IB Chemistry Practice Test 7

Correct: (B)

Isotopes have the same atomic number, so their electron arrangements and chemical properties are almost identical, but different neutron numbers give them different mass numbers.

Correct: (D)

Chromium has 24 electrons. The half-filled 3d subshell arrangement [Ar] 4s^1 3d^5 is the observed ground-state configuration.

Correct: (A)

Number of molecules = 0.250 × 6.02 × 10^23 = 1.51 × 10^23.

Correct: (C)

V = nRT / P = (0.400 × 8.31 × 300) / 200000 = 0.00499 m^3 = 4.99 dm^3.

Correct: (B)

The amount of acid stays constant: n = 0.200 × 0.0250 = 0.00500 mol. Concentration = 0.00500 / 0.2500 = 0.0200 mol dm^-3.

Correct: (D)

Mole ratio C : H : O = 40.0/12.0 : 6.7/1.0 : 53.3/16.0 = 3.33 : 6.7 : 3.33 = 1 : 2 : 1, giving CH2O.

Correct: (A)

There is a very large increase between the second and third ionization energies, so the atom has two electrons in its outer shell that are much easier to remove than the core electrons.

Correct: (C)

The n = 5 to n = 3 transition has the smallest energy difference of the choices. Since wavelength is inversely proportional to energy, it gives the longest wavelength.

Correct: (D)

Mass of oxygen in one mole = 3 × 16.00 = 48.00 g. Percentage = 48.00 / 100.09 × 100 = 48.0%.

Correct: (B)

Each formula unit produces three sulfate ions, so [SO4^2-] = 3 × 0.150 = 0.450 mol dm^-3.

Correct: (A)

In an ionic compound, electrons are transferred from metal atoms to non-metal atoms and the resulting oppositely charged ions attract each other throughout the lattice.

Correct: (C)

Carbon has two electron domains and no lone pairs in CO2, so the molecule is linear with a 180° bond angle.

Correct: (D)

Ethanol contains an O-H bond, so hydrogen bonds form between the hydrogen atom of one molecule and a lone pair on oxygen in another molecule.

Correct: (B)

Each carbon atom forms three covalent bonds, leaving one electron per atom delocalised within the layer. These electrons can carry charge along the layers.

Correct: (C)

Xenon has four bonding pairs and two lone pairs. The six electron domains are arranged octahedrally, and the four fluorine atoms lie in one plane, giving a square planar shape.

Correct: (A)

BF3 is symmetrical and trigonal planar, so the three equal bond dipoles point 120° apart and cancel.

Correct: (D)

Melting an ionic compound requires disrupting many strong attractions between oppositely charged ions in three dimensions.

Correct: (B)

Bond length decreases as bond order increases. A triple bond is shorter than a double bond, which is shorter than a single bond.

Correct: (C)

Metal atoms lose their outer electrons into a delocalised sea, and the attraction between this sea and the positive ions holds the metal together.

Correct: (A)

Diamond has a giant covalent network in which every carbon atom forms four single covalent bonds in a tetrahedral arrangement.

Correct: (D)

Across a period, protons are added to the nucleus but electrons enter the same main shell, so the stronger nuclear attraction pulls the electron cloud closer.

Correct: (B)

Transition metals have closely spaced 4s and 3d electrons, so they commonly form ions with variable oxidation states and often form coloured complexes.

Correct: (A)

CH3COOCH3 contains the ester linkage -COO- joining an acyl group to an alkoxy group.

Correct: (C)

A homologous series has the same functional group and general formula; each successive member contains one additional CH2 unit.

Correct: (D)

Down the group, the valence electron occupies a shell farther from the nucleus and experiences more shielding, so it is removed more easily.

Correct: (B)

CH3CH2NH2 contains an -NH2 group attached to an ethyl group, so it is the primary amine ethylamine.

Correct: (A)

Iron is [Ar] 4s^2 3d^6. The two 4s electrons are removed first when Fe^2+ forms, leaving [Ar] 3d^6.

Correct: (C)

Aluminium oxide reacts with both strong acids and strong bases, so it is amphoteric. Na2O is basic, while SO3 and CO2 are acidic oxides.

