Practice neco chemistry 2024 Past Questions | Edcrib

Question 1 View Details

Three non‑metals—phosphorus (P), sulfur (S) and chlorine (Cl)—form the oxides \(\mathrm{P_4O_{10}}\), \(\mathrm{SO_2}\) and \(\mathrm{Cl_2O_7}\) respectively. Using the oxidation state of the central atom in each oxide, determine which oxide is the most acidic. State the molecular formula of that oxide.

A. P4O10

B. SO2

Correct C. Cl2O7

D. ClO2

Correct Answer: C

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Question 2 View Details

Ten point zero grams of nitrogen gas reacts with excess oxygen to give nitrogen dioxide according to \(\mathrm{N_2 + 2\,O_2 \rightarrow 2\,NO_2}\). The nitrogen dioxide then reacts with water: \(\mathrm{3\,NO_2 + H_2O \rightarrow 2\,HNO_3 + NO}\). Assuming an overall 80 % yield for \(\mathrm{HNO_3}\) based on the amount of nitrogen originally present, calculate the mass of \(\mathrm{HNO_3}\) obtained (in grams, three significant figures).

A. 20.0

B. 30.0

C. 28.0

Correct D. 24.0

Correct Answer: D

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Question 3 View Details

In acidic solution potassium permanganate reacts with oxalic acid according to the overall equation \(\mathrm{MnO_4^- + C_2O_4^{2-} \rightarrow Mn^{2+} + CO_2}\). Balance the redox equation using the half‑reaction method and then determine the volume of \(\mathrm{CO_2}\) (at STP, 22.4 L mol⁻¹) produced when 0.025 mol of \(\mathrm{KMnO_4}\) reacts completely.

Correct A. 2.80

B. 2.00

C. 2.24

D. 3.36

Correct Answer: A

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Question 4 View Details

A weak acid HA has \(K_a = 1.8\times10^{-5}\). Its sodium salt NaA is completely soluble. In 1.00 L of water 0.50 g of HA (M = 60 g mol⁻¹) and 0.735 g of NaA (M = 82 g mol⁻¹) are dissolved.\n(a) Calculate the pH of the resulting buffer solution using the Henderson-Hasselbalch equation.\n(b) How many milliliters of 0.10 M HCl must be added to lower the pH by exactly 0.50 units? Give your answer to two significant figures.

Correct A. 45

B. 40

C. 55

D. 60

Correct Answer: A

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Question 5 View Details

Acetic acid (CH₃COOH) has \(K_a = 1.8\times10^{-5}\). It reacts with ethanol to give ethyl acetate and water. At 298 K the equilibrium constant for the esterification (in concentration terms) is \(K = 4.5\). If 0.10 mol of acetic acid and 0.10 mol of ethanol are placed in 1.00 L of water with no ester initially present, calculate the fraction of the original acetic acid that is converted to ethyl acetate at equilibrium (give the answer as a decimal to two significant figures).

A. 0.82

B. 0.12

C. 0.45

Correct D. 0.68

Correct Answer: D

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Question 6 View Details

A 0.500 g sample containing only Fe2O3 and Fe3O4 is dissolved in acid and titrated with 0.100 M KMnO4. The reaction in acidic medium is \(\mathrm{MnO_4^- + 5Fe^{2+} + 8H^+ \rightarrow Mn^{2+} + 5Fe^{3+} + 4H_2O}\). The titration required 2.00 mL of KMnO4 solution. Assuming all Fe2+ present originates from the FeO component of Fe3O4, calculate the percentage by mass of Fe3O4 in the original sample. Give your answer to two decimal places.

Correct A. 46.35 %

B. 44.20 %

C. 52.10 %

D. 48.75 %

Correct Answer: A

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Question 7 View Details

Equal volumes (0.500 L each) of a 0.010 M BaCl2 solution and a 0.020 M Na2SO4 solution are mixed at 25 °C. BaSO4 precipitates and solid remains in equilibrium with the solution. Given \(K_{sp}(\mathrm{BaSO_4}) = 1.1\times10^{-10}\), determine (a) the mass of BaSO4 that precipitates, and (b) the equilibrium concentration of Ba^{2+} ions in the final solution. Express the mass in grams to three significant figures and the concentration in mol L^{-1} in scientific notation.

A. 1.05 g; 2.2×10⁻⁸ M

B. 1.17 g; 1.1×10⁻⁸ M

C. 1.23 g; 2.2×10⁻⁸ M

Correct D. 1.17 g; 2.2×10⁻⁸ M

Correct Answer: D

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Question 8 View Details

A wastewater containing phenol at an initial concentration of 50 mg L^{-1} is treated by batch adsorption onto activated carbon. The adsorption follows the Langmuir isotherm \[ q_e = \frac{Q_{\max} K_L C_e}{1+K_L C_e} \] where \(Q_{\max}=250\) mg g^{-1} and \(K_L = 0.015\) L mg^{-1}. The reactor uses 2.0 g of carbon per litre of wastewater. Assuming a volume of 1 L, calculate the equilibrium phenol concentration \(C_e\) (in mg L^{-1}) after treatment. Solve for \(C_e\) and give your answer to one decimal place.

