically occurs between a metal and a nonmetal.

Question 35Matter and Its Structure

Which of the following molecules has a trigonal planar molecular geometry?

  1. BF₃ Correct answer.
  2. NH₃
  3. CH₄
  4. H₂O

Explanation

BF₃ has three bonding pairs and no lone pairs on the central boron atom, so the electron pairs arrange in a trigonal planar geometry. NH₃ is trigonal pyramidal, CH₄ is tetrahedral, and H₂O is bent.

Question 36Matter and Its Structure

Which of the following statements about hydrogen bonding is correct?

  1. It is a strong covalent bond within a molecule.
  2. It occurs when hydrogen is bonded to a highly electronegative atom (N, O, or F) and interacts with a lone pair on another such atom. Correct answer.
  3. It occurs between any molecule containing hydrogen and any electronegative atom.
  4. It is weaker than London dispersion forces.

Explanation

Hydrogen bonding is a strong dipole–dipole interaction that requires hydrogen to be covalently bonded to N, O, or F and to approach a lone pair on another N, O, or F atom. It is an intermolecular force, not a covalent bond.

Question 37Matter and Its Structure

Which of the following statements correctly describes the difference between sigma (σ) and pi (π) bonds?

  1. Sigma bonds are weaker than pi bonds and are always formed first.
  2. Pi bonds can rotate freely, while sigma bonds cannot.
  3. Sigma bonds are formed by end-to-end overlap of orbitals and are generally stronger than pi bonds. Correct answer.
  4. Sigma bonds result from side-to-side overlap of p orbitals, while pi bonds result from end-to-end overlap.

Explanation

Sigma bonds arise from end-to-end overlap of orbitals along the internuclear axis, allowing greater overlap and typically stronger bonding. Pi bonds arise from side-to-side overlap of parallel p orbitals, which is less effective and restricts rotation.

Question 38Matter and Its Structure

Which of the following is the correct electron configuration for a neutral chromium atom (Z = 24) in its ground state?

  1. [Ar] 4s² 3d⁴
  2. [Ar] 4s¹ 3d⁵ Correct answer.
  3. [Ar] 3d⁶
  4. [Ar] 4s² 3d⁶

Explanation

Chromium is an exception to the Aufbau principle: a half-filled 3d subshell and a half-filled 4s orbital give a lower-energy arrangement. Thus one 4s electron is promoted to 3d, giving [Ar] 4s¹ 3d⁵.

Question 39Matter and Its Structure

Which of the following is the correct Lewis structure for carbon dioxide (CO₂)?

  1. O≡C–O with a triple and a single bond
  2. O=C–O with a double and a single bond
  3. O–C–O with single bonds only
  4. O=C=O Correct answer.

Explanation

Carbon dioxide has 16 valence electrons. A valid Lewis structure places two double bonds between carbon and each oxygen, using all 16 electrons and giving each atom a complete octet without formal charges.

Question 40Matter and Its Structure

Which of the following best explains why the first ionization energy of magnesium is higher than that of aluminum?

  1. Aluminum has a larger atomic radius than magnesium.
  2. Magnesium has a higher effective nuclear charge than aluminum.
  3. Aluminum has a filled 3s subshell, making electron removal more difficult.
  4. The electron removed from aluminum comes from a 3p orbital, which is higher in energy and better shielded than the 3s orbital from which magnesium's electron is removed. Correct answer.

Explanation

In magnesium, the first electron removed is from the filled 3s orbital, which is relatively low in energy and closer to the nucleus. In aluminum, the electron is removed from the 3p orbital, which is higher in energy and more shielded by the 3s electrons, so less energy is required.

Question 41Chemical Reactions and Energetics

Which statement best describes the first law of thermodynamics as applied to a chemical reaction?

  1. The total energy of an isolated system remains constant. Correct answer.
  2. Heat always flows from a colder body to a warmer body.
  3. Energy can be created when bonds form during a reaction.
  4. The entropy of the universe always decreases in a spontaneous reaction.

Explanation

The first law states that energy is conserved: it can be transferred or converted but not created or destroyed, so the total energy of an isolated system is constant. Bond formation releases energy that was stored, it does not create new energy. Heat flows spontaneously from hot to cold, and entropy of the universe increases, not decreases.

Question 42Chemical Reactions and Energetics

In the reaction 2 H2(g) + O2(g) -> 2 H2O(l), which species is oxidized?

