Angola 12th Grade National Examination — Chemistry (Química)

How many protons, neutrons, and electrons are present in a neutral atom of phosphorus-31 (atomic number Z = 15, mass number A = 31)?

• A15 protons, 16 neutrons, 15 electrons
• B15 protons, 31 neutrons, 15 electrons
• C16 protons, 15 neutrons, 16 electrons
• D15 protons, 16 neutrons, 16 electrons

Which of the following represents a valid set of quantum numbers (n, l, ml, ms) for an electron in a 3d subshell?

• An = 3, l = 2, ml = -1, ms = +1/2
• Bn = 3, l = 3, ml = 0, ms = -1/2
• Cn = 3, l = 1, ml = +2, ms = +1/2
• Dn = 2, l = 2, ml = -2, ms = -1/2

What is the correct ground-state electron configuration of a neutral chromium atom (Cr, Z = 24)?

• A[Ar] 4s1 3d5
• B[Ar] 4s2 3d4
• C[Ar] 3d6
• D[Ar] 4s2 4p4

Why is the first ionization energy of nitrogen (N, Z = 7) higher than that of oxygen (O, Z = 8), despite oxygen having a higher nuclear charge?

• AOxygen has a paired electron in one of its 2p orbitals, resulting in increased electron-e…
• BNitrogen has a smaller atomic radius than oxygen, holding its outer electrons more tightly
• COxygen's outer electrons experience significantly greater shielding from inner core elect…
• DNitrogen has a completely filled 2p subshell, giving it noble gas stability

Which of the following correctly arranges the isoelectronic species S2-, Cl-, K+, and Ca2+ in order of decreasing ionic radius (largest to smallest)?

• AS2- > Cl- > K+ > Ca2+
• BCa2+ > K+ > Cl- > S2-
• CK+ > Ca2+ > S2- > Cl-
• DCl- > S2- > Ca2+ > K+

An unknown Period 3 element X exhibits the following successive ionization energies: IE1 = 738 kJ/mol, IE2 = 1451 kJ/mol, IE3 = 7733 kJ/mol, IE4 = 10543 kJ/mol. In which group of the Periodic Table is element X located?

• AGroup 2 (Alkaline earth metals)
• BGroup 1 (Alkali metals)
• CGroup 13 (Boron group)
• DGroup 14 (Carbon group)

A naturally occurring element X consists of two stable isotopes: 63X (isotopic mass = 62.93 amu, abundance = 69.17%) and 65X (isotopic mass = 64.93 amu, abundance = 30.83%). What is the relative atomic mass of element X?

• A63.55 amu
• B63.93 amu
• C64.00 amu
• D64.31 amu

Why does chlorine (Cl) have a more exothermic electron affinity (-349 kJ/mol) than fluorine (F, -328 kJ/mol), contrary to the general periodic trend?

• AFluorine's 2p subshell is extremely compact, resulting in high electron-electron repulsio…
• BChlorine has a higher electronegativity than fluorine on the Pauling scale
• CFluorine has more electron shielding than chlorine
• DChlorine's incoming electron enters an empty 3d orbital rather than a 3p orbital

According to the Bohr model of the hydrogen atom, an electron transitions from the n = 4 energy level to the n = 2 energy level. Given Rydberg's constant energy factor RH = 2.18 x 10^-18 J and Planck's constant h = 6.626 x 10^-34 J·s, what is the frequency of the emitted photon?

• A6.17 x 10^14 Hz
• B4.09 x 10^14 Hz
• C2.06 x 10^14 Hz
• D8.22 x 10^14 Hz

Which of the following stable molecules contains an atom that possesses an incomplete (sub-octet) valence shell of six electrons?

• ABF3 (Boron trifluoride)
• BCH4 (Methane)
• CNH3 (Ammonia)
• DSF6 (Sulfur hexafluoride)

According to Valence Shell Electron Pair Repulsion (VSEPR) theory, what is the molecular geometry of sulfur tetrafluoride (SF4)?

• ASee-saw
• BTetrahedral
• CSquare planar
• DTrigonal pyramidal

In the propadiene (allene) molecule, H2C=C=CH2, what is the hybridization of the central carbon atom (C2) and the terminal carbon atoms (C1 and C3)?

• ACentral carbon is sp; terminal carbons are sp2
• BCentral carbon is sp2; terminal carbons are sp3
• CCentral carbon is sp3; terminal carbons are sp2
• DCentral carbon is sp2; terminal carbons are sp

Why does trans-1,2-dichloroethene have a net molecular dipole moment of zero (μ = 0 D), whereas cis-1,2-dichloroethene is a polar molecule with a non-zero dipole moment?

