SENIOR ELEMENTARY TEACHER CHEMISTRY PAST PAPER 2020 TO DATE
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چار آپشن میں سے کسی ایک پر کلک کرنے سے جواب سرخ ہو جائے گا۔
- 1.5
- 2
- 2.5
- None of these
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Explanation
- Bond order is calculated using Molecular Orbital Theory.
- Formula: Bond order = (Bonding electrons − Antibonding electrons) / 2
- O₂ has bond order = 2.
- O₂⁺ means one electron is removed from antibonding orbital.
- Removing antibonding electron increases bond order by 0.5.
- So, bond order of O₂⁺ = 2.5.
Last verified on 19-05-2026
مستقل درجہ حرارت اور دباؤ پر ردِ عمل کب خود بخود ہوتا ہے؟
- ΔG > 0
- ΔG = 0
- ΔG < 0
- None of these
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Explanation
- Spontaneity of a reaction is decided by Gibbs free energy (ΔG).
- At constant temperature and pressure, ΔG is the key factor.
- A reaction proceeds on its own when energy is released.
- This means the system moves to a lower energy state.
- So, Gibbs free energy must be negative.
Last verified on 19-05-2026
ایتھانوائل کلورائیڈ کے نیوکلیوفیلک ایسیل سبسٹی ٹیوشن میں لیونگ گروپ کون سا ہوتا ہے؟
- OH⁻
- Cl⁻
- CH₃⁻
- None of these
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Explanation
- Nucleophilic acyl substitution occurs in acid derivatives like acyl chlorides.
- Ethanoyl chloride reacts by replacing the halogen atom.
- The carbonyl carbon is attacked by a nucleophile.
- The group that leaves must be stable after departure.
- Chloride ion (Cl⁻) is a good leaving group due to its stability.
Last verified on 19-05-2026
- Propanol
- 1-bromopropane
- 2-bromopropane
- None of these
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Explanation
- Markovnikov’s rule explains addition of HX to unsymmetrical alkenes.
- Hydrogen (H) attaches to the carbon with more hydrogens already.
- Bromine (Br) attaches to the more substituted carbon.
- Propene (CH₃–CH=CH₂) follows this rule during addition of HBr.
- This forms a more stable secondary carbocation intermediate.
Last verified on 19-05-2026
کریٹیکل درجہ حرارت کیا ہے؟ اس سے اوپر کیا ہوتا ہے؟
- Gas cannot be liquefied by pressure alone
- Gas freezes immediately
- Gas becomes plasma
- None of these
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Explanation
- Critical temperature is a key concept in gas liquefaction.
- It is the highest temperature at which a gas can be liquefied.
- Above this temperature, gas particles have too much kinetic energy.
- No amount of pressure alone can bring molecules close enough to liquefy.
- So liquefaction becomes impossible above this point.
Last verified on 19-05-2026
ان میں سب سے مضبوط لیگنڈ کون سا ہے؟
- Cl⁻
- H₂O
- CN⁻
- None of these
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Explanation
- Ligands are ions or molecules that donate electron pairs to metal ions.
- Their strength is explained by the spectrochemical series.
- Strong ligands cause greater splitting of d-orbitals.
- CN⁻ produces strong crystal field splitting and electron pairing.
- Therefore, it is considered a strong field ligand.
Last verified on 19-05-2026
- Square planar and paramagnetic
- Tetrahedral and paramagnetic
- Square planar and diamagnetic
- None of these
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Explanation
- In this complex, nickel is in +2 oxidation state.
- Ni²⁺ has electronic configuration: 3d⁸.
- CN⁻ is a strong field ligand and causes electron pairing.
- This leads to dsp² hybridization with square planar geometry.
- All electrons become paired, making the complex diamagnetic.
Last verified on 19-05-2026
کون سا آئن سب سے زیادہ غیر جفت الیکٹران رکھتا ہے؟
- Mn²⁺
- Fe²⁺
- Co²⁺
- None of these
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Explanation
- Unpaired electrons depend on d-orbital electronic configuration.
- Mn²⁺ has configuration: [Ar] 3d⁵.
- According to Hund’s rule, all five d-electrons remain unpaired.
Last verified on 19-05-2026
کون سا مالیکیول 120° کے قریب بانڈ اینگل رکھتا ہے؟
- CH₄
- NH₃
- BF₃
- None of these
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Explanation
- Bond angle depends on molecular geometry and electron pair repulsion.
- Trigonal planar molecules have bond angles close to 120°.
- BF₃ has no lone pair on central atom, so perfect trigonal planar shape.
Last verified on 19-05-2026
سب سے زیادہ آئنک خصوصیت کس میں ہے؟
- KBr
- LiF
- NaCl
- None of these
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Explanation
- Ionic character depends on electronegativity difference between atoms.
- Greater difference → more ionic nature.
- Lithium (Li) and Fluorine (F) have a very large electronegativity gap.
- Small size of Li⁺ also increases polarizing power, but F⁻ is very electronegative.
- So LiF shows the strongest ionic bond among given options.
Last verified on 19-05-2026
