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TASC Chemistry Level 4 study materials
TASC Chemistry Level 4 study resources

TASC Chemistry Level 4 practice questions

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Question 1Knowledge and Understanding of Chemistry

A magnesium ion, Mg²⁺, has the electron configuration:

  1. 1s² 2s² 2p⁴
  2. 1s² 2s² 2p⁶ Correct answer.
  3. 1s² 2s² 2p⁶ 3s²
  4. 1s² 2s² 2p⁶ 3s² 3p²

Explanation

A neutral magnesium atom contains 12 electrons, giving the configuration 1s² 2s² 2p⁶ 3s². Forming a 2+ ion removes the two outermost 3s electrons, so the ion retains ten electrons in the neon-like arrangement 1s² 2s² 2p⁶.

Question 2Knowledge and Understanding of Chemistry

How many moles of sodium hydroxide are present in 8.0 g of NaOH? (M(NaOH) = 40 g mol⁻¹)

  1. 5.0 mol
  2. 0.40 mol
  3. 0.10 mol
  4. 0.20 mol Correct answer.

Explanation

The amount in moles is found by dividing the mass by the molar mass, so n = m ÷ M = 8.0 g ÷ 40 g mol⁻¹. This gives 0.20 mol of sodium hydroxide.

Question 3Knowledge and Understanding of Chemistry

Which of the following compounds is held together by ionic bonding?

  1. Methane, CH₄
  2. Sodium chloride, NaCl Correct answer.
  3. Water, H₂O
  4. Carbon dioxide, CO₂

Explanation

Ionic bonding arises when a metal transfers electrons to a non-metal, producing oppositely charged ions held in a lattice. Sodium is a metal and chlorine is a non-metal, so NaCl is an ionic compound, while the other three are molecular covalent substances.

Question 4Knowledge and Understanding of Chemistry

In the reaction 2H₂(g) + O₂(g) → 2H₂O(g), 4.0 mol of hydrogen is mixed with 1.0 mol of oxygen. What is the maximum amount of water that can form?

  1. 1.0 mol
  2. 4.0 mol
  3. 3.0 mol
  4. 2.0 mol Correct answer.

Explanation

The equation shows that 2 mol of hydrogen reacts with 1 mol of oxygen to give 2 mol of water. The 4.0 mol of hydrogen would require 2.0 mol of oxygen, so oxygen is the limiting reagent and 1.0 mol of O₂ produces 2.0 mol of water.

Question 5Knowledge and Understanding of Chemistry

For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Which change would increase the equilibrium yield of ammonia?

  1. Removing nitrogen gas
  2. Adding a catalyst
  3. Increasing the total pressure Correct answer.
  4. Increasing the temperature

Explanation

Four moles of gas on the left form two moles of gas on the right, so raising the pressure shifts the position of equilibrium towards the side with fewer gas molecules, producing more ammonia. Temperature and concentration changes must be judged separately.

Question 6Knowledge and Understanding of Chemistry

What is the pH of a 0.010 mol L⁻¹ solution of hydrochloric acid?

  1. pH = 2 Correct answer.
  2. pH = 1
  3. pH = 0.01
  4. pH = 12

Explanation

Hydrochloric acid is a strong acid that dissociates completely, so the hydrogen ion concentration equals 0.010 mol L⁻¹, or 1 × 10⁻². The pH is the negative logarithm of that concentration, giving pH = 2.

Question 7Knowledge and Understanding of Chemistry

The compound with the condensed structural formula CH₃CH₂OH is named:

  1. ethane
  2. ethanoic acid
  3. ethanal
  4. ethanol Correct answer.

Explanation

The formula shows a two-carbon chain with an −OH group attached to one carbon, and the −OH group is the alcohol functional group. A two-carbon alcohol is named ethanol, with the suffix -ol indicating the hydroxyl group.

Question 8Knowledge and Understanding of Chemistry

What is the oxidation state of manganese in potassium permanganate, KMnO₄?

  1. +4
  2. +6
  3. +7 Correct answer.
  4. +2

Explanation

The compound is neutral, so its oxidation states must sum to zero. Potassium is +1 and each of the four oxygen atoms is −2, giving a total of +1 − 8 = −7, so manganese must be +7.

Question 9Knowledge and Understanding of Chemistry

For H₂(g) + I₂(g) ⇌ 2HI(g) at a certain temperature, an equilibrium mixture contains [H₂] = 0.20 mol L⁻¹, [I₂] = 0.20 mol L⁻¹ and [HI] = 1.6 mol L⁻¹. What is the value of Kc?

