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European Baccalaureate Chemistry study materials
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European Baccalaureate Chemistry practice questions

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Question 1Atomic structure and the periodic table

Which statement correctly describes the arrangement of particles in the Rutherford–Bohr model of the atom?

  1. Electrons are embedded in a uniform sphere of positive charge.
  2. A dense nucleus containing protons and neutrons is surrounded by electrons in discrete energy levels. Correct answer.
  3. Electrons occupy a single continuous orbit at a fixed distance from the nucleus.
  4. Protons and neutrons are distributed evenly throughout the whole atomic volume.

Explanation

The Rutherford–Bohr model places nearly all the mass, as protons and neutrons, in a small central nucleus, with electrons occupying separate quantised shells around it. This explains both the large-angle alpha scattering observed by Rutherford and the line spectra described by Bohr.

Question 2Atomic structure and the periodic table

An atom of an element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In which group and period of the periodic table does this element lie?

  1. Group 16, period 4
  2. Group 16, period 3 Correct answer.
  3. Group 6, period 3
  4. Group 14, period 3

Explanation

The highest occupied shell is n = 3, so the element is in period 3. The outer configuration 3s² 3p⁴ gives six valence electrons, which places the element in group 16, the chalcogens.

Question 3Atomic structure and the periodic table

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

  1. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ Correct answer.
  2. 1s² 2s² 2p⁶ 3s² 3p⁵ 3d⁵ 4s¹
  3. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²
  4. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶

Explanation

Chromium is an exception to the aufbau order: a half-filled 3d⁵ set with a single 4s electron is more stable than 3d⁴ 4s². The total of 24 electrons is preserved in the configuration 3d⁵ 4s¹.

Question 4Atomic structure and the periodic table

Successive ionisation energies of an element are 578, 1817, 2745, 11575 and 14830 kJ mol⁻¹. Between which two ionisations is the largest jump, and what does it indicate?

  1. Between the 1st and 2nd, indicating removal of a valence electron.
  2. Between the 4th and 5th, indicating a change of subshell within the same shell.
  3. Between the 2nd and 3rd, indicating the presence of a lone pair.
  4. Between the 3rd and 4th, indicating removal from a new inner shell. Correct answer.

Explanation

The jump from 2745 to 11575 kJ mol⁻¹ is by far the largest, showing that the fourth electron must be taken from a filled inner shell much closer to the nucleus. The element therefore has three valence electrons and belongs to group 13.

Question 5Atomic structure and the periodic table

Which property increases across period 3 from sodium to chlorine?

  1. Number of occupied electron shells
  2. Atomic radius
  3. Metallic character
  4. First ionisation energy Correct answer.

Explanation

Across a period the nuclear charge rises while electrons enter the same shell, so the effective nuclear charge felt by outer electrons increases and first ionisation energy generally rises. Atomic radius and metallic character both decrease in the same direction.

Question 6Atomic structure and the periodic table

Which pair of elements shows the greatest similarity in chemical properties?

  1. Chlorine and argon
  2. Potassium and caesium Correct answer.
  3. Sodium and magnesium
  4. Carbon and nitrogen

Explanation

Potassium and caesium are both group 1 alkali metals, so they share the same valence configuration ns¹ and react in closely similar ways. Elements in the same group have analogous outer-shell arrangements and therefore similar chemistry.

Question 7Atomic structure and the periodic table

Why is the first ionisation energy of magnesium higher than that of sodium?

  1. Magnesium has a greater nuclear charge while the electron removed comes from the same 3s level. Correct answer.
  2. Magnesium has a half-filled 3p subshell that confers extra stability.
  3. Magnesium has one more occupied electron shell than sodium.
  4. Magnesium has a larger atomic radius, so its outer electron is held less tightly.

Explanation

Both elements lose a 3s electron, but magnesium has one extra proton, so the effective nuclear charge attracting that electron is larger. The greater attraction means more energy is needed to remove the electron.

Question 8Atomic structure and the periodic table

Which of the following species has the electron configuration 1s² 2s² 2p⁶?

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

Explanation

A sodium atom (Z = 11) loses one electron to give Na⁺ with 10 electrons, exactly the configuration 1s² 2s² 2p⁶. This is isoelectronic with neon.