• AIn the trans isomer, the two polar C-Cl bond dipoles point in exactly opposite directions…
• BThe C-Cl bonds in the trans isomer are non-polar due to equal electronegativity
• CThe cis isomer contains ionic bonds while the trans isomer contains purely covalent bonds
• DThe trans isomer is non-planar and therefore lacks a permanent dipole moment

Which of the following pure substances exhibits intermolecular hydrogen bonding in the liquid state?

• AEthanol (CH3CH2OH)
• BDimethyl ether (CH3OCH3)
• CFluoroethane (CH3CH2F)
• DPropane (CH3CH2CH3)

Why is the lattice energy of magnesium oxide (MgO, -3791 kJ/mol) significantly more exothermic than that of sodium chloride (NaCl, -786 kJ/mol)?

• AMg2+ and O2- ions carry double the ionic charges (+2 and -2) and have smaller ionic radii…
• BMgO has a higher molecular weight than NaCl
• CMagnesium has a lower first ionization energy than sodium
• DOxygen is more electronegative than chlorine, creating a purely covalent lattice

What is the hybridization of the xenon atom and the molecular geometry of xenon tetrafluoride (XeF4)?

• Asp3d2 hybridization; square planar geometry
• Bsp3d hybridization; see-saw geometry
• Csp3d2 hybridization; octahedral geometry
• Dsp3 hybridization; tetrahedral geometry

Which model best explains the high electrical and thermal conductivity, malleability, and ductility of metallic copper?

• AA lattice of positive metal cations surrounded by a delocalized 'sea' of mobile valence e…
• BRigid localized directional covalent bonds between adjacent copper atoms
• CAlternating positive and negative ions held rigidly by strong electrostatic forces
• DDiscrete neutral copper molecules held together by weak London dispersion forces

Arrange the following compounds with comparable molecular weights in order of increasing normal boiling point: butane (M = 58), propan-2-one (M = 58), propan-1-ol (M = 60), and ethanoic acid (M = 60).

• AButane < Propan-2-one < Propan-1-ol < Ethanoic acid
• BEthanoic acid < Propan-1-ol < Propan-2-one < Butane
• CButane < Propan-1-ol < Propan-2-one < Ethanoic acid
• DPropan-2-one < Butane < Propan-1-ol < Ethanoic acid

How many total oxygen atoms are present in a 44.0 g sample of carbon dioxide gas (CO2, molar mass = 44.01 g/mol)? (Avogadro's constant NA = 6.022 x 10^23 mol^-1)

• A1.20 x 10^24 atoms
• B6.02 x 10^23 atoms
• C3.01 x 10^23 atoms
• D2.40 x 10^24 atoms

A pure organic compound is analyzed and found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is the empirical formula of this compound? (Atomic masses: C = 12.01, H = 1.008, O = 16.00)

• ACH2O
• BC2H4O
• CCHO2
• DC2H6O

Aluminium reacts with chlorine gas according to the equation: 2Al(s) + 3Cl2(g) -> 2AlCl3(s). If 5.40 g of Al (molar mass = 26.98 g/mol) reacts completely with 21.3 g of Cl2 (molar mass = 70.90 g/mol), what is the theoretical mass of aluminium chloride (AlCl3, molar mass = 133.34 g/mol) produced?

• A26.7 g
• B13.3 g
• C40.0 g
• D53.4 g

In the industrial synthesis of acetylsalicylic acid (aspirin, C9H8O4, M = 180.16 g/mol), 13.8 g of salicylic acid (C7H6O3, M = 138.12 g/mol) is reacted with excess acetic anhydride: C7H6O3 + C4H6O3 -> C9H8O4 + C2H4O2. If 14.4 g of pure aspirin is isolated, what is the percentage yield of the reaction?

• A80.0%
• B75.0%
• C85.5%
• D90.0%

What is the molar concentration (molarity) of a solution prepared by dissolving 10.6 g of anhydrous sodium carbonate (Na2CO3, molar mass = 106.0 g/mol) in sufficient distilled water to make exactly 250.0 mL of solution?

• A0.400 mol/L
• B0.100 mol/L
• C0.040 mol/L
• D0.800 mol/L

What volume of concentrated 6.00 M hydrochloric acid (HCl) stock solution is needed to prepare 500.0 mL of a 0.300 M HCl solution by dilution with distilled water?

• A25.0 mL
• B50.0 mL
• C10.0 mL
• D15.0 mL

Calcium carbonate decomposes thermally according to: CaCO3(s) -> CaO(s) + CO2(g). What volume of carbon dioxide gas measured at standard temperature and pressure (STP: 0°C, 1.00 atm, molar volume = 22.4 L/mol) is released by the complete decomposition of 20.0 g of CaCO3 (molar mass = 100.1 g/mol)?