A. 5.9 mg\/L

B. 7.3 mg\/L

Correct C. 6.4 mg/L

D. 9.0 mg\/L

Correct Answer: C

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Question 9 View Details

The vapor pressure of a volatile liquid at two temperatures is measured as follows: at 298 K the pressure is 0.40 atm, and at 308 K the pressure is 0.80 atm. Assuming the Clausius-Clapeyron relation \(\ln(P_2/P_1) = -\Delta H_{vap}/R\,(1/T_2 - 1/T_1)\) with \(R = 8.314\) J mol^{-1}\,K^{-1}, calculate the enthalpy of vapourisation \(\Delta H_{vap}\) in kJ mol^{-1}. Give your answer to two significant figures.

Correct A. 53 kJ/mol

B. 45 kJ/mol

C. 62 kJ/mol

D. 78 kJ/mol

Correct Answer: A

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Question 10 View Details

A 0.250 g sample of a solid mixture of CaCO3 and MgCO3 is dissolved in excess 0.1500 M HCl (25.00 mL). The excess acid is back‑titrated with 0.1000 M NaOH, requiring 12.50 mL of NaOH. After neutralisation, the solution is treated with excess ammonium oxalate, precipitating calcium oxalate monohydrate of mass 0.150 g. Determine the percentage by mass of CaCO3 in the original sample. Give your answer to one decimal place.

A. 52.3 %

B. 45.0 %

C. 38.5 %

Correct D. 41.2 %

Correct Answer: D

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Question 11 View Details

An unknown inorganic salt is dissolved in distilled water to give a clear solution. The following observations are recorded: 1. Flame test of the solid gives a brick‑red colour. 2. Adding a few drops of dilute \( \text{HCl} \) to the solution produces no precipitate. 3. Adding \( \text{AgNO}_3 \) to the solution yields a white precipitate that dissolves in dilute \( \text{NH}_3 \) solution. 4. Adding \( \text{BaCl}_2 \) to a separate portion of the same solution produces a white precipitate which dissolves on addition of dilute \( \text{HCl} \). Based on these observations, identify the cation and the anion present in the salt, and write the formula of the salt.

Correct A. The conclusion is incorrect; the observations indicate the presence of both chloride and carbonate ions, so the sample is a mixture of calcium chloride (\( \text{CaCl}_2 \)) and calcium carbonate (\( \text{CaCO}_3 \)).

B. The observations indicate chloride and sulfate ions, so the sample is a mixture of calcium chloride (\\( \\text{CaCl}_2 \\)) and calcium sulfate (\\( \\text{CaSO}_4 \\)).

C. The observations point to calcium chloride (\\( \\text{CaCl}_2 \\)) as the sole constituent.

D. The observations are consistent with a single salt, calcium carbonate (\\( \\text{CaCO}_3 \\)).

Correct Answer: A

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Question 12 View Details

A galvanic cell is constructed with the half‑cells \( \text{Fe}^{3+}/\text{Fe}^{2+} \) (standard potential \( E^{\circ}=+0.771\ \text{V} \)) and \( \text{Cu}^{2+}/\text{Cu} \) (standard potential \( E^{\circ}=+0.340\ \text{V} \)). The cell contains \(0.0100\ \text{M}\) \( \text{Fe}^{3+} \) and \(0.100\ \text{M}\) \( \text{Fe}^{2+} \) in one half‑cell, and \(0.0100\ \text{M}\) \( \text{Cu}^{2+} \) in the other half‑cell at \(25^{\circ}\text{C}\). The measured cell EMF is \(0.430\ \text{V}\). Calculate the equilibrium constant \(K\) for the overall redox reaction.

Correct A. K \approx 3.7 \times 10^{14}

B. K \approx 5.0 \times 10^{15}

C. K \approx 1.0 \times 10^{12}

D. K \approx 2.5 \times 10^{13}

Correct Answer: A

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Question 13 View Details

Which element has the electron configuration \([\text{Ar}]\,4s^{2}\,3d^{10}\)?

A. Nickel (Ni)

B. Gallium (Ga)

Correct C. Zinc (Zn)

D. Copper (Cu)

Correct Answer: C

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Question 14 View Details

Using molecular‑orbital theory, determine the bond order of carbon monoxide (\( \text{CO} \)) and state whether the molecule is paramagnetic or diamagnetic. Show the key steps of the MO electron‑filling.