Question 9Atomic structure and the periodic table

Which statement about the emission spectrum of hydrogen is correct?

  1. It arises from electrons being removed completely from the atom.
  2. It contains only one line, corresponding to the transition from n = 2 to n = 1.
  3. It consists of discrete lines because photons are emitted when electrons fall between quantised energy levels. Correct answer.
  4. It is continuous because the electron can lose any amount of energy.

Explanation

Bohr's model requires electrons to occupy fixed energy levels, so emission occurs only for photons whose energy matches the difference between two levels. Each allowed transition therefore produces a line of a specific wavelength.

Question 10Atomic structure and the periodic table

An electron in a hydrogen atom falls from n = 4 to n = 2. Which region of the electromagnetic spectrum contains the emitted photon?

  1. Microwave
  2. Visible Correct answer.
  3. Infrared
  4. Ultraviolet

Explanation

Transitions ending at n = 2 form the Balmer series, and the n = 4 to n = 2 line lies in the visible region. The energy gap is smaller than for Lyman transitions (which end at n = 1 and are ultraviolet) but larger than for infrared Paschen lines.

Question 11Atomic structure and the periodic table

Which is the correct condensed electron configuration of the Fe³⁺ ion (Z = 26)?

  1. [Ar] 3d³ 4s²
  2. [Ar] 3d⁵ Correct answer.
  3. [Ar] 3d⁶
  4. [Ar] 3d⁴ 4s¹

Explanation

A neutral iron atom is [Ar] 3d⁶ 4s². When transition metals ionise, the 4s electrons are removed first, so Fe³⁺ loses both 4s electrons and one 3d electron, giving [Ar] 3d⁵.

Question 12Atomic structure and the periodic table

Which element has the highest first ionisation energy?

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

Explanation

Neon has a small radius and a full 2p subshell with a high effective nuclear charge, so its first ionisation energy is the highest of these four. The values fall in the order Ne > Ar > Na > K.

Question 13Atomic structure and the periodic table

Which explanation best accounts for the decrease in first ionisation energy down group 2?

  1. The atoms become more electronegative down the group.
  2. The number of valence electrons decreases down the group.
  3. The outer electron occupies a shell further from the nucleus and is more shielded. Correct answer.
  4. The nuclear charge decreases down the group.

Explanation

Down a group each element has its outer electron in a higher principal shell, farther from the nucleus and shielded by more inner electrons. Both effects weaken the attraction, so less energy is needed to remove the electron.

Question 14Atomic structure and the periodic table

Which statement about the shapes of atomic orbitals is correct?

  1. An s orbital has two lobes, while a p orbital is spherical.
  2. A d orbital is always spherical, and a p orbital has three lobes.
  3. Both s and p orbitals are spherical, differing only in size.
  4. An s orbital is spherical, while a p orbital has two lobes separated by a nodal plane. Correct answer.

Explanation

s orbitals are spherically symmetric about the nucleus, whereas each p orbital consists of two lobes on opposite sides of the nucleus with a nodal plane passing through it. The three p orbitals are oriented along mutually perpendicular axes.

Question 15Atomic structure and the periodic table

How many orbitals in total are available in the n = 3 shell, and what is its maximum electron capacity?

  1. 9 orbitals, holding 18 electrons Correct answer.
  2. 6 orbitals, holding 12 electrons
  3. 5 orbitals, holding 10 electrons
  4. 3 orbitals, holding 6 electrons

Explanation

For n = 3 there is one 3s, three 3p and five 3d orbitals, giving 9 orbitals in total. Since each orbital holds two electrons with opposite spins, the shell accommodates 18 electrons.

Question 16Atomic structure and the periodic table

Which set of quantum numbers (n, l, mₗ, mₛ) is not permitted for an electron?

  1. (2, 1, 0, +½)
  2. (2, 2, 0, +½) Correct answer.
  3. (3, 2, −2, −½)
  4. (1, 0, 0, +½)

Explanation

The azimuthal quantum number l can take values from 0 to n − 1, so for n = 2 the only permitted values are 0 and 1. The set with n = 2 and l = 2 is therefore impossible because it would describe a 2d orbital, which does not exist.

Question 17Atomic structure and the periodic table

Which factor best explains why the second ionisation energy of an element is always greater than its first?

