Practice neco model questions vol1 chemistry 2025 Past Questions

Question 1 View Details

A 150 g sample of water at \(25^{\circ}\text{C}\) is heated in a calorimeter by a heater delivering \(500\ \text{W}\) for \(10\) minutes. No heat is lost to the surroundings. The specific heat capacity of water is \(4.18\ \text{J g}^{-1}\text{°C}^{-1}\) and the latent heat of vaporisation is \(2260\ \text{J g}^{-1}\). Determine the final state of the system (liquid water only, a mixture of liquid water and steam, or steam only) and the temperature of the system after heating.

A. mixture of liquid water and steam at 120 °C

B. liquid water only at 100 °C

C. steam only at 100 °C

Correct D. mixture of liquid water and steam at 100 °C

Correct Answer: D

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Question 2 View Details

A \(0.5\ \text{mol}\) sample of an unknown monohydric alcohol with molecular formula \(\mathrm{C}_{n}\mathrm{H}_{2n+2}\mathrm{O}\) is dehydrated with concentrated \(\mathrm{H_{2}SO_{4}}\) to give a single alkene. The alkene is then hydrogenated over a Pt catalyst with excess \(\mathrm{H_{2}}\) to give an alkane of molecular weight \(58\ \text{g mol}^{-1}\). Identify the alcohol (its molecular formula and IUPAC name). Subsequently, when the original \(0.5\ \text{mol}\) of this alcohol is completely combusted, calculate the mass of \(\mathrm{CO_{2}}\) produced.

A. C3H8O (propan-1-ol); 66 g CO₂

Correct B. C4H10O (2‑methyl‑2‑propanol); 88 g CO₂

C. C5H12O (2‑methyl‑1‑butanol); 110 g CO₂

D. C4H10O (tert‑butanol); 80 g CO₂

Correct Answer: B

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Question 3 View Details

In the diagram of a gas‑collection‑over‑water apparatus, a gas is collected at a volume of \(250.0\ \text{mL}\) at \(25.0^{\circ}\text{C}\). The barometric pressure is \(760\ \text{mmHg}\) and the vapour pressure of water at this temperature is \(23.8\ \text{mmHg}\). After drying, the gas volume is measured as \(242.0\ \text{mL}\) at \(1.00\ \text{atm}\) and \(298\ \text{K}\). Determine the number of moles of the original gas collected over water.

A. 1.1×10⁻³ mol

B. 1.0×10⁻² mol

C. 8.5×10⁻³ mol

Correct D. 9.9×10⁻³ mol

Correct Answer: D

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Question 4 View Details

The reaction \(\mathrm{2Al + 6HCl \rightarrow 2AlCl_{3} + 3H_{2}}\) is carried out using \(5.0\ \text{g}\) of aluminium and \(10.0\ \text{g}\) of hydrochloric acid. Calculate the theoretical mass of hydrogen gas produced and the percent yield if \(0.20\ \text{g}\) of hydrogen is actually collected.

A. 0.300 g H₂ (theoretical); 66.7 % yield

Correct B. 0.276 g H₂ (theoretical); 72.5 % yield

C. 0.260 g H₂ (theoretical); 76.9 % yield

D. 0.250 g H₂ (theoretical); 80.0 % yield

Correct Answer: B

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Question 5 View Details

A mixture of propane (\(\mathrm{C_{3}H_{8}}\)) and butane (\(\mathrm{C_{4}H_{10}}\)) is combusted completely. The combustion produces \(176\ \text{g}\) of carbon dioxide and \(93.6\ \text{g}\) of water. Determine the mass percent of each hydrocarbon in the original mixture.

A. 55.0 % propane; 45.0 % butane

B. 70.2 % propane; 29.8 % butane

C. 62.5 % propane; 37.5 % butane

Correct D. 60.3 % propane; 39.7 % butane

Correct Answer: D

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Question 6 View Details

A galvanic cell is constructed as \(\text{Zn(s)}| \text{Zn}^{2+}(0.010\ \text{M}) \;||\; \text{Cu}^{2+}(0.10\ \text{M})| \text{Cu(s)}\). The standard reduction potentials are \(E^{\circ}_{\text{Zn}^{2+}/\text{Zn}} = -0.76\ \text{V}\) and \(E^{\circ}_{\text{Cu}^{2+}/\text{Cu}} = +0.34\ \text{V}\).\n(a) Calculate the cell potential at 25 °C using the Nernst equation.\n(b) If a total charge of \(0.500\ \text{C}\) passes through the cell, determine the mass of copper deposited on the copper electrode.

