Every Book Back multiple-choice question from p-Block Elements-II (12th Standard Chemistry, Samacheer Kalvi) — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
In which of the following, \(NH_3\) is not used?
- A. Nessler's reagentCorrect
- B. Reagent for analysis of \(Cu^{2+}\) ions
- C. Manufacture of fertilizers
- D. Tollen's reagent
Explanation. Ammonia is used to prepare fertilizers and complex salts, and is a part of Tollen's reagent, but Nessler's reagent is \(K_2[HgI_4]\).
Q2
Which is true regarding nitrogen?
- A. least electronegative element
- B. has low ionisation enthalpy than oxygen
- C. d- orbitals available
- D. ability to form \(p\pi-p\pi\) bonds with itselfCorrect
Explanation. Nitrogen's small size and high electronegativity allow it to form strong triple bonds using \(p\pi-p\pi\) overlap.
Q3
An element belongs to group 15 and 3rd period of the periodic table, its electronic configuration would be
- A. \(1s^2 2s^2 2p^4\)
- B. \(1s^2 2s^2 2p^3\)
- C. \(1s^2 2s^2 2p^6 3s^2 3p^2\)
- D. \(1s^2 2s^2 2p^6 3s^2 3p^3\)Correct
Explanation. Group 15 elements have a \(ns^2 np^3\) valence shell; phosphorus in the third period has \(n=3\).
Q4
Solid (A) reacts with strong aqueous NaOH liberating a foul smelling gas(B) which spontaneously burn in air giving smoky rings. A and B are respectively
- A. \(P_4\) (red) and \(PH_3\)
- B. \(P_4\) (white) and \(PH_3\)Correct
- C. \(S_8\) and \(H_2S\)
- D. \(P_4\) (white) and \(P_2H_4\)
Explanation. White phosphorus reacts with sodium hydroxide to produce phosphine gas, which forms smoke rings of \(P_4O_{10}\) when it ignites.
Q5
On heating with concentrated NaOH solution in an inert atmosphere of \(CO_2\), white phosphorus gives a gas (B) and a compound (C). The oxidation state of phosphorus in (B) and (C) respectively are
- A. -3, +1Correct
- B. +3, +1
- C. +1, +1
- D. -3, -3
Explanation. In the disproportionation of white phosphorus, phosphorus is reduced to \(PH_3\) (oxidation state -3) and oxidized to \(NaH_2PO_2\) (+1).
Q6
Which of the following is the strongest acid?
- A. \(H_3PO_2\)Correct
- B. \(H_3PO_3\)
- C. \(H_3PO_4\)
- D. \(H_4P_2O_7\)
Explanation. Hypophosphorous acid is the strongest because the acid strength of phosphorus oxoacids increases as the oxidation state of phosphorus decreases.
Q7
Which of the following is correct for \(P_4\) molecule of white phosphorus?
- A. It contains 6 P-P single bondsCorrect
- B. It contains 4 P-P single bonds
- C. It contains 4 lone pairs of electrons
- D. It has a planar structure
Explanation. White phosphorus has a tetrahedral structure with four phosphorus atoms at the corners, resulting in six P-P covalent bonds.
Q8
Which of the following noble gas is used in magnetic resonance imaging (MRI) systems?
- A. HeliumCorrect
- B. Neon
- C. Argon
- D. Krypton
Explanation. Liquid helium is used as a coolant to maintain superconducting magnets in MRI machines at extremely low temperatures.
Q9
In the case of nitrogen, \(NCl_3\) is possible but not \(NCl_5\) while in case of phosphorus, \(PCl_3\) as well as \(PCl_5\) are possible. It is due to
- A. Availability of vacant d orbitals in P but not in N
- B. Lower electronegativity of P than N
- C. Lower ionisation energy of P than N
- D. all of theseCorrect
Explanation. Nitrogen cannot expand its octet because it lacks vacant d-orbitals in its second energy shell, preventing the formation of five bonds.
Q10
The basicity of pyrophosphoric acid (\(H_4P_2O_7\)) is
- A. 2
- B. 4Correct
- C. 3
- D. 5
Explanation. Pyrophosphoric acid contains four replaceable hydrogen atoms attached to oxygen, making it a tetrabasic acid.
Q11
The molarity of given orthophosphoric acid is 2M. Its normality is
- A. 2N
- B. 6NCorrect
- C. 4N
- D. 0.6N
Explanation. Normality equals molarity multiplied by basicity; orthophosphoric acid is tribasic, so \(2 M \times 3 = 6 N\).
Q12
Assertion: \(PCl_5\) is ionic in solid state. Reason: In solid state, it exists as \([PCl_4]^+\) and \([PCl_6]^-\).
- A. Both A and R are true and R is not the correct explanation
- B. A is true but R is false
- C. Both A and R are false
- D. Both A and R are true and R is the correct explanationCorrect
Explanation. Phosphorus pentachloride exists as a salt composed of tetrahedral \([PCl_4]^+\) cations and octahedral \([PCl_6]^-\) anions in its crystalline form.
Q13
Which of the following is the correct order of increasing oxidizing power?
- A. \(F_2 < Cl_2 < Br_2 < I_2\)
- B. \(F_2 > Cl_2 > Br_2 > I_2\)
- C. \(Cl_2 < Br_2 < F_2 < I_2\)
- D. \(I_2 < Br_2 < Cl_2 < F_2\)Correct
Explanation. Oxidizing power increases up the halogen group because standard reduction potential becomes more positive from iodine to fluorine.
Q14
The magnetic property of the oxygen molecule is
- A. diamagnetic
- B. ferromagnetic
- C. paramagneticCorrect
- D. ferrimagnetic
Explanation. Molecular orbital theory shows that oxygen has two unpaired electrons in antibonding \(\pi^*\) orbitals, making it paramagnetic.
Q15
Which of the following statement is true for the structure of ozone?
- A. \(O-O\) bond lengths are equalCorrect
- B. it is linear
- C. \(O-O-O\) bond angle is \(120^\circ\)
- D. none of these
Explanation. Due to resonance, the two oxygen-oxygen bonds in ozone are identical in length, lying between single and double bond values.
Q16
The correct order of the thermal stability of hydrogen halides is
- A. \(HI > HBr > HCl > HF\)
- B. \(HF > HCl > HBr > HI\)
- C. \(HCl > HF > HBr > HI\)
- D. \(HI < HBr < HCl < HF\)Correct
Explanation. Bond dissociation energy decreases from HF to HI as the halogen size increases, making HF the most thermally stable.
Q17
Which of the following is the strongest oxidizing agent?
- A. \(Cl_2\)
- B. \(Br_2\)
- C. \(I_2\)
- D. \(F_2\)Correct
Explanation. Fluorine is the most electronegative element and has the highest reduction potential, making it the most powerful oxidizing agent.
Q18
In which of the following compounds, the oxidation state of oxygen is +2?
- A. \(H_2O_2\)
- B. \(O_2F_2\)
- C. \(OF_2\)Correct
- D. \(H_2O\)
Explanation. Oxygen is more electronegative than all elements except fluorine; in \(OF_2\), fluorine takes the -1 state, leaving oxygen at +2.