  1. The remaining electrons are held by the same nuclear charge now shared among fewer electrons. Correct answer.
  2. The nucleus gains a proton each time an electron is removed.
  3. Repulsion between electrons increases after the first electron is removed.
  4. The electron removed second comes from a shell of lower principal quantum number in every case.

Explanation

After one electron is removed the ion has the same nuclear charge but one fewer electron, so each remaining electron experiences a greater effective nuclear charge. More energy is therefore required to remove the next electron.

Question 18Atomic structure and the periodic table

Across period 3, the melting points rise from sodium to silicon and then fall sharply at phosphorus, sulfur, chlorine and argon. Which explanation is correct?

  1. The bonding changes from ionic through metallic to giant covalent.
  2. All the elements form simple molecular structures whose sizes decrease across the period.
  3. The atoms become heavier, so melting point must fall steadily.
  4. The bonding changes from metallic through giant covalent to simple molecular with weak intermolecular forces. Correct answer.

Explanation

Sodium, magnesium and aluminium form metals, silicon forms a giant covalent lattice with very strong bonds, and the remaining elements form simple molecules held together by weak van der Waals forces. Melting point therefore peaks at silicon and drops sharply afterwards.

Question 19Atomic structure and the periodic table

Which comparison of the ionic radii of Na⁺, Mg²⁺, F⁻ and O²⁻ is correct?

  1. F⁻ > O²⁻ > Mg²⁺ > Na⁺
  2. O²⁻ > F⁻ > Na⁺ > Mg²⁺ Correct answer.
  3. Na⁺ > Mg²⁺ > O²⁻ > F⁻
  4. Mg²⁺ > Na⁺ > F⁻ > O²⁻

Explanation

All four ions are isoelectronic with 10 electrons. O²⁻ has the smallest nuclear charge (Z = 8) and Mg²⁺ the largest (Z = 12), so the electrons are pulled in most strongly for Mg²⁺. Radius therefore decreases in the order O²⁻ > F⁻ > Na⁺ > Mg²⁺.

Question 20Atomic structure and the periodic table

An element has successive ionisation energies showing a large jump after the second electron is removed, and its oxide has the formula XO. Which conclusion is consistent with both observations?

  1. X is a group 13 metal forming the ion X³⁺.
  2. X is a group 2 metal forming the ion X²⁺. Correct answer.
  3. X is a group 1 metal forming the ion X⁺.
  4. X is a group 16 non-metal forming the ion X²⁻.

Explanation

A large jump after the second ionisation indicates only two easily removed valence electrons, characteristic of group 2. The oxide XO requires a 2+ cation to balance O²⁻, which confirms that X forms X²⁺.

Question 21Chemical bonding and structure

Which statement correctly describes the bonding in a crystal of sodium chloride?

  1. Electrostatic attraction between a lattice of cations and a lattice of anions Correct answer.
  2. Attraction between discrete NaCl molecules held by dipole forces
  3. Delocalised electrons moving freely through a lattice of sodium ions
  4. Sharing of electron pairs between adjacent sodium and chlorine atoms

Explanation

Sodium chloride is an ionic solid: electrons are transferred from sodium to chlorine, giving Na+ and Cl- ions that pack into a giant three-dimensional lattice. The lattice is held together by the electrostatic attraction between oppositely charged ions, not by shared electron pairs or discrete molecules.

Question 22Chemical bonding and structure

Which molecule contains a bond angle of approximately 104.5 degrees?

  1. BF3
  2. CO2
  3. H2O Correct answer.
  4. CH4

Explanation

Water has two bonding pairs and two lone pairs on oxygen, giving a bent shape with the lone pairs compressing the tetrahedral angle to about 104.5 degrees. The other molecules have no lone pairs on the central atom, so their angles are governed only by bonding-pair repulsion.

Question 23Chemical bonding and structure

Which type of intermolecular force is responsible for the relatively high boiling point of water compared with hydrogen sulfide?

  1. Permanent dipole-dipole forces
  2. Instantaneous dipole-induced dipole forces
  3. Hydrogen bonding Correct answer.
  4. Ionic attraction between molecules

Explanation

Oxygen is much more electronegative than sulfur, so water forms hydrogen bonds between the O-H donor and the oxygen lone pair of a neighbouring molecule. These are far stronger than the dipole-dipole and dispersion forces available to H2S, so more energy is needed to separate water molecules.