Correct: (B)

Standard enthalpy of combustion refers to complete combustion of one mole of a substance in oxygen under standard conditions.

Correct: (D)

q = mcΔT = 50.0 × 4.18 × 8.0 = 1672 J = 1.67 kJ.

Correct: (C)

ΔH = [(-394) + 2(-286)] – [-75] = -966 + 75 = -891 kJ for the equation as written.

Correct: (A)

Bonds broken require 436 + 242 = 678 kJ. Bonds formed release 2 × 431 = 862 kJ. ΔH = 678 – 862 = -184 kJ.

Correct: (D)

In an exothermic reaction, thermal energy is transferred to the surroundings, so the products have lower enthalpy than the reactants.

Correct: (B)

CaCO3(s) → CaO(s) + CO2(g) produces a gas from a solid, greatly increasing the dispersal of particles and energy.

Correct: (C)

ΔG = ΔH – TΔS = -100 – 298(-0.100) = -100 + 29.8 = -70.2 kJ.

Correct: (A)

The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.

Correct: (B)

Reaction rate measures how quickly concentration changes, commonly in mol dm^-3 s^-1.

Correct: (D)

At a higher temperature the distribution broadens and shifts to higher energies while the total area stays constant because the number of molecules is unchanged.

Correct: (C)

A fourfold rate increase when [A] doubles means the reaction is second order in A. No change when [B] doubles means it is zero order in B.

Correct: (A)

A catalyst provides a different reaction pathway with a lower activation energy, so a greater fraction of collisions are successful.

Correct: (D)

For a first-order reaction, k = ln 2 / t1/2 = 0.693 / 20.0 = 0.0347 s^-1.

Correct: (B)

For rate = k[A]^2, k = rate / [A]^2, giving dm^3 mol^-1 s^-1.

Correct: (C)

Product concentrations are divided by reactant concentrations, with each concentration raised to its stoichiometric coefficient.

Correct: (A)

Increasing temperature favours the endothermic reverse reaction, so the equilibrium shifts left and Kc for the exothermic forward reaction decreases.

Correct: (D)

Kc = [B] / [A] = 0.400 / 0.200 = 2.00.

Correct: (B)

Particles move faster at higher temperature, collide more often, and a much larger fraction have energy at least equal to the activation energy.

Correct: (C)

Since 2^2 = 4, a doubling of concentration causing a fourfold rate increase indicates second-order dependence on NO.

Correct: (A)

Equilibrium is dynamic: both processes continue, but at equal rates, so there is no net change in composition.

Correct: (D)

HCl ionizes completely, so [H^+(aq)] = 0.0100 mol dm^-3 and pH = -log(0.0100) = 2.00.

Correct: (C)

NH4^+ can donate one proton to form NH3, and NH3 can accept one proton to form NH4^+, so they differ by exactly one proton.

Correct: (B)

A buffer needs substantial amounts of both a weak acid and its conjugate base so it can neutralise added acid or base.

Correct: (A)

Let the oxidation state of sulfur be x. x + 4(-2) = -2, so x = +6.

Correct: (D)

Zinc atoms lose electrons and their oxidation state increases from 0 to +2, so Zn is oxidised and acts as the reducing agent.

Correct: (C)

At the cathode, Pb^2+ ions gain electrons and are reduced to lead metal.

Correct: (B)

For a weak acid, [H^+] is approximately sqrt(Ka × c) = sqrt(1.8 × 10^-5 × 0.100) = 1.34 × 10^-3, so pH = 2.87.

Correct: (A)

At equivalence, the solution contains the conjugate base of the weak acid. It accepts protons from water, producing OH^- and making the solution alkaline.

Correct: (D)

The C=C bond breaks and a bromine atom adds to each carbon atom, forming a dibromoalkane.

Correct: (C)

Magnesium has the most negative electrode potential, so Mg atoms lose electrons most readily and Mg is the strongest reducing agent listed.

Correct: (B)

pH = pKa + log([base] / [acid]) = 4.76 + log(0.200 / 0.100) = 4.76 + 0.30 = 5.06.

Correct: (A)

A nucleophile is an electron-pair donor attracted to an electron-deficient centre. It may be negatively charged or neutral with a lone pair.