  1. 8.0
  2. 64 Correct answer.
  3. 0.25
  4. 40

Explanation

The equilibrium constant expression is Kc = [HI]² ÷ ([H₂][I₂]) because the coefficient of HI is 2. Substituting gives (1.6)² ÷ (0.20 × 0.20) = 2.56 ÷ 0.040 = 64.

Question 10Knowledge and Understanding of Chemistry

The standard electrode potentials are E°(Zn²⁺/Zn) = −0.76 V and E°(Cu²⁺/Cu) = +0.34 V. What is the standard cell potential of the galvanic cell made from these two half-cells?

  1. −1.10 V
  2. 1.10 V Correct answer.
  3. 0.76 V
  4. 0.42 V

Explanation

The cell potential is found from E°(cell) = E°(cathode) − E°(anode), with copper as the cathode and zinc as the anode. Substituting gives 0.34 V − (−0.76 V) = 1.10 V, a positive value consistent with a spontaneous reaction.

Question 11Knowledge and Understanding of Chemistry

An atom of chlorine-37 has atomic number 17. How many neutrons does one atom contain?

  1. 17
  2. 54
  3. 37
  4. 20 Correct answer.

Explanation

The mass number equals protons plus neutrons, and the atomic number gives the proton count. For chlorine-37, 37 − 17 = 20 neutrons.

Question 12Knowledge and Understanding of Chemistry

What is the correct chemical formula for calcium chloride?

  1. CaCl
  2. Ca₂Cl₂
  3. CaCl₂ Correct answer.
  4. Ca₂Cl

Explanation

Calcium forms Ca²⁺ ions and chlorine forms Cl⁻ ions. Charge balance requires two chloride ions for each calcium ion, giving CaCl₂.

Question 13Knowledge and Understanding of Chemistry

Which technique separates a dissolved salt from its solvent by removing the solvent as a gas?

  1. evaporation Correct answer.
  2. filtration
  3. chromatography
  4. distillation

Explanation

Evaporation removes solvent by turning it into a gas, leaving the dissolved salt behind as a solid. The solid can then be collected from the evaporating basin.

Question 14Knowledge and Understanding of Chemistry

What volume is occupied by 0.50 mol of an ideal gas at room temperature and pressure, where the molar volume is 24 dm³ mol⁻¹?

  1. 12 dm³ Correct answer.
  2. 48 dm³
  3. 24 dm³
  4. 0.021 dm³

Explanation

At room temperature and pressure, one mole of an ideal gas occupies about 24 dm³. Multiplying 0.50 mol by 24 dm³ mol⁻¹ gives 12 dm³.

Question 15Knowledge and Understanding of Chemistry

Which substance has the highest boiling point?

  1. H₂Se
  2. H₂O Correct answer.
  3. H₂Te
  4. H₂S

Explanation

Water molecules form strong hydrogen bonds because hydrogen is attached to highly electronegative oxygen. These hydrogen bonds require more energy to overcome than the weaker intermolecular forces in the other hydrides.

Question 16Knowledge and Understanding of Chemistry

25.0 cm³ of 0.100 mol L⁻¹ sulfuric acid exactly neutralises 20.0 cm³ of sodium hydroxide solution. What is the concentration of the sodium hydroxide solution?

  1. 0.500 mol L⁻¹
  2. 0.250 mol L⁻¹ Correct answer.
  3. 0.125 mol L⁻¹
  4. 0.100 mol L⁻¹

Explanation

Sulfuric acid reacts with sodium hydroxide in a 1:2 mole ratio: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Moles of acid = 0.0250 × 0.100 = 0.00250 mol, so NaOH moles = 0.00500 mol; dividing by 0.0200 dm³ gives 0.250 mol L⁻¹.

Question 17Knowledge and Understanding of Chemistry

Which compound is an isomer of butane, C₄H₁₀?

  1. but-1-ene
  2. 2-methylpropane Correct answer.
  3. propane
  4. 2,2-dimethylpropane

Explanation

Isomers have the same molecular formula but different structural arrangements. Butane and 2-methylpropane both have the formula C₄H₁₀.

Question 18Knowledge and Understanding of Chemistry

In the reaction Mg + 2HCl → MgCl₂ + H₂, which species is oxidised?

  1. H₂
  2. HCl
  3. MgCl₂
  4. Mg Correct answer.

Explanation

Magnesium starts with oxidation state 0 and becomes Mg²⁺ in MgCl₂, so it loses electrons. Loss of electrons is oxidation.