Question 24Chemical bonding and structure

Which species has a trigonal pyramidal shape?

  1. NO3-
  2. BF3
  3. NH3 Correct answer.
  4. SO3

Explanation

Ammonia has three bonding pairs and one lone pair on nitrogen, so the four electron regions are tetrahedral and the molecular shape is trigonal pyramidal. The other species have three bonding regions and no lone pair on the central atom, giving trigonal planar geometry.

Question 25Chemical bonding and structure

Which pair of elements is most likely to form a compound with predominantly covalent character?

  1. Carbon and chlorine Correct answer.
  2. Potassium and chlorine
  3. Sodium and fluorine
  4. Magnesium and oxygen

Explanation

Covalent character increases when the two bonded atoms have similar electronegativities. Carbon and chlorine differ by only about 0.6 on the Pauling scale, so the bond is polar covalent, whereas the other pairs combine a metal with a very electronegative non-metal and give ionic solids.

Question 26Chemical bonding and structure

In terms of the kinetic theory, why does a liquid have a fixed volume but no fixed shape?

  1. Particles are far apart and move rapidly in straight lines
  2. Particles are close together but can slide past one another Correct answer.
  3. Particles are held in a rigid lattice by strong electrostatic forces
  4. Particles are locked in fixed positions but can vibrate about them

Explanation

In a liquid the particles are still close enough for attractive forces to keep the volume constant, but they have enough kinetic energy to move past each other. This mobility allows a liquid to flow and adopt the shape of its container.

Question 27Chemical bonding and structure

Which statement best explains why graphite conducts electricity while diamond does not?

  1. Graphite has weaker covalent bonds, so electrons can break free more easily
  2. Graphite has delocalised electrons between its layers, whereas diamond has all four valence electrons localised in sigma bonds Correct answer.
  3. Graphite contains mobile ions, whereas diamond contains only neutral atoms
  4. Graphite has a lower melting point, so its electrons move more freely at room temperature

Explanation

Each carbon in graphite is bonded to three neighbours in a trigonal planar sheet, leaving one electron in a delocalised pi system that can move along the layers. In diamond each carbon forms four sigma bonds in a rigid tetrahedral network, so all valence electrons are localised and cannot carry current.

Question 28Chemical bonding and structure

Which molecule is polar overall despite containing polar bonds?

  1. CO2
  2. CCl4
  3. NH3 Correct answer.
  4. BF3

Explanation

Ammonia has polar N-H bonds arranged asymmetrically because of the lone pair, so the bond dipoles do not cancel and the molecule has a net dipole moment. The other molecules are highly symmetrical, so their individual bond dipoles cancel to give zero net dipole.

Question 29Chemical bonding and structure

An element of the third period forms a chloride with the formula XCl3 that is a covalent liquid at room temperature. Which element is most likely to be X?

  1. Sodium
  2. Silicon
  3. Aluminium Correct answer.
  4. Phosphorus

Explanation

Aluminium is a metal but its small size and high charge give the Al-Cl bond appreciable covalent character, and AlCl3 exists as covalent molecular dimers that form a low-melting solid or liquid. The other third-period elements either give ionic chlorides (sodium) or chlorides with different stoichiometry and behaviour.

Question 30Chemical bonding and structure

Which observation provides the strongest evidence that magnesium chloride has significant covalent character?

  1. It has a high melting point
  2. It conducts electricity when molten
  3. It is soluble in ethanol and has a relatively low melting point Correct answer.
  4. It dissolves readily in water

Explanation

Ionic compounds typically dissolve in polar solvents such as water and conduct when molten, so those observations are consistent with ionic bonding. Solubility in a less polar solvent such as ethanol, together with a melting point lower than expected for a 2+ / 1- lattice, indicates that the bonding has appreciable covalent character.

Question 31Chemical bonding and structure

Which species has a bond angle closest to 120 degrees?

  1. CH4
  2. SO2 Correct answer.
  3. H2O
  4. NH4+

Explanation

Sulfur dioxide has two bonding regions and one lone pair on sulfur, giving a trigonal planar electron geometry with an O-S-O angle of about 119 degrees, close to 120. The other species have tetrahedral electron geometries with angles near 104.5 or 109.5 degrees.

