Chemistry Proposed Question Answers and explanation

 This are proposed Questions for chemistry Post-utme Preparation.

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1. The ideal gas equation is (A) PV = nRT.

   - Explanation: The ideal gas equation is a fundamental relation in chemistry and physics that describes the behavior of an ideal gas. The equation is PV = nRT, where P is pressure, V is volume, n is the number of moles of gas, R is the ideal gas constant, and T is the temperature in Kelvin.

2. The number of atoms in one mole of a substance is equal to (B) the Avogadro’s number.

   - Explanation: Avogadro's number (6.022 × 10^23) is the number of atoms, ions, or molecules in one mole of a substance. It is a fundamental constant in chemistry.

3. The number of electrons in the outermost shell of the atom represents its (C) group.

   - Explanation: The group number in the periodic table represents the number of valence electrons in the outermost shell of an atom for the main group elements.

4. The number of protons or electrons of an atom represents its (E) atomic number.

   - Explanation: The atomic number of an element is the number of protons in the nucleus of an atom. It also equals the number of electrons in a neutral atom.

5. The partial pressure of oxygen in a sample of air is 500 mmHg. If the total pressure is 780 mmHg, what is the mole fraction of the oxygen? (A) 0.64

   - Explanation: The mole fraction of a gas in a mixture is the ratio of the partial pressure of the gas to the total pressure. Here, the mole fraction of oxygen is 500 mmHg / 780 mmHg = 0.64.

6. The periodic classification of the elements is an arrangement of the elements in order of their (D) atomic numbers.

   - Explanation: The modern periodic table is arranged by increasing atomic number, which is the number of protons in the nucleus of an atom.

7. The process of changing one element into another is called (D) transmutation.

   - Explanation: Transmutation is the process of changing one chemical element into another through nuclear reactions.

8. The shapes of CO2, H2O, and CH4 respectively are (D) linear, bent, and tetrahedral.

   - Explanation: CO2 is linear, H2O is bent (angular), and CH4 is tetrahedral in shape.

9. This compound exhibits ionic bonding (C) KCl.

   - Explanation: KCl (potassium chloride) is an ionic compound formed by the transfer of electrons from potassium to chlorine.

10. This type of bonding involves overlapping of orbitals during which electrons are shared (B) covalent bonding.

    - Explanation: Covalent bonding occurs when atoms share pairs of electrons through the overlapping of their orbitals.

11. Which one of the following changes is physical? (D) Cooling water to obtain ice.

    - Explanation: Cooling water to obtain ice is a physical change because it involves a change of state without altering the chemical composition of the substance.

12. A side effect of soft water is that (D) it attacks lead contained in pipes.

    - Explanation: Soft water can be more corrosive to plumbing materials like lead pipes, leading to potential contamination.

13. Ammonia is very soluble in water because it is a/an (B) polar molecule.

    - Explanation: Ammonia (NH3) is polar due to its molecular geometry and the difference in electronegativity between nitrogen and hydrogen, leading to strong interactions with water molecules.

14. Citrus fruits such as lemon and grape taste sour because they contain (A) ascorbic acid and citric acid.

    - Explanation: The sour taste of citrus fruits is primarily due to the presence of ascorbic acid (vitamin C) and citric acid.

15. Soap lather is an example of a colloid in which a (C) gas is dispersed in a liquid.

    - Explanation: Soap lather is a colloid where gas bubbles are dispersed in a liquid (soapy water).

16. The air around a petroleum refinery is most likely to contain (C) SO2, CO, and NO2.

    - Explanation: Emissions from petroleum refineries commonly include sulfur dioxide (SO2), carbon monoxide (CO), and nitrogen dioxide (NO2).

17. The difference between colloids and suspensions is brought out clearly by the fact that while colloids (D) do not settle out on standing, suspensions do.

    - Explanation: Colloids remain evenly dispersed and do not settle out over time, whereas suspensions contain larger particles that settle out under the influence of gravity.

18. The following substances are non-electrolytes except (E) C and D.

Explanation: Both acetic acid (C) and NaCl (D) are electrolytes. Acetic acid is a weak electrolyte, meaning it partially dissociates into ions in solution, while NaCl is a strong electrolyte that fully dissociates into ions in solution. Chloroform (A) and sugar cane (B) do not dissociate into ions in solution, making them non-electrolytes.