Question 19Knowledge and Understanding of Chemistry

For the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), the forward reaction is exothermic. Which change increases the numerical value of Kc?

  1. lowering the temperature Correct answer.
  2. increasing the pressure
  3. increasing the temperature
  4. adding a catalyst

Explanation

Kc changes only when temperature changes. The forward reaction is exothermic, so lowering the temperature shifts the equilibrium to the right and increases Kc.

Question 20Knowledge and Understanding of Chemistry

Why does graphite conduct electricity while diamond does not?

  1. Graphite has a lower density than diamond.
  2. Graphite has delocalised electrons. Correct answer.
  3. Diamond contains free protons that carry charge.
  4. Graphite contains mobile ions between its layers.

Explanation

In graphite, each carbon atom bonds to three others, leaving one delocalised electron per atom. These delocalised electrons can move along the layers and carry current, whereas diamond's four bonding electrons are localised.

Question 21Knowledge and Understanding of Chemistry

An element has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁵. To which group of the periodic table does this element belong?

  1. Group 15
  2. Group 13
  3. Group 17 Correct answer.
  4. Group 2

Explanation

The 3s and 3p subshells together contain seven electrons, so the atom has seven valence electrons in its outer shell. Elements with seven valence electrons occupy Group 17, the halogens, and this configuration corresponds to chlorine.

Question 22Knowledge and Understanding of Chemistry

Which of the following processes is exothermic?

  1. Melting ice at 0 °C
  2. Dissolving ammonium nitrate in water
  3. Boiling water at 100 °C
  4. Burning methane in oxygen Correct answer.

Explanation

An exothermic process releases energy to the surroundings, usually as heat, so the surroundings warm up. Combustion of methane releases energy because the bonds formed in carbon dioxide and water are stronger than those broken in methane and oxygen.

Question 23Knowledge and Understanding of Chemistry

Which quantity fixes the chemical identity of an element?

  1. The number of nucleons
  2. The number of neutrons
  3. The number of electrons
  4. The number of protons Correct answer.

Explanation

The atomic number equals the number of protons in the nucleus and is unique to each element, fixing its position in the periodic table. Changing the proton count produces a different element, whereas changing neutrons or electrons gives an isotope or an ion of the same element.

Question 24Knowledge and Understanding of Chemistry

A fixed mass of gas is kept at constant temperature. Its volume is 200 cm³ when the pressure is 100 kPa. What volume does the gas occupy when the pressure is raised to 150 kPa?

  1. 133 cm³ Correct answer.
  2. 300 cm³
  3. 100 cm³
  4. 150 cm³

Explanation

At constant temperature, pressure and volume are inversely proportional, so P₁V₁ = P₂V₂. Rearranging gives V₂ = (100 kPa × 200 cm³) ÷ 150 kPa, which rounds to 133 cm³.

Question 25Knowledge and Understanding of Chemistry

Marble chips of calcium carbonate react with dilute hydrochloric acid. Which change raises the initial rate of reaction without altering the total volume of gas finally collected?

  1. Using a single large chip
  2. Grinding the chips to powder Correct answer.
  3. Cooling the acid to 10 °C
  4. Diluting the acid with distilled water

Explanation

Rate depends on the frequency of collisions between reacting particles. Grinding exposes a much larger surface area to the acid, so more particles collide each second, yet the same total amount of carbonate reacts, leaving the final gas volume unchanged.

Question 26Knowledge and Understanding of Chemistry

What mass of potassium nitrate is needed to prepare 250 cm³ of a 0.200 mol L⁻¹ solution? (M(KNO₃) = 101 g mol⁻¹)

  1. 5.05 g Correct answer.
  2. 50.5 g
  3. 20.2 g
  4. 1.26 g

Explanation

Convert the volume to litres, so 250 cm³ = 0.250 dm³. The amount needed is c × V = 0.200 mol L⁻¹ × 0.250 L = 0.0500 mol, and multiplying by the molar mass gives 0.0500 mol × 101 g mol⁻¹ = 5.05 g.

Question 27Knowledge and Understanding of Chemistry

Which of the following molecules possesses a permanent dipole moment?

  1. Methane, CH₄
  2. Ammonia, NH₃ Correct answer.
  3. Carbon dioxide, CO₂
  4. Boron trifluoride, BF₃

Explanation

A molecule has a permanent dipole only if its bond dipoles fail to cancel. In ammonia the polar N–H bonds point towards the corners of a trigonal pyramid, so their dipoles add to give a net dipole, while the other three molecules are symmetrical and their bond dipoles cancel.

