List of top Chemistry Questions asked in CBSE CLASS XII

In what way is the electronic configuration of the transition elements different from that of the non-transition elements?

What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

What may be the stable oxidation state of the transition element with the following d electron configurations in the ground state of their atoms : 3d3 ,3d5 ,3d8 and 3d4?

To what extent do the electronic configurations decide the stability of oxidation states in the first series of the transition elements? Illustrate your answer with examples.

Why are Mn2+ compounds more stable than Fe2+ towards oxidation to their +3 state?

Henry’s law constant for the molality of methane in benzene at 298 K is \(4.27 \times 105\, mm\, Hg\). Calculate the solubility of methane in benzene at 298 K under 760 mm Hg.
Vapour pressure of water at 293 K is 17.535 mm Hg. Calculate the vapour pressure of water at 293 K when 25 g of glucose is dissolved in 450 g of water.
19.5 g of \(CH_2 FCOOH\) is dissolved in 500 g of water. The depression in the freezing point of water observed is \(1.00\degree C\). Calculate the van’t Hoff factor and dissociation constant of fluoroacetic acid.
Calculate the depression in the freezing point of water when 10 g of \(CH_3CH_2CHClCOOH\) is added to 250 g of water. \(K_a = 1.4 × 10^{–3} , K_f = 1.86 K kg mol^{–1} .\)
Although phenoxide ion has more number of resonating structures than carboxylate ion,carboxylic acid is a stronger acid than phenol.Why?
The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.
Calculate the amount of benzoic acid \((C_6H_5COOH)\) required for preparing 250 mL of 0.15 M solution in methanol.
Nalorphene \( (C_{19}H_{21}NO_3 )\), similar to morphine, is used to combat withdrawal symptoms in narcotic users. Dose of nalorphene generally given is 1.5 mg. Calculate the mass of \(1.5 \times 10^{–3} m\) aqueous solution required for the above dose.
Calculate the mass percentage of aspirin \((C_9H_8O_4 )\) in acetonitrile \((CH_3CN)\) when 6.5 g of \(C_9H_8O_4\) is dissolved in 450 g of \(CH_3CN.\)

When 3-methyl butane-2-ol is treated with HBr, the following reaction takes place:
Give a mechanism for this reaction. (Hint: The secondary carbocation formed in step II rearranges to a more stable tertiary carbocation by a hydride ion shift from 3rd carbon atom

Show how would you synthesise the following alcohols from appropriate alkenes?

If the solubility product of CuS is \(6 × 10^{–16}\), calculate the maximum molarity of CuS in aqueous solution.
An organic compound contains 69.77% carbon,11.63% hydrogen and rest oxygen.The molecular mass of the compound is 86.It does not reduce Tollens’ reagent but forms an addition compound with sodium hydrogensulphite and give positive iodoform test.On vigorous oxidation it gives ethanoic andpropanoic acid.Writethe possible structure of the compound.

Write the mechanism of the reaction of HI with methoxymethane.

If the density of some lake water is \(1.25g mL^{–1}\) and contains 92 g of \(Na^+\) ions per kg of water, calculate the molality of \(Na^+\) ions in the lake

Explain the fact that in aryl alkyl ethers

(i)the alkoxy group activates the benzene ring towards electrophilic substitution and 

(ii)it directs the incoming substituents to ortho and para positions in the benzene ring.

Give plausible explanation for each of the following:

(i) Cyclohexanone forms cyanohydrin in good yield but 2,2,6-trimethylcyclohexanone does not.

(ii)There are two –NH2 groups in semicarbazide.However,only one is involved in the formation of semicarbazones.

(iii)During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst,the water or the ester should be removed as soon as it is formed.
 

Amongst the following compounds, identify which are insoluble, partially soluble and highly soluble in water? 
(i) phenol (ii) toluene (iii) formic acid 
(iv) ethylene glycol (v) chloroform (vi) pentanol.

Complete each synthesis by giving missing starting material, reagent or products.

 

Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.