• A4.48 L
• B2.24 L
• C8.96 L
• D22.4 L

A 25.00 mL aliquot of an unknown sulfuric acid solution (H2SO4) requires exactly 37.50 mL of a standardized 0.200 M sodium hydroxide solution (NaOH) to reach the stoichiometric equivalence point in a titration: H2SO4(aq) + 2NaOH(aq) -> Na2SO4(aq) + 2H2O(l). What is the molar concentration of the sulfuric acid solution?

• A0.150 M
• B0.300 M
• C0.075 M
• D0.267 M

A 500.0 g environmental water sample collected near an industrial facility in Luanda is analyzed and found to contain 2.50 mg of dissolved lead(II) ions (Pb2+). What is the concentration of Pb2+ in parts per million (ppm)?

• A5.00 ppm
• B2.50 ppm
• C0.50 ppm
• D50.0 ppm

A sealed 2.00 L vessel contains 4.40 g of an unknown pure gas at a temperature of 27.0°C (300.15 K) and a pressure of 1.23 atm. Assuming ideal gas behavior (R = 0.0821 L·atm/(mol·K)), what is the molar mass of the gas?

• A44.0 g/mol
• B28.0 g/mol
• C32.0 g/mol
• D58.0 g/mol

A student mixes 100.0 mL of 0.200 M silver nitrate (AgNO3) with 100.0 mL of 0.100 M calcium chloride (CaCl2). What is the maximum mass of silver chloride precipitate (AgCl, molar mass = 143.32 g/mol) that can form? (2AgNO3 + CaCl2 -> 2AgCl + Ca(NO3)2)

• A2.87 g
• B1.43 g
• C5.73 g
• D0.72 g

According to general aqueous solubility rules, which of the following ionic compounds is virtually insoluble in water at 25°C?

• ABarium sulfate (BaSO4)
• BSodium carbonate (Na2CO3)
• CAmmonium chloride (NH4Cl)
• DCalcium nitrate (Ca(NO3)2)

Complete quantitative combustion of a 0.220 g sample of an unknown gaseous hydrocarbon produces exactly 0.660 g of CO2 (M = 44.01 g/mol) and 0.360 g of H2O (M = 18.02 g/mol). What is the empirical formula of the hydrocarbon?

• AC3H8
• BCH2
• CC2H5
• DC4H10

Why does dissolving 1.00 mol of sodium chloride (NaCl) into 1.00 kg of pure water produce approximately twice the freezing point depression of dissolving 1.00 mol of glucose (C6H12O6) into the same mass of water?

• ANaCl is a strong electrolyte that dissociates into two moles of ions (Na+ and Cl-, van 't…
• BNaCl has a much smaller molar mass than glucose
• CGlucose reacts with water molecules while NaCl remains inert
• DNaCl increases the vapor pressure of water whereas glucose lowers it

For the gas-phase decomposition reaction 2NO2(g) -> 2NO(g) + O2(g), how is the rate of disappearance of NO2 related to the rate of appearance of O2?

• AThe rate of disappearance of NO2 is twice the rate of appearance of O2: -d[NO2]/dt = 2 *…
• BThe rate of disappearance of NO2 is equal to the rate of appearance of O2
• CThe rate of disappearance of NO2 is half the rate of appearance of O2
• DThe rate of disappearance of NO2 is four times the rate of appearance of O2

For the reaction 2A + B -> C, experimental initial rate data show that doubling [A] while holding [B] constant quadruples the initial rate, while doubling [B] while holding [A] constant doubles the initial rate. What is the rate law and the overall reaction order?

• ARate = k[A]^2[B]; overall order = 3
• BRate = k[A][B]; overall order = 2
• CRate = k[A]^2[B]^2; overall order = 4
• DRate = k[A][B]^2; overall order = 3

What are the SI units of the specific rate constant k for a chemical reaction that follows an overall second-order rate law: Rate = k[A]^2?

• AL·mol^-1·s^-1 (or M^-1·s^-1)
• Bs^-1
• Cmol·L^-1·s^-1 (or M·s^-1)
• DL^2·mol^-2·s^-1 (or M^-2·s^-1)

According to collision theory and the Maxwell-Boltzmann distribution, why does an increase in temperature cause a substantial increase in reaction rate?