A. Bond order = 3; the molecule is paramagnetic.

B. Bond order = 2; the molecule is diamagnetic.

Correct C. Bond order = 3; the molecule is diamagnetic.

D. Bond order = 4; the molecule is diamagnetic.

Correct Answer: C

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Question 15 View Details

A 0.500 g sample of an impure solid is suspected to contain calcium carbonate (\( \text{CaCO}_3 \)). The sample is treated with excess \(0.1500\ \text{M}\) hydrochloric acid, using \(25.00\ \text{mL}\) of the acid. After the reaction, the excess acid is back‑titrated with \(0.1000\ \text{M}\) sodium hydroxide, requiring \(12.35\ \text{mL}\) of the base. Calculate the percentage purity of calcium carbonate in the sample.

A. ≈ 27.8 % purity

B. ≈ 22.5 % purity

Correct C. ≈ 25.2 % purity

D. ≈ 30.1 % purity

Correct Answer: C

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Question 16 View Details

A 10.0 g mixture of benzoic acid (C\(_7\)H\(_6\)O\(_2\), M = 122.12 g·mol\(^{-1}\)) and acetic acid (C\(_2\)H\(_4\)O\(_2\), M = 60.05 g·mol\(^{-1}\)) is completely combusted. The combustion produces 0.500 mol of \(\mathrm{CO_2}\). Assuming complete combustion, calculate the mass of benzoic acid and the mass of acetic acid present in the mixture.

A. Benzoic acid: 6.50 g; Acetic acid: 3.50 g

Correct B. Benzoic acid: 6.96 g; Acetic acid: 3.04 g

C. Benzoic acid: 7.20 g; Acetic acid: 2.80 g

D. Benzoic acid: 5.00 g; Acetic acid: 5.00 g

Correct Answer: B

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Question 17 View Details

In the reaction \(2\,\mathrm{NaCl} + \mathrm{H_2SO_4} \rightarrow \mathrm{Na_2SO_4} + 2\,\mathrm{HCl}\), 5.0 g of NaCl are mixed with 4.0 g of \(\mathrm{H_2SO_4}\). Determine which reactant is the limiting reagent, calculate the mass of \(\mathrm{Na_2SO_4}\) that can be formed, and state the percent excess of the other reagent.

A. Limiting reagent: \\(\\mathrm{NaCl}\\); \\(\\mathrm{Na_2SO_4}\\) formed = 5.80 g; NaCl excess = 0 %

Correct B. Limiting reagent: \(\mathrm{H_2SO_4}\); \(\mathrm{Na_2SO_4}\) formed = 5.80 g; NaCl excess = 4.6 %

C. Limiting reagent: \\(\\mathrm{NaCl}\\); \\(\\mathrm{Na_2SO_4}\\) formed = 4.90 g; NaCl excess = 2.1 %

D. Limiting reagent: \\(\\mathrm{H_2SO_4}\\); \\(\\mathrm{Na_2SO_4}\\) formed = 6.30 g; NaCl excess = 3.8 %

Correct Answer: B

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Question 18 View Details

The solubility product of calcium fluoride at 25 °C is \(K_{sp}=1.5\times10^{-10}\). A solution already contains 0.010 M NaF. Calculate the maximum concentration of \(\mathrm{Ca^{2+}}\) that can exist in this solution at equilibrium and give the mass of CaF\(_2\) that can dissolve per litre of solution.

A. [Ca^{2+}] = 1.5×10⁻⁸ M; CaF₂ that can dissolve = 1.17×10⁻⁶ g L⁻¹ (≈0.00117 mg L⁻¹)

B. [Ca^{2+}] = 6.0×10⁻⁶ M; CaF₂ that can dissolve = 4.68×10⁻⁴ g L⁻¹ (≈0.468 mg L⁻¹)

C. [Ca^{2+}] = 1.2×10⁻⁵ M; CaF₂ that can dissolve = 9.37×10⁻⁴ g L⁻¹ (≈0.937 mg L⁻¹)

Correct D. [Ca^{2+}] = 1.5×10⁻⁶ M; CaF₂ that can dissolve = 1.17×10⁻⁴ g L⁻¹ (≈0.117 mg L⁻¹)

Correct Answer: D

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Question 19 View Details

2‑Butanol (C\(_4\)H\(_{10}\)O) is oxidised with excess acidified potassium dichromate to give butan‑2‑one. If 5.00 g of 2‑butanol are used and the isolated mass of butan‑2‑one obtained is 4.20 g, calculate the percentage yield of the reaction.

A. 78.5

B. 90.1

Correct C. 86.4

D. 84.2

Correct Answer: C

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Question 20 View Details

A 2.50 L container at 298 K holds a mixture of nitrogen and oxygen gases at a total pressure of 1.20 atm. When the mixture is passed over heated copper, nitrogen is removed, and the pressure falls to 0.78 atm (temperature unchanged). Determine the mole fraction of oxygen in the original mixture and the mass of oxygen present initially.