Question 32Chemical bonding and structure

Which statement about the strength of intermolecular forces in the halogens is correct?

  1. Ionic forces increase from fluorine to iodine because the atoms become larger
  2. Dispersion forces increase from fluorine to iodine because the number of electrons increases Correct answer.
  3. Dipole-dipole forces decrease from fluorine to iodine because bond polarity falls
  4. Hydrogen bonding increases from fluorine to iodine because the molecules become more polar

Explanation

All the halogens are non-polar diatomic molecules, so the only intermolecular force is the instantaneous dipole-induced dipole force. Its strength grows with the number of electrons and the polarisability of the molecule, which is why iodine is a solid and fluorine a gas at room temperature.

Question 33Chemical bonding and structure

Which property is characteristic of a substance with a giant covalent structure such as silicon dioxide?

  1. It dissolves in water to give a conducting solution
  2. It conducts electricity when molten
  3. It is insoluble in water and has a very high melting point Correct answer.
  4. It has a low melting point and soft texture

Explanation

In a giant covalent lattice every atom is joined to its neighbours by strong covalent bonds that must be broken for melting, so the melting point is very high. There are no free ions or delocalised electrons, so the solid is insoluble in water and does not conduct.

Question 34Chemical bonding and structure

Which explanation correctly accounts for the very high melting point of magnesium oxide compared with sodium chloride?

  1. MgO contains hydrogen bonds between its ions
  2. MgO has more delocalised electrons than NaCl
  3. MgO has a giant covalent structure, whereas NaCl is ionic
  4. MgO contains 2+ and 2- ions, so the electrostatic attraction is stronger than in NaCl Correct answer.

Explanation

Both oxides and chlorides here are ionic, but the charges on Mg2+ and O2- are double those on Na+ and Cl-, and the ions are also smaller. The electrostatic attraction is proportional to the product of the charges divided by the distance between the ion centres, so the MgO lattice is much stronger.

Question 35Chemical bonding and structure

Which statement about the shapes of molecules with lone pairs is correct?

  1. Lone pairs occupy more space than bonding pairs, so bond angles are reduced below the ideal value Correct answer.
  2. Lone pairs always make the molecule linear regardless of the number of bonding pairs
  3. Lone pairs have no effect on bond angles because they are not bonded to atoms
  4. Lone pairs occupy less space than bonding pairs, so bond angles are increased above the ideal value

Explanation

A lone pair is held by only one nucleus, so its electron cloud is more diffuse and repels adjacent bonding pairs more strongly than a bonding pair does. This greater repulsion closes the angle between the bonding pairs, as seen in water and ammonia.

Question 36Chemical bonding and structure

Which molecule is expected to have the largest bond angle at the central atom?

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

Explanation

Carbon dioxide has only two electron regions around carbon and no lone pairs, so the bonding pairs arrange themselves as far apart as possible, giving a linear molecule with an angle of 180 degrees. The other molecules have four electron regions and angles of about 104.5 to 109.5 degrees.

Question 37Chemical bonding and structure

A solid melts at 801 degrees Celsius, dissolves in water to give a conducting solution, and does not conduct electricity when solid. Which type of structure is most consistent with these observations?

  1. A simple molecular covalent solid
  2. A metallic lattice
  3. A giant covalent network
  4. A giant ionic lattice Correct answer.

Explanation

A high melting point, solubility in water with a conducting solution, and no conduction in the solid state are the classic signature of an ionic lattice. The ions are fixed in the solid but become mobile when the lattice is broken by melting or by dissolution.

Question 38Chemical bonding and structure

Which feature of a molecule of the type XY4 would make it non-polar even though each X-Y bond is polar?

  1. A tetrahedral arrangement in which the four bond dipoles cancel by symmetry Correct answer.
  2. A see-saw arrangement with one lone pair on X
  3. A square planar arrangement with two lone pairs on X
  4. A trigonal pyramidal arrangement with one lone pair on X

Explanation

When four identical polar bonds are arranged tetrahedrally, the dipoles point towards the corners of a regular tetrahedron and their vector sum is zero. The molecule is therefore non-polar overall, as in carbon tetrachloride or methane with polar substituents.

Question 39Chemical bonding and structure

Which comparison of the thermal stability and the bond strength of the hydrogen halides is correct?

