List of top Chemistry Questions asked in CBSE CLASS XII

The half-life period of a radioactive element is \(1.5 \times 10^{10}\) years. Calculate the time in which the activity of the element is reduced to 75% of its original value. \[ \text{Given : } \log 2 = 0.30,\; \log 3 = 0.48,\; \log 4 = 0.60 \]
State Kohlrausch's law of independent migration of ions. With the help of a curve, explain why it is not easy to determine \(\Lambda_m^\circ\) for weak electrolytes by extrapolating the concentration--molar conductivity curve, as it is for strong electrolytes.
How will you obtain the following from benzene diazonium chloride ? Write chemical equations involved : (b) Iodobenzene
How will you obtain the following from benzene diazonium chloride ? Write chemical equations involved : (a) Phenol
Give reasons for the following : (c) CH$_3$--I is more reactive than CH$_3$--Br towards SN2 reactions.
How will you obtain the following from benzene diazonium chloride ? Write chemical equations involved : (c) p-Aminoazobenzene
An organic compound (A) with molecular formula C$_3$H$_5$N on reaction with C$_6$H$_5$MgBr followed by hydrolysis, gives a compound (B). Compound (B) forms an orange-red precipitate with 2,4-DNP reagent and does not give iodoform test. It neither reduces Tollens' or Fehling's reagent nor does it decolourise bromine water. On drastic oxidation with chromic acid it gives a carboxylic acid (C) having molecular formula C$_7$H$_6$O$_2$. Identify the compounds (A), (B) and (C). Write the reactions of compound (A) with C$_6$H$_5$MgBr followed by hydrolysis to give compound (B).
An antifreeze solution is prepared by dissolving 31 g of ethylene glycol (Molar mass = 62 g mol$^{-1}$) in 600 g of water. Calculate the freezing point of the solution. (K$_f$ for water = 1.86 K kg mol$^{-1}$)
Give reasons for the following : (b) p-Dichlorobenzene has higher melting point than those of o- and m-isomers.
Give reasons for the following : (a) Alkyl halides, though polar, yet their solubility in water is very low.
How do you explain the following ? (a) Presence of a carbonyl group in glucose (b) Presence of five --OH groups in glucose
For a reaction A + B $\rightarrow$ Products, the rate law is : \[ \text{Rate} = k[A]^{\frac{3}{2}}[B] \] Write the overall order of the reaction. Can this reaction be an elementary reaction ? Give reason in support of your answer.
For the compound having molecular formula C$_4$H$_9$Br, write : (a) the isomer which is most reactive towards SN1 displacement. (b) the isomer which, on reacting with Na metal in the presence of dry ether, gives 2,5-Dimethylhexane.
Assertion (A) : Phenol is more acidic than 4-methylphenol. Reason (R) : The presence of an electron releasing group in phenol makes it more acidic.
Assertion (A) : Order of reaction is not applicable for elementary reaction but applicable for complex reaction. Reason (R) : Order of reaction is an experimental quantity.
Explain why, on addition of 1 mol of KCl to 1 litre of water, the boiling point of water increases, while the addition of 1 mol of methyl alcohol to 1 litre of water decreases the boiling point.
Write IUPAC names of the following compounds : \[ (i)\ [PtCl_2(en)_2]SO_4 \] \[ (ii)\ (NH_4)_2[CoF_4] \]
Assertion (A) : Aromatic primary amines can easily be prepared by Gabriel phthalimide synthesis. Reason (R) : Gabriel phthalimide synthesis is used to prepare only aliphatic primary amines.
Assertion (A) : Separation of Zr and Hf is difficult. Reason (R) : Zr and Hf have almost identical atomic and ionic radii due to lanthanoid contraction.
For the reaction \(2A \rightarrow 3B\), the rate of reaction \(\frac{d[B]}{dt}\) is equal to :
The boiling point of an azeotropic mixture of water and nitric acid is more than that of pure water and nitric acid. The mixture shows :
On hydrolysis, which of the following carbohydrates gives only glucose ?
In aqueous acidified solution, CrO$_4^{2-}$ ion converts to which of the following ?
The dehydration of 1$^\circ$, 2$^\circ$ and 3$^\circ$ alcohols follows the order :
Which reagent is used to distinguish between C$_2$H$_5$NH$_2$ and (C$_2$H$_5$)$_2$NH ?