19. 200 cm³ of air was passed over heated copper in a syringe several times to produce copper (II) oxide. When cooled, the final volume of air recorded was 158 cm³. Estimate the percentage of oxygen in the air. (C) 21%

   - Explanation: The volume of oxygen reacted is 200 cm³ - 158 cm³ = 42 cm³. The percentage of oxygen is (42 cm³ / 200 cm³) * 100 = 21%.

20. 30 cm³ of oxygen at 10 atmosphere pressures is placed in a 20 dm³ container. Calculate the new pressure if temperature is kept constant. (B)  15.0 atm

   - Explanation: Using Boyle's Law (P₁V₁ = P₂V₂), we have (10 atm)(30 cm³) = P₂(20 dm³). Converting cm³ to dm³ (30 cm³ = 0.03 dm³), we get P₂ = (10 atm * 0.03 dm³) / 20 dm³ = 0.015 

21. Bond dissociation energy of 500 KJ mol⁻¹ may be assigned to (A)

22. A mixture of iron and sulphur can be separated by dissolving the mixture in (B) dilute hydrochloric acid.

   - Explanation: Iron reacts with dilute hydrochloric acid to form soluble iron chloride, while sulfur remains undissolved.

23. A mixture of sand, ammonium chloride, and sodium chloride is best separated by (A) sublimation followed by addition of water and filtration.

   - Explanation: Ammonium chloride sublimates upon heating, leaving sand and sodium chloride, which can then be separated by dissolving sodium chloride in water and filtering out the sand.

24. A pure solid usually melts (B) over a narrow range of temperature.

   - Explanation: A pure substance has a specific melting point and thus melts over a narrow temperature range.

25. A small quantity of solid ammonium chloride was heated gently in a test tube; the solid gradually disappeared to produce a mixture of two gases. Later a white cloudy deposit was observed on the cooler part of the test tube. The ammonium chloride is said to have undergone (B) sublimation.

   - Explanation: Sublimation is the process where a solid turns directly into a gas without passing through the liquid state, and then condenses back to a solid on cooling.

26. CH₄ has this geometry: (C) tetrahedral.

   - Explanation: Methane (CH₄) has a tetrahedral geometry due to the sp³ hybridization of the carbon atom.

27. Chlorine, consisting of two isotopes of mass numbers 35 and 37, has an atomic mass of 35.5. The relative abundance of the isotope of mass number 37 is (B) 25.

   - Explanation: The atomic mass is the weighted average of the isotopes. With an average atomic mass of 35.5, the relative abundance of Cl-37 can be calculated to be 25%.

28. Elements P, Q, R, S, have 6, 11, 15, and 17 electrons respectively, therefore (D) Q will form an electrovalent bond with S.

    - Explanation: Q (sodium, 11 electrons) will form an electrovalent (ionic) bond with S (chlorine, 17 electrons).

29. In the oil drop experiment, Millikan determined the (C) charge of the electron.

    - Explanation: Millikan's oil drop experiment was designed to measure the electric charge of a single electron.

30. One of these atomic shells is the most stable (C) K.

    - Explanation: The K shell (first energy level) is the closest to the nucleus and thus the most stable.

31. Sieving is a technique used to separate mixtures containing (C) sand and ammonium chloride.

    - Explanation: Sieving separates components based on size; sand and ammonium chloride can be separated if they are of different particle sizes.

32. The abnormally high boiling point of water is primarily due to (E) hydrogen bonding.

    - Explanation: Hydrogen bonding between water molecules leads to a higher boiling point compared to other molecules of similar size.

33. The energy required to remove an electron from the isolated gaseous atom is known as (C) ionisation energy.

    - Explanation: Ionisation energy is the energy needed to remove an electron from a gaseous atom or ion.

34. The group that oxygen belongs to is collectively called (C) chalcogens.

    - Explanation: Oxygen is part of the chalcogen group, which includes elements in Group 16 of the periodic table.

35. The hydrolysis of NH4Cl salt will give (A) acidic solution

Explanation: When NH4Cl (ammonium chloride) hydrolyzes in water, it forms NH4+ and Cl-. NH4+ acts as a weak acid, donating protons to water and producing H3O+ ions, making the solution acidic.

36. The pollutants that are likely to be present in an industrial environment are (A) H2S, SO2 and oxides of nitrogen

Explanation: Industrial environments often emit hydrogen sulfide (H2S), sulfur dioxide (SO2), and various nitrogen oxides due to combustion processes and chemical reactions.