Question 28Knowledge and Understanding of Chemistry

A weak monoprotic acid HA has Ka = 1.0 × 10⁻⁵ mol L⁻¹. What is the pH of a 0.10 mol L⁻¹ aqueous solution of HA?

  1. 3.0 Correct answer.
  2. 1.0
  3. 2.0
  4. 5.0

Explanation

For a weak acid, [H⁺] ≈ √(Ka × c) = √(1.0 × 10⁻⁵ × 0.10) = 1.0 × 10⁻³ mol L⁻¹. Taking the negative logarithm gives pH = 3.0, and the approximation holds because only a small fraction of the acid is ionised.

Question 29Knowledge and Understanding of Chemistry

Use these data: (1) C(s) + O₂(g) → CO₂(g), ΔH = −394 kJ mol⁻¹; (2) CO(g) + ½O₂(g) → CO₂(g), ΔH = −283 kJ mol⁻¹. What is ΔH for C(s) + ½O₂(g) → CO(g)?

  1. −677 kJ mol⁻¹
  2. +111 kJ mol⁻¹
  3. −394 kJ mol⁻¹
  4. −111 kJ mol⁻¹ Correct answer.

Explanation

Reverse equation (2) so that CO becomes a product, changing the sign of its enthalpy to +283 kJ mol⁻¹. Adding the reversed equation to equation (1) cancels the CO₂ on both sides and gives the target equation, so ΔH = −394 + 283 = −111 kJ mol⁻¹.

Question 30Knowledge and Understanding of Chemistry

The reaction CH₂=CH₂ + H₂ → CH₃CH₃, carried out over a nickel catalyst, is best classified as:

  1. a neutralisation reaction
  2. an elimination reaction
  3. an addition reaction Correct answer.
  4. a substitution reaction

Explanation

The two carbon atoms of ethene already share a double bond, and hydrogen adds across it so that each carbon gains one extra hydrogen, converting the double bond into a single bond. No atom leaves the molecule, which rules out substitution and elimination.

Question 31Knowledge and Understanding of Chemistry

Which element is a Group 2 metal in Period 3 of the periodic table?

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

Explanation

Group 2 elements have two electrons in their outer shell, and Period 3 elements have electrons in the first three shells. Magnesium has atomic number 12 and electron configuration 2,8,2, placing it in Group 2 and Period 3. Sodium is in Group 1, aluminium is in Group 3, and silicon is in Group 4.

Question 32Knowledge and Understanding of Chemistry

Which statement best describes a covalent bond?

  1. The electrostatic attraction between oppositely charged ions
  2. The attraction between metal cations and delocalised electrons
  3. The attraction between the positive end of one polar molecule and the negative end of another
  4. The sharing of one or more pairs of electrons between atoms Correct answer.

Explanation

A covalent bond forms when two atoms share a pair of electrons so that each atom gains a filled outer shell. This sharing usually occurs between non-metal atoms with similar electronegativities. Ionic bonding, metallic bonding and intermolecular attractions are described by the other options.

Question 33Knowledge and Understanding of Chemistry

A student has a mixture of ethanol and water. Which technique is most suitable for separating the two liquids?

  1. Fractional distillation Correct answer.
  2. Chromatography
  3. Filtration
  4. Crystallisation

Explanation

Ethanol and water are miscible liquids with different boiling points, so fractional distillation can separate them. The mixture is heated and the more volatile component vaporises first, then condenses in the fractionating column. Filtration removes insoluble solids, crystallisation forms crystals from solution, and chromatography is mainly used for separating small amounts of dissolved substances.

Question 34Knowledge and Understanding of Chemistry

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

  1. CH₂O Correct answer.
  2. C₂H₄O₂
  3. C₂H₆O
  4. CH₄O

Explanation

Assume 100 g of compound, giving 40.0 g C, 6.7 g H and 53.3 g O. Divide each mass by its molar mass: C 40.0/12.0 = 3.33 mol, H 6.7/1.0 = 6.7 mol and O 53.3/16.0 = 3.33 mol. Dividing by the smallest value gives the ratio C:H:O = 1:2:1, so the empirical formula is CH₂O.

Question 35Knowledge and Understanding of Chemistry

A fixed mass of gas occupies 300 cm³ at 27 °C. What volume will it occupy at 127 °C if the pressure remains constant?