  1. HCl is more thermally stable than HF because chlorine is larger than fluorine
  2. All four hydrogen halides have equal thermal stability because they are isoelectronic
  3. HF is the most thermally stable because the H-F bond is the strongest Correct answer.
  4. HI is the most thermally stable because the H-I bond is the strongest

Explanation

Thermal stability depends on the strength of the H-X bond, which decreases as the halogen becomes larger and the overlap with the hydrogen 1s orbital becomes poorer. The H-F bond is therefore the strongest and HF the most thermally stable, decomposing only at very high temperatures.

Question 40Chemical bonding and structure

Which statement best explains why the first ionisation energy of aluminium is lower than that of magnesium?

  1. Aluminium has a smaller atomic radius, which increases the ionisation energy
  2. Magnesium has one more proton than aluminium, so its outer electron is harder to remove
  3. The outer electron of aluminium is in a 3p orbital, which is higher in energy and better shielded than the 3s orbital of magnesium Correct answer.
  4. Aluminium has a larger nuclear charge, which attracts the outer electron more strongly

Explanation

Magnesium loses a 3s electron, while aluminium loses a 3p electron that lies further from the nucleus and is shielded by the filled 3s subshell. Despite the higher nuclear charge of aluminium, the 3p electron is easier to remove, so its first ionisation energy is lower.

Question 41States of matter and stoichiometry

A sample of argon gas occupies 250 cm³ at 1.00 × 10⁵ Pa. Assuming the temperature stays constant, what volume does it occupy at 2.50 × 10⁵ Pa?

  1. 62.5 cm³
  2. 625 cm³
  3. 100 cm³ Correct answer.
  4. 250 cm³

Explanation

Boyle's law states that at constant temperature the pressure and volume of a fixed amount of gas are inversely proportional, so P₁V₁ = P₂V₂. Rearranging gives V₂ = P₁V₁/P₂ = (1.00 × 10⁵ × 250)/(2.50 × 10⁵) = 100 cm³. The pressure rises by a factor of 2.5, so the volume must fall by the same factor.

Question 42States of matter and stoichiometry

Which statement describes the arrangement and motion of particles in a liquid?

  1. Particles are far apart and arranged in a regular, repeating pattern.
  2. Particles are closely packed in a regular lattice and vibrate about fixed positions.
  3. Particles are closely packed but irregularly arranged, and can slide past one another. Correct answer.
  4. Particles are far apart and move rapidly in straight lines, colliding occasionally.

Explanation

In a liquid the particles are still close together and in contact, but they are not held in fixed positions, so the substance keeps a definite volume while taking the shape of its container. The particles have enough energy to move past each other, which explains why liquids flow. A regular lattice with vibration about fixed sites describes a solid, not a liquid.

Question 43States of matter and stoichiometry

What amount of substance, in moles, is present in 8.8 g of carbon dioxide, CO₂? (M: C = 12.0, O = 16.0 g mol⁻¹)

  1. 0.20 mol Correct answer.
  2. 5.0 mol
  3. 0.50 mol
  4. 2.0 mol

Explanation

The molar mass of CO₂ is 12.0 + 2(16.0) = 44.0 g mol⁻¹. Dividing the mass by the molar mass gives n = 8.8/44.0 = 0.20 mol. Always compute the molar mass from the formula before dividing, since forgetting the two oxygen atoms is the commonest error here.

Question 44States of matter and stoichiometry

Which of the following is the correct empirical formula of a compound containing 40.0 % carbon, 6.7 % hydrogen and 53.3 % oxygen by mass? (M: H = 1.0, C = 12.0, O = 16.0 g mol⁻¹)

  1. C₂H₆O
  2. CH₂O Correct answer.
  3. C₂H₄O₂
  4. CHO

Explanation

Dividing each percentage by the atomic mass gives C: 40.0/12.0 = 3.33, H: 6.7/1.0 = 6.7, O: 53.3/16.0 = 3.33. Dividing by the smallest value (3.33) gives the ratio 1 : 2 : 1, so the empirical formula is CH₂O. The empirical formula must contain the simplest whole-number ratio, so any multiple of it is not acceptable.

Question 45States of matter and stoichiometry

Under the same conditions of temperature and pressure, which sample contains the greatest number of molecules?