A. E = 1.13 V; mass of Cu deposited = 2.5×10⁻⁴ g

B. E = 1.25 V; mass of Cu deposited = 2.0×10⁻⁴ g

Correct C. E = 1.13 V; mass of Cu deposited = 1.6×10⁻⁴ g

D. E = 0.96 V; mass of Cu deposited = 1.2×10⁻⁴ g

Correct Answer: C

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Question 7 View Details

In the molecule \(\text{SF}_4\) the axial S-F bonds are 164 pm long while the equatorial S-F bonds are 154 pm long. Assuming bond length is inversely proportional to the percentage of s‑character in the hybrid orbital used for the bond, estimate the ratio of s‑character in the axial S-F hybrid orbitals to that in the equatorial S-F hybrid orbitals.

A. 0.78 (axial s‑character is about 78 % of equatorial)

B. 0.85 (axial s‑character is about 85 % of equatorial)

C. 1.06 (axial s‑character is about 106 % of equatorial)

Correct D. 0.94 (axial s‑character is about 94 % of equatorial)

Correct Answer: D

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Question 8 View Details

The diagram shows a water molecule. Identify the hybridisation of the oxygen atom and state the approximate bond angle shown.

A. sp³ hybridisation; bond angle ≈ 109.5°

Correct B. sp³ hybridisation; bond angle ≈ 104.5°

C. sp hybridisation; bond angle ≈ 180°

D. sp² hybridisation; bond angle ≈ 120°

Correct Answer: B

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Question 9 View Details

A 0.025 M solution of an unknown monoprotic weak acid HA (a carboxylic acid) is titrated with 0.10 M NaOH. At the half‑equivalence point the measured pH is 4.75.\n(a) Determine the pK_a of HA.\n(b) Given that the molecular formula of HA is C₃H₆O₂, identify the acid.\n(c) Calculate the pH of a 0.10 M solution of this acid at 25 °C.

A. pK_a = 5.00; acid = propanoic acid; pH of 0.10 M solution ≈ 2.88

B. pK_a = 4.75; acid = butanoic acid; pH of 0.10 M solution ≈ 2.70

C. pK_a = 4.75; acid = propanoic acid; pH of 0.10 M solution ≈ 3.10

Correct D. pK_a = 4.75; acid = propanoic acid; pH of 0.10 M solution ≈ 2.88

Correct Answer: D

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Question 10 View Details

A buffer is prepared by dissolving 0.20 mol of acetic acid (CH₃COOH, K_a = 1.8 × 10⁻⁵) and 0.15 mol of sodium acetate (CH₃COONa) in 1.00 L of water.\n(a) Calculate the initial pH of the buffer.\n(b) After adding 0.050 mol of HCl, calculate the new pH.\n(c) How many moles of NaOH must be added to restore the pH to its original value?

Correct A. Initial pH = 4.62; after HCl pH = 4.34; NaOH needed ≈ 0.051 mol

B. Initial pH = 4.50; after HCl pH = 4.25; NaOH needed ≈ 0.040 mol

C. Initial pH = 4.55; after HCl pH = 4.30; NaOH needed ≈ 0.060 mol

D. Initial pH = 4.70; after HCl pH = 4.20; NaOH needed ≈ 0.045 mol

Correct Answer: A

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Question 11 View Details

An unknown metal forms a +2 cation and its atomic number is 20 more than that of sodium. Identify the element (provide its name and symbol).

A. Aluminum (Al)

Correct B. Gallium (Ga)

C. Germanium (Ge)

D. Calcium (Ca)

Correct Answer: B

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Question 12 View Details

When 2‑methyl‑2‑butene is treated with cold dilute potassium permanganate, a colourless product is obtained. State the IUPAC name of the product and give its molecular formula.

Correct A. 2‑methyl‑2,3‑butanediol, C5H12O2

B. 2-methyl-1,2-butanediol, C5H12O2

C. 2-ethyl-2,3-butanediol, C6H14O2

D. 2-methyl-2,3-butanediol, C5H10O2

Correct Answer: A

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Question 13 View Details

A 1.80 g sample of an unknown compound containing only carbon, hydrogen and oxygen is found to contain 0.72 g of carbon. Complete combustion of the sample produces 0.90 g of water. Determine the empirical formula of the compound.