• AA much larger fraction of colliding molecules possess kinetic energy equal to or greater…
• BThe activation energy of the reaction is significantly lowered by the temperature increase
• CThe concentration of reactant molecules increases due to thermal expansion
• DEvery collision becomes perfectly oriented at higher temperatures

How does the addition of a positive chemical catalyst accelerate the rate of a chemical reaction without being consumed?

• AIt provides an alternative reaction pathway with a lower activation energy, without alter…
• BIt increases the average kinetic energy of the reactant molecules
• CIt makes the overall enthalpy change of the reaction (ΔH) more negative
• DIt shifts the chemical equilibrium position to produce a higher equilibrium yield of prod…

A first-order chemical reaction has a half-life (t1/2) of 20.0 minutes. What percentage of the initial reactant concentration remains unreacted after 60.0 minutes?

• A12.5%
• B25.0%
• C6.25%
• D33.3%

A proposed reaction mechanism for the gas-phase reaction NO2(g) + CO(g) -> NO(g) + CO2(g) consists of two elementary steps: Step 1: NO2 + NO2 -> NO3 + NO (slow, rate-determining) Step 2: NO3 + CO -> NO2 + CO2 (fast) What is the experimentally expected rate law and which species acts as a reaction intermediate?

• ARate = k[NO2]^2; NO3 is the reaction intermediate
• BRate = k[NO2][CO]; CO is the reaction intermediate
• CRate = k[NO2]^2[CO]; NO is the reaction intermediate
• DRate = k[NO3][CO]; NO2 is the reaction intermediate

A chemical reaction has an activation energy of Ea = 50.0 kJ/mol. Using the Arrhenius equation ln(k2/k1) = (Ea/R) * (1/T1 - 1/T2) with R = 8.314 J/(mol·K), by what factor does the rate constant k increase when the temperature rises from 300.0 K to 310.0 K?

• AApproximately 1.91 (almost doubles)
• BApproximately 1.03
• CApproximately 3.82
• DApproximately 10.0

Which of the following industrial chemical processes represents an example of heterogeneous catalysis?

• ASynthesis of ammonia from nitrogen and hydrogen gases over solid iron catalyst: N2(g) + 3…
• BAcid-catalyzed hydrolysis of aqueous ethyl ethanoate using liquid hydrochloric acid
• CDestruction of atmospheric ozone catalyzed by gaseous chlorine radicals: Cl(g) + O3(g) ->…
• DOxidation of aqueous iodide ions by peroxodisulfate catalyzed by aqueous Fe2+ ions

What fundamental condition characterizes a closed chemical system that has reached dynamic chemical equilibrium?

• AThe rates of the forward and reverse reactions are equal, and the macroscopic concentrati…
• BThe concentrations of all reactants and all products become equal
• CAll chemical reactions have completely ceased
• DThe mass of products exceeds the mass of reactants

For the gaseous equilibrium N2(g) + 3H2(g) <=> 2NH3(g) in a 1.0 L container at 400°C, the equilibrium concentrations are found to be [N2] = 0.10 mol/L, [H2] = 0.20 mol/L, and [NH3] = 0.40 mol/L. What is the value of the equilibrium constant Kc?

• A200
• B100
• C50
• D10

For the gas-phase equilibrium 2SO2(g) + O2(g) <=> 2SO3(g), what is the correct mathematical relationship between the pressure equilibrium constant Kp and the concentration equilibrium constant Kc?

• AKp = Kc * (RT)^-1
• BKp = Kc * (RT)^1
• CKp = Kc * (RT)^-2
• DKp = Kc

For the exothermic industrial synthesis of sulfur trioxide: 2SO2(g) + O2(g) <=> 2SO3(g) (ΔH = -198 kJ/mol), which combination of changes will shift the equilibrium position to the right to maximize the equilibrium yield of SO3?

• AIncreasing total pressure and decreasing temperature
• BDecreasing total pressure and increasing temperature
• CIncreasing total pressure and adding an inert gas at constant volume
• DIncreasing temperature and removing O2 gas continuously

What is the pH of an aqueous 0.0050 M barium hydroxide solution, Ba(OH)2, at 25°C assuming complete dissociation?

• A12.00
• B11.70
• C2.00
• D12.30

Calculate the pH of a 0.100 M aqueous solution of acetic acid (CH3COOH) at 25°C, given its acid dissociation constant Ka = 1.8 x 10^-5.

• A2.87
• B1.00
• C4.74
• D3.37

A buffer solution is prepared by mixing 0.20 M acetic acid (CH3COOH) and 0.30 M sodium acetate (CH3COONa). Given the acid dissociation constant Ka(CH3COOH) = 1.8 x 10^-5 (pKa = 4.74), what is the pH of this buffer solution?

• A4.92
• B4.74
• C4.56
• D5.12