  1. 533 cm³
  2. 225 cm³
  3. 300 cm³
  4. 400 cm³ Correct answer.

Explanation

Convert the temperatures to kelvin: 27 °C = 300 K and 127 °C = 400 K. Charles's law states that V/T is constant at fixed pressure, so V₂ = V₁ × T₂/T₁ = 300 cm³ × 400 K / 300 K = 400 cm³.

Question 36Knowledge and Understanding of Chemistry

What volume of 2.0 mol L⁻¹ hydrochloric acid is required to prepare 250 cm³ of 0.50 mol L⁻¹ hydrochloric acid?

  1. 62.5 cm³ Correct answer.
  2. 31.3 cm³
  3. 250 cm³
  4. 125 cm³

Explanation

Use the dilution relationship C₁V₁ = C₂V₂, where C is concentration and V is volume. Rearranging gives V₁ = (0.50 mol L⁻¹ × 250 cm³) / 2.0 mol L⁻¹ = 62.5 cm³. The concentrated acid is then diluted with water to a final volume of 250 cm³.

Question 37Knowledge and Understanding of Chemistry

A 0.10 mol L⁻¹ solution of a weak acid is compared with a 0.10 mol L⁻¹ solution of a strong acid. Which statement is correct?

  1. Both solutions have the same pH because their concentrations are equal.
  2. The weak acid solution has a higher pH and a lower electrical conductivity. Correct answer.
  3. The weak acid solution has a lower pH and a higher electrical conductivity.
  4. The weak acid solution has a higher pH and a higher electrical conductivity.

Explanation

A weak acid only partially ionises in water, so it releases fewer hydrogen ions than a strong acid of the same concentration. Fewer H⁺ ions mean a higher pH, so the solution is less acidic, and fewer mobile ions mean lower electrical conductivity. A strong acid is almost fully ionised and therefore has a lower pH and higher conductivity.

Question 38Knowledge and Understanding of Chemistry

For the equilibrium N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹, which change shifts the position of equilibrium to the right?

  1. Increasing the temperature Correct answer.
  2. Decreasing the temperature
  3. Adding a catalyst
  4. Increasing the pressure

Explanation

The forward reaction is endothermic, since ΔH is positive, and it produces more gas molecules, going from 1 mol to 2 mol. Increasing the temperature favours the endothermic direction, shifting the equilibrium to the right. Increasing pressure would shift it left towards fewer gas molecules, and a catalyst speeds up both directions equally without changing the position.

Question 39Knowledge and Understanding of Chemistry

In the electrolysis of concentrated aqueous sodium chloride, which products are formed at the cathode and at the anode, respectively?

  1. Sodium and oxygen
  2. Hydrogen and chlorine Correct answer.
  3. Hydrogen and oxygen
  4. Sodium and chlorine

Explanation

In concentrated brine, water and chloride ions compete at the electrodes. At the cathode, hydrogen ions from water are reduced in preference to sodium ions because sodium is too reactive to be deposited from aqueous solution. At the anode, chloride ions are oxidised to chlorine in preference to water because the chloride concentration is high.

Question 40Knowledge and Understanding of Chemistry

Which pair of compounds are functional group isomers of each other?

  1. Propan-1-ol and propan-2-ol
  2. But-1-ene and but-2-ene
  3. Ethane and ethene
  4. Ethanol and dimethyl ether Correct answer.

Explanation

Functional group isomers have the same molecular formula but different functional groups. Ethanol, C₂H₆O, contains a hydroxyl group, while dimethyl ether, also C₂H₆O, contains an ether linkage, so they are functional group isomers. Propan-1-ol and propan-2-ol are positional isomers, but-1-ene and but-2-ene are positional isomers, and ethane and ethene have different molecular formulas.

Question 41Knowledge and Understanding of Chemistry

Two atoms have the same atomic number but different mass numbers. Which count differs between the two atoms?

  1. Number of occupied shells
  2. Electron count in the neutral atom
  3. Proton count
  4. Neutron count Correct answer.

Explanation

Atomic number is the proton count, and mass number is protons plus neutrons. Atoms with the same atomic number but different mass numbers are isotopes, so their neutron counts differ while proton and neutral-electron counts stay the same.

Question 42Knowledge and Understanding of Chemistry

Which formula represents a saturated hydrocarbon?

  1. C2H2
  2. C2H6 Correct answer.
  3. C6H6
  4. C2H4

Explanation

A saturated hydrocarbon contains only single carbon–carbon bonds and follows the general formula CnH2n+2. Ethane, C2H6, is the only option that fits this pattern.