Correct A. C3H5O3

B. C2H4O2

C. C4H6O3

D. C3H6O2

Correct Answer: A

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Question 14 View Details

A lake contains dissolved calcium at a concentration of 2.5 mg L⁻¹ as Ca²⁺ and bicarbonate at 150 mg L⁻¹ as HCO₃⁻. The water pH is 8.5. Using the second dissociation constant of carbonic acid K₂ = 4.7×10⁻¹¹ and the solubility product of calcite Ksp = 4.8×10⁻⁹ at 25 °C, calculate the saturation index (SI) for calcite and state whether the water is undersaturated, saturated or supersaturated with respect to calcite.

A. SI ≈ -0.32; the water is supersaturated with respect to calcite.

B. SI ≈ -0.10; the water is undersaturated with respect to calcite.

Correct C. SI ≈ -0.32; the water is undersaturated with respect to calcite.

D. SI ≈ 0.15; the water is supersaturated with respect to calcite.

Correct Answer: C

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Question 15 View Details

A 0.500 mol sample of a non‑volatile solute is dissolved in 1.00 kg of water. The solution's boiling point is observed to be 100.8 °C at 1 atm. Given the ebullioscopic constant of water Kb = 0.512 °C·kg mol⁻¹, calculate the van't Hoff factor i of the solute. Based on the value of i, indicate whether the solute is likely a nonelectrolyte, a 2‑ion electrolyte, or a 3‑ion electrolyte.

A. i ≈ 3.5; the solute is likely a 3‑ion electrolyte

B. i ≈ 1.0; the solute is likely a nonelectrolyte

Correct C. i ≈ 3.1; the solute is likely a 3‑ion electrolyte.

D. i ≈ 2.0; the solute is likely a 2‑ion electrolyte

Correct Answer: C

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Question 16 View Details

A wastewater treatment plant receives 5000 m³ per day of effluent containing nitrate ions at a concentration of 30 mg L⁻¹. The treatment consists of two consecutive steps: (i) a biological denitrification stage that removes 60 % of the nitrate present, but 10 % of the nitrate removed is converted to nitrite, which is not removed later; (ii) an ion‑exchange column that removes 80 % of the nitrate remaining after the first stage. Assuming the volume of water does not change, calculate the final concentration of nitrate ions in the discharged water (in mg L⁻¹, to one decimal place).

A. 2.0 mg L⁻¹

B. 1.8 mg L⁻¹

Correct C. 2.4 mg L⁻¹

D. 3.2 mg L⁻¹

Correct Answer: C

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Question 17 View Details

Chlorine gas reacts with excess sodium hydroxide according to \(\ce{Cl2 + 2 NaOH -> NaCl + NaOCl + H2O}\). The NaOCl formed is later acidified with excess sulphuric acid, giving \(\ce{NaOCl + 2 H2SO4 -> Na2SO4 + Cl2 + H2O + O2}\). If 5.0 g of \(\ce{Cl2}\) are initially used, what volume of \(\ce{O2}\) gas is produced at STP (0 °C, 1 atm)? Give your answer in litres to two decimal places.

Correct A. 1.58 L

B. 1.45 L

C. 1.70 L

D. 1.30 L

Correct Answer: A

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Question 18 View Details

The diagram shows a 3 × 3 section of the periodic table with the elements arranged as follows: top row Na, Mg, Al; middle row K, Ca, Sc; bottom row Rb, Sr, Y. Based on the periodic trends illustrated, which element in the diagram has the highest first ionisation energy? State the element symbol.

Correct A. Mg

B. K

C. Na

D. Ca

Correct Answer: A

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Question 19 View Details

The diagram depicts a Bohr model of an atom with three electron shells containing 2, 8 and 5 electrons respectively. Identify the element and write its ground‑state electron configuration in subshell notation.

A. Sulfur; 1s^2 2s^2 2p^6 3s^2 3p^4

B. Chlorine; 1s^2 2s^2 2p^6 3s^2 3p^5

C. Silicon; 1s^2 2s^2 2p^6 3s^2 3p^2

Correct D. Phosphorus; 1s^2 2s^2 2p^6 3s^2 3p^3

Correct Answer: D

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Question 20 View Details

Nylon‑6,6 is produced by step‑growth polymerisation of adipic acid (\(\ce{C6H10O4}\), M = 146.14 g mol⁻¹) and hexamethylenediamine (\(\ce{C6H16N2}\), M = 116.21 g mol⁻¹). In a sample the number‑average degree of polymerisation is 500. (a) Calculate the number‑average molecular weight \(M_n\) of the polymer. (b) Determine the mass percentage of nitrogen in the polymer. Give \(M_n\) in scientific notation (g mol⁻¹) and the nitrogen percentage to one decimal place.