A. Mole fraction O₂ = 0.70; Mass of O₂ = 2.79 g

B. Mole fraction O₂ = 0.35; Mass of O₂ = 1.37 g

Correct C. Mole fraction O₂ = 0.65; Mass of O₂ = 2.55 g

D. Mole fraction O₂ = 0.55; Mass of O₂ = 2.20 g

Correct Answer: C

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Question 21 View Details

An aqueous sample contains an unknown metal ion. The following observations are made sequentially: (i) Addition of dilute \\(\\mathrm{HCl}\\) produces no precipitate. (ii) Addition of excess \\(\\mathrm{NH_4OH}\\) yields a blue precipitate that dissolves on further addition of \\(\\mathrm{NH_4OH}\\). (iii) Passing \\(\\mathrm{H_2S}\\) gas through the solution acidified with \\(\\mathrm{HCl}\\) gives a black precipitate which does not dissolve in dilute \\(\\mathrm{HCl}\\). (iv) A flame test shows a green colour. Identify the metal ion present.

A. Fe^{2+}

B. Zn^{2+}

Correct C. Cu^{2+}

D. Ni^{2+}

Correct Answer: C

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Question 22 View Details

A solution contains 0.010 M NaCl. The solubility product of AgCl at 25 °C is \\(K_{sp}=1.8\times10^{-10}\\). Calculate the molar solubility of AgCl in this solution. If the temperature is raised such that \\(K_{sp}\\) doubles, what is the new solubility? Give both solubilities in scientific notation to two significant figures.

A. s₁ = 2.5×10⁻⁸ M; s₂ = 5.0×10⁻⁸ M

Correct B. s₁ = 1.8×10⁻⁸ M; s₂ = 3.6×10⁻⁸ M

C. s₁ = 1.8×10⁻⁹ M; s₂ = 3.6×10⁻⁹ M

D. s₁ = 1.3×10⁻⁸ M; s₂ = 2.6×10⁻⁸ M

Correct Answer: B

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Question 23 View Details

2‑Methyl‑1‑butene (\\(CH_2=CH‑CH(CH_3)‑CH_3\\)) is first treated with ozone (\\(O_3\\)) at \-78 °C followed by reductive work‑up with zinc and water. The resulting carbonyl compounds are then oxidised with pyridinium chlorochromate (PCC) and finally esterified with methanol in the presence of catalytic sulfuric acid. Identify the final organic product obtained.

A. Methyl 3‑methyl‑butanoate (methyl isovalerate)

B. Methyl 2‑methyl‑butanoate (methyl isopentanoate)

C. Methyl 2‑ethyl‑propanoate (methyl ethylacetate)

Correct D. Methyl 2‑methyl‑propanoate (methyl isobutyrate)

Correct Answer: D

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Question 24 View Details

A buffer solution is prepared by mixing 250 L of 0.20 M acetic acid (\\(CH_3COOH\\)) with 250 L of 0.15 M sodium acetate (\\(CH_3COONa\\)). After the buffer is formed, 0.010 mol of 0.1 M HCl is added. Given \\(K_a = 1.8\times10^{-5}\\) for acetic acid, calculate the pH of the solution after the acid addition. Express the pH to two decimal places.

A. 4.68

Correct B. 4.41

C. 4.23

D. 4.55

Correct Answer: B

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Question 25 View Details

A blend of poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) is prepared with weight fractions 0.60 PVC and 0.40 PMMA. The glass‑transition temperatures are \\(T_{g,\text{PVC}} = 80^{\circ}C\\) and \\(T_{g,\text{PMMA}} = 105^{\circ}C\\). Using the Fox equation \\(\frac{1}{T_g}= \frac{w_1}{T_{g1}}+\frac{w_2}{T_{g2}}\\) (temperatures in Kelvin), calculate the glass‑transition temperature of the blend in °C. Both monomers polymerise by step‑growth with extent of reaction \(p = 0.98\). The molar masses of the monomers are 62.5 g mol(^{-1}) for vinyl chloride and 100.1 g mol(^{-1}) for methyl methacrylate, and the initial moles of the two monomers are equal. Determine the number‑average molecular weight (\(M_n\)) of the resulting polymer blend. Give \(T_g\) (°C) to one decimal place and \(M_n\) (g mol(^{-1})) to the nearest integer.

A. Tg = 90.1 °C; Mn = 3950 g mol⁻¹

Correct B. Tg = 89.5 °C; Mn = 3877 g mol⁻¹

C. Tg = 85.0 °C; Mn = 3800 g mol⁻¹

D. Tg = 92.3 °C; Mn = 4000 g mol⁻¹

Correct Answer: B

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