  1. O2
  2. O2 and H2O equally
  3. H2 Correct answer.
  4. H2O

Explanation

Oxidation is a loss of electrons (an increase in oxidation number). Hydrogen goes from oxidation state 0 in H2 to +1 in H2O, so H2 is oxidized. Oxygen goes from 0 in O2 to -2 in H2O, so O2 is reduced.

Question 43Chemical Reactions and Energetics

Which of the following is a strong electrolyte when dissolved in water?

  1. Ethanol
  2. Glucose
  3. Sodium chloride Correct answer.
  4. Acetic acid

Explanation

Strong electrolytes dissociate essentially completely into ions in water. Sodium chloride is an ionic compound that fully dissociates into Na+ and Cl-, making it a strong electrolyte. Acetic acid is a weak acid, while glucose and ethanol are molecular compounds that do not ionize appreciably.

Question 44Chemical Reactions and Energetics

A reaction has a negative enthalpy change and a positive entropy change. What is true about its spontaneity?

  1. It is nonspontaneous at all temperatures.
  2. It is spontaneous only at high temperature.
  3. It is spontaneous only at low temperature.
  4. It is spontaneous at all temperatures. Correct answer.

Explanation

Spontaneity is governed by the Gibbs free energy change, dG = dH - T dS. With dH negative and dS positive, both terms favor a negative dG at every temperature, so the reaction is spontaneous at all temperatures.

Question 45Chemical Reactions and Energetics

What is the oxidation number of manganese in the permanganate ion, MnO4-?

  1. +4
  2. +2
  3. -1
  4. +7 Correct answer.

Explanation

Each oxygen is assigned -2, giving a total of -8 from the four oxygens. Because the ion carries an overall charge of -1, manganese must balance to +7: (+7) + (-8) = -1.

Question 46Chemical Reactions and Energetics

For the equilibrium N2(g) + 3 H2(g) <-> 2 NH3(g), which change shifts the equilibrium toward the products?

  1. Adding a catalyst
  2. Increasing the pressure Correct answer.
  3. Increasing the volume of the container
  4. Removing some N2

Explanation

Le Chatelier's principle predicts that increasing pressure favors the side with fewer moles of gas. The reaction has 4 moles of gas on the left and 2 on the right, so increased pressure shifts the equilibrium toward NH3.

Question 47Chemical Reactions and Energetics

Which quantity is defined as the heat required to raise the temperature of one gram of a substance by one degree Celsius?

  1. Specific heat capacity Correct answer.
  2. Heat of fusion
  3. Molar enthalpy
  4. Activation energy

Explanation

Specific heat capacity is the heat needed per gram per degree of temperature change. Molar enthalpy refers to heat per mole for a process, heat of fusion is the energy to melt one mole or gram, and activation energy is the barrier to reaction.

Question 48Chemical Reactions and Energetics

Which of the following reactions is a precipitation reaction?

  1. HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
  2. Zn(s) + 2 HCl(aq) -> ZnCl2(aq) + H2(g)
  3. AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq) Correct answer.
  4. 2 H2O2(aq) -> 2 H2O(l) + O2(g)

Explanation

A precipitation reaction forms an insoluble solid from ions in solution. Silver chloride is insoluble and forms as a solid when AgNO3 and NaCl solutions are combined. The other reactions are neutralization, single replacement, and decomposition.

Question 49Chemical Reactions and Energetics

Given the reaction 2 H2(g) + O2(g) -> 2 H2O(l), if 4.0 moles of H2 react completely with excess O2, how many moles of water are produced?

  1. 1.0 mol
  2. 8.0 mol
  3. 4.0 mol Correct answer.
  4. 2.0 mol

Explanation

The balanced equation shows a 2:2 ratio between H2 and H2O. Thus 4.0 moles of H2 produce 4.0 moles of water.

Question 50Chemical Reactions and Energetics

Which factor increases the rate of a chemical reaction between a solid and a solution?

  1. Decreasing the temperature of the mixture
  2. Adding more solvent to dilute the solution
  3. Increasing the surface area of the solid Correct answer.
  4. Increasing the activation energy

Explanation

Reaction rate depends on the frequency of effective collisions. Increasing the surface area of the solid exposes more particles to the solution, raising the collision frequency and thus the rate.