  1. 0.5 dm³ of carbon dioxide, CO₂
  2. 1.5 dm³ of oxygen, O₂
  3. 1.0 dm³ of chlorine, Cl₂
  4. 2.0 dm³ of nitrogen, N₂ Correct answer.

Explanation

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, regardless of the identity of the gas. The number of molecules is therefore proportional only to the volume, and 2.0 dm³ is the largest volume listed. The molar mass of the gas does not affect the molecule count under these conditions.

Question 46States of matter and stoichiometry

Which change of state is correctly described as sublimation?

  1. A liquid changing to a vapour at its boiling point.
  2. A solid passing directly to the vapour without forming a liquid. Correct answer.
  3. A gas condensing directly to a solid without forming a liquid.
  4. A solid melting to a liquid at its melting point.

Explanation

Sublimation is the direct conversion of a solid into a vapour without passing through the liquid state, as happens with iodine or solid carbon dioxide at ordinary pressure. The reverse process, vapour to solid, is called deposition. Melting and boiling both involve an intermediate or final liquid phase and are therefore different transitions.

Question 47States of matter and stoichiometry

What volume of 0.100 mol dm⁻³ sodium hydroxide solution is required to neutralise exactly 25.0 cm³ of 0.200 mol dm⁻³ hydrochloric acid?

  1. 50.0 cm³ Correct answer.
  2. 100 cm³
  3. 12.5 cm³
  4. 25.0 cm³

Explanation

The reaction HCl + NaOH → NaCl + H₂O has a 1 : 1 mole ratio, so the moles of acid and base must be equal at the end point. Moles of acid = 0.200 × 0.0250 = 5.00 × 10⁻³ mol, so the same amount of base is needed. Dividing by the base concentration gives V = 5.00 × 10⁻³/0.100 = 0.0500 dm³ = 50.0 cm³.

Question 48States of matter and stoichiometry

A 2.00 g sample of an impure carbonate is treated with excess dilute acid and the carbon dioxide evolved is collected. If 0.0200 mol of CO₂ is obtained, what is the percentage by mass of carbonate in the sample, assuming one mole of carbonate gives one mole of CO₂ and M(carbonate) = 100 g mol⁻¹?

  1. 20.0 %
  2. 100 % Correct answer.
  3. 80.0 %
  4. 50.0 %

Explanation

Since each mole of carbonate releases one mole of CO₂, the sample contains 0.0200 mol of carbonate. Its mass is 0.0200 × 100 = 2.00 g, which equals the whole sample mass, so the purity is 100 %. The calculation shows the sample is entirely carbonate by this method.

Question 49States of matter and stoichiometry

A gaseous hydrocarbon contains 85.7 % carbon and 14.3 % hydrogen by mass and has a molar mass of 42 g mol⁻¹. What is its molecular formula? (M: H = 1.0, C = 12.0 g mol⁻¹)

  1. C₄H₈
  2. C₂H₄
  3. C₃H₆ Correct answer.
  4. CH₂

Explanation

Dividing by atomic masses gives C: 85.7/12.0 = 7.14 and H: 14.3/1.0 = 14.3, a ratio of 1 : 2, so the empirical formula is CH₂ with empirical mass 14 g mol⁻¹. The molecular mass of 42 g mol⁻¹ is three times this, so the molecular formula is (CH₂)₃ = C₃H₆. The molecular formula must be a whole-number multiple of the empirical formula and match the given molar mass.

Question 50States of matter and stoichiometry

Which sample contains the greatest number of atoms? (M: H = 1.0, O = 16.0, Ne = 20.2, C = 12.0 g mol⁻¹; N_A = 6.02 × 10²³ mol⁻¹)

  1. 1.0 g of oxygen gas, O₂
  2. 1.0 g of carbon, C
  3. 1.0 g of neon, Ne
  4. 1.0 g of hydrogen gas, H₂ Correct answer.

Explanation

For 1.0 g of each substance the mole amounts are H₂: 0.50 mol, Ne: 0.0495 mol, O₂: 0.031 mol and C: 0.083 mol. Hydrogen gas gives 0.50 × 2 = 1.0 mol of atoms, far more than carbon (0.083 mol), oxygen (0.062 mol) or neon (0.0495 mol). Low molar mass and a diatomic formula both raise the atom count for a fixed mass.