A. M_n = 1.13×10^5 g mol⁻¹; nitrogen % ≈ 11.0 %

Correct B. M_n = 1.13×10^5 g mol⁻¹; nitrogen % ≈ 12.4 %

C. M_n = 9.8×10^4 g mol⁻¹; nitrogen % ≈ 11.6 %

D. M_n = 1.05×10^5 g mol⁻¹; nitrogen % ≈ 13.2 %

Correct Answer: B

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Question 21 View Details

A sample containing 12.0 g of iron (Fe) is reacted with 10.0 g of chlorine gas (Cl₂) according to the equation \(2 \text{Fe} + 3 \text{Cl}_2 \rightarrow 2 \text{FeCl}_3\). The reaction gives an actual yield of 78 % based on the theoretical amount of FeCl₃. Calculate (a) the mass of FeCl₃ actually obtained and (b) the mass of iron that remains unreacted.

A. Mass of FeCl₃ obtained = 13.5 g; mass of unreacted Fe = 5.20 g

B. Mass of FeCl₃ obtained = 10.2 g; mass of unreacted Fe = 8.30 g

Correct C. Mass of FeCl₃ obtained = 11.9 g; mass of unreacted Fe = 6.75 g

D. Mass of FeCl₃ obtained = 9.8 g; mass of unreacted Fe = 9.00 g

Correct Answer: C

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Question 22 View Details

A solution is prepared by dissolving 2.00 g of an unknown monohydric alcohol in 100.0 g of water. The freezing point of the solution is observed to be -0.81 °C. Given the cryoscopic constant of water \(K_f = 1.86\;^\circ\text{C·kg·mol}^{-1}\), determine (a) the molar mass of the alcohol and (b) identify the alcohol from the list: methanol (CH₃OH), ethanol (CH₃CH₂OH), propan‑1‑ol (CH₃CH₂CH₂OH), butan‑1‑ol (CH₃CH₂CH₂CH₂OH).

A. Molar mass ≈ 32 g·mol⁻¹; the alcohol is methanol (CH₃OH)

B. Molar mass ≈ 58 g·mol⁻¹; the alcohol is propan‑1‑ol (CH₃CH₂CH₂OH)

Correct C. Molar mass ≈ 46 g·mol⁻¹; the alcohol is ethanol (CH₃CH₂OH)

D. Molar mass ≈ 74 g·mol⁻¹; the alcohol is butan‑1‑ol (CH₃CH₂CH₂CH₂OH)

Correct Answer: C

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Question 23 View Details

A buffer solution is prepared by mixing 0.250 mol of acetic acid (\(\text{CH}_3\text{COOH}\), \(K_a = 1.8 \times 10^{-5}\)) with 0.150 mol of sodium acetate (\(\text{CH}_3\text{COONa}\)) in 1.00 L of water. (a) Calculate the initial pH of the buffer. (b) After adding 0.050 mol of hydrochloric acid (HCl) to the solution, calculate the new pH.

Correct A. Initial pH ≈ 4.52; pH after HCl addition ≈ 4.27

B. Initial pH ≈ 5.00; pH after HCl addition ≈ 4.80

C. Initial pH ≈ 4.30; pH after HCl addition ≈ 4.10

D. Initial pH ≈ 4.75; pH after HCl addition ≈ 4.50

Correct Answer: A

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Question 24 View Details

The molecular formula C₅H₁₂O corresponds to a monofunctional alcohol. (a) How many distinct structural isomers are possible for this formula? (b) One of these isomers, when treated with pyridinium chlorochromate (PCC), yields an aldehyde without any skeletal rearrangement. Identify this isomer.

A. 6 isomers; the isomer is 3‑methyl‑1‑butanol

Correct B. 7 isomers; the isomer is 2‑methyl‑1‑butanol

C. 8 isomers; the isomer is 1‑butanol

D. 7 isomers; the isomer is 1‑methyl‑2‑butanol

Correct Answer: B

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Question 25 View Details

Determine the hybridisation of the central nitrogen atom in the nitrate ion (\(\text{NO}_3^{-}\)).

A. sp³d

B. sp

Correct C. sp²

D. sp³

Correct Answer: C

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