Question 43Knowledge and Understanding of Chemistry

What is the formula of magnesium chloride?

  1. MgCl3
  2. MgCl
  3. MgCl2 Correct answer.
  4. Mg2Cl

Explanation

Magnesium forms Mg2+ ions and chloride forms Cl− ions. Charge balance requires two chloride ions for each magnesium ion, giving the formula MgCl2.

Question 44Knowledge and Understanding of Chemistry

Nitrogen and hydrogen react as follows: N2 + 3H2 → 2NH3. If 2.0 mol N2 reacts with 3.0 mol H2, which reactant is limiting and how many moles of NH3 form?

  1. N2 is limiting; 4.0 mol NH3
  2. H2 is limiting; 3.0 mol NH3
  3. H2 is limiting; 2.0 mol NH3 Correct answer.
  4. N2 is limiting; 2.0 mol NH3

Explanation

The equation needs 3 mol H2 per mol N2, so 2.0 mol N2 would require 6.0 mol H2; only 3.0 mol H2 is present. Hydrogen is limiting, and 3.0 mol H2 produces 2.0 mol NH3 by the 3:2 ratio.

Question 45Knowledge and Understanding of Chemistry

A 2.0 L vessel contains 0.50 mol of gas at 300 K. What pressure does the gas exert? Use R = 8.31 kPa L mol−1 K−1.

  1. 0.62 kPa
  2. 312 kPa
  3. 1246 kPa
  4. 623 kPa Correct answer.

Explanation

Use the ideal gas equation in the form P = nRT/V. Substituting gives P = (0.50 × 8.31 × 300) / 2.0, which is approximately 623 kPa.

Question 46Knowledge and Understanding of Chemistry

Which pair of aqueous solutions, when mixed, forms a precipitate?

  1. Hydrochloric acid and sodium hydroxide
  2. Potassium nitrate and sodium sulfate
  3. Silver nitrate and sodium chloride Correct answer.
  4. Sodium chloride and potassium nitrate

Explanation

Silver chloride is insoluble in water, so mixing silver nitrate and sodium chloride solutions produces a white precipitate. The other pairs contain only soluble ions and remain clear unless a weak acid/base reaction occurs.

Question 47Knowledge and Understanding of Chemistry

Which atom has the largest atomic radius?

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

Explanation

Atomic radius decreases across a period because the effective nuclear charge increases while electrons enter the same shell. Sodium is farthest left among these Period 3 elements, so it has the largest radius.

Question 48Knowledge and Understanding of Chemistry

Which reagent converts ethanol to ethanoic acid under suitable conditions?

  1. Hydrogen with a nickel catalyst
  2. Concentrated sulfuric acid at 170 °C
  3. Aqueous sodium chloride
  4. Acidified dichromate(VI) Correct answer.

Explanation

Acidified potassium dichromate(VI) is an oxidizing agent that converts a primary alcohol such as ethanol to ethanoic acid under suitable warming. The other reagents either dehydrate, reduce, or fail to oxidize the alcohol.

Question 49Knowledge and Understanding of Chemistry

For the equilibrium A + B ⇌ 2C, Kc = 4.0 at a certain temperature. In a mixture at that temperature, [A] = [B] = 0.50 mol L−1 and [C] = 1.5 mol L−1. Which statement describes the net change?

  1. It shifts left because Q < Kc
  2. It shifts right because Q < Kc
  3. It shifts left because Q > Kc Correct answer.
  4. It is at equilibrium because Q = Kc

Explanation

Calculate Q using the same form as Kc: Q = [C]²/([A][B]) = (1.5)²/(0.50 × 0.50) = 9.0. Since Q is greater than Kc, the reverse reaction is favored and the system shifts left.

Question 50Knowledge and Understanding of Chemistry

Use these average bond enthalpies: H–H = 436 kJ mol−1, Cl–Cl = 242 kJ mol−1, H–Cl = 431 kJ mol−1. What is the estimated enthalpy change for H2 + Cl2 → 2HCl?

  1. −184 kJ mol−1 Correct answer.
  2. +96 kJ mol−1
  3. −96 kJ mol−1
  4. +184 kJ mol−1

Explanation

Bonds broken cost 436 + 242 = 678 kJ mol−1, and bonds formed release 2 × 431 = 862 kJ mol−1. Therefore ΔH = 678 − 862 = −184 kJ mol−1.