37. What is the basicity of tetraoxosulphate (VI) acid? (A) 2

Explanation: Tetraoxosulphate (VI) acid (H2SO4) can donate two protons per molecule, giving it a basicity of 2.

38. Which of the following gases is the most dangerous pollutant? (D) Carbon (II) oxide

Explanation: Carbon (II) oxide (CO) is highly toxic because it binds with hemoglobin in the blood more effectively than oxygen, reducing the blood's oxygen-carrying capacity.

39. Which of the following ions is a pollutant in drinking water even in trace amounts? (B) Hg2+

Explanation: Mercury ions (Hg2+) are highly toxic and can cause serious health issues even at very low concentrations, making them dangerous pollutants in drinking water.

40. Which of the following is an example of a double salt (B) alum

Explanation: Alum is a double salt consisting of two different cations, usually aluminum and a monovalent cation, and it crystallizes from a solution containing the two salts.

41. 0.16g of methane when burnt increases the temperature of 100g of water by 40 degrees Celsius, what is the heat of combustion of methane if the heat capacity of water is 4.2 Jg-1°C-1? (E) 1,680kJmol-1

42. A charged car battery has (A) chemical energy that can be converted into another energy called electrical energy

Explanation: A charged car battery stores chemical energy, which is converted into electrical energy when the battery is used to power electrical components in the car.

43. Copper oxide is heated with charcoal to produce carbon monoxide and copper. The reaction is an example of (A) both oxidation and reduction

Explanation: This reaction is a redox reaction where copper oxide is reduced to copper and charcoal (carbon) is oxidized to carbon monoxide.

44. For each oxygen atom in hydrogen peroxide which acts as an oxidant, there is an oxygen atom which acts as a/an (B) reductant

Explanation: In hydrogen peroxide (H2O2), one oxygen atom can act as an oxidant while the other acts as a reductant in redox reactions.

45. Given the change of phase: CO2(g) changes to CO2(s), the entropy of the system (A) decreases

Explanation: The transition from gas to solid involves a decrease in entropy because the molecules in the solid state have less freedom of movement compared to the gas state.

46. In which of the following is the entropy change positive? (C) Dissolution of sodium metal in water

Explanation: The dissolution of sodium metal in water increases the disorder (entropy) of the system as the solid metal dissolves and forms ions in solution.

47. One of these elements is the best reducing agent. (B) Rb

Explanation: Rubidium (Rb) is a very reactive alkali metal and a strong reducing agent, easily losing electrons to form positive ions.

48. The name of the gas driven off at the negative electrode during the electrolysis of brine is (B) chlorine 

49. The oxidation state of P in H2P2O7 2- is (D) +5

Explanation: In the pyrophosphate ion (P2O7 4-), each phosphorus atom has an oxidation state of +5.

50. The oxidation state of S in Ca(HSO3)2 is (C) +4

Explanation: In calcium bisulfite (Ca(HSO3)2), the oxidation state of sulfur in the bisulfite ion (HSO3 -) is +4.

51. The oxidizing agent in the reaction, 3Br2 + 6OH- = BrO3- + 5Br- + 3H2O is (A) Br2

Explanation: In this reaction, bromine (Br2) is reduced to bromide ions (Br-) and acts as the oxidizing agent.

52. When heat is absorbed during a chemical reaction, the reaction is said to be (D) endothermic

Explanation: An endothermic reaction absorbs heat from its surroundings, resulting in a temperature decrease in the surroundings.

53. Given the reaction at equilibrium: 2CO(g) + O2(g) ↔ 2CO2(g). When the reaction is subjected to stress, a change will occur in the concentration of (C) both reactants and products

Explanation: According to Le Chatelier's Principle, when a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will adjust its position to counteract the change, affecting the concentrations of both reactants and products.

54. Given the reaction at equilibrium: N2(g) + O2(g) ↔ 2NO(g). As the concentration of N2(g) increases, the concentration of O2(g) will (A) decrease

Explanation: Increasing the concentration of N2(g) will shift the equilibrium to the right, consuming more O2(g) to form NO(g), thus decreasing the concentration of O2(g).

55. If a reaction is exothermic and there is a great disorder, it means that (B) There will be a large increase in free energy