 JEE Mains Previous Years Questions with Solutions

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1

JEE Main 2013 (Offline)

Stability of the species Li2, $Li_2^−$ and $Li_2^+$ increases in the order of:
A
Li2 < $Li_2^+$ < $Li_2^-$
B
$Li_2^+$ < $Li_2^-$ < Li2
C
$Li_2^-$ <$Li_2^+$ < Li2
D
$Li_2^-$ < Li2 < $Li_2^+$

Explanation

Li2 = ${\sigma _{1{s^2}}}\,\,\sigma _{1{s^2}}^ * \,$ ${\sigma _{2{s^2}}} \,$

$\therefore$ Bond order = ${1 \over 2}\left( {4 - 2} \right)$ = 1

$Li_2^+$ = ${\sigma _{1{s^2}}}\,\,\sigma _{1{s^2}}^ * \,$ ${\sigma _{2{s^1}}} \,$

$\therefore$ Bond order = ${1 \over 2}\left( {3 - 2} \right)$ = 0.5

$Li_2^-$ = ${\sigma _{1{s^2}}}\,\,\sigma _{1{s^2}}^ * \,$ ${\sigma _{2{s^2}}} \,\,\sigma _{1{s^1}}^ * \,$

$\therefore$ Bond order = ${1 \over 2}\left( {4 - 3} \right)$ = 0.5

The bond order of $Li_2^+$ and $Li_2^-$ is same but $Li_2^+$ is more stable than $Li_2^-$ because $Li_2^+$ is smaller in size and has 2 electrons in antibonding orbitals whereas $Li_2^-$ has 3 electrons in antibonding orbitals. Hence $Li_2^+$ is more stable than $Li_2^-$.
2

JEE Main 2013 (Offline)

In which of the following pairs of molecules/ions, both the species are not likely to exist?
A
$H_2^+$ , $He_2^{2-}$
B
$H_2^-$ , $He_2^{2-}$
C
$H_2^{2+}$ , $He_2$
D
$H_2^-$ , $He_2^{2+}$

Explanation

$H_2^{2+}$ , $He_2$

Both have zero bond order. Thus, they do not exist.
3

AIEEE 2012

In which of the following pairs the two species are not isostructural ?
A
$CO_3^{2-}$ and $NO_3^-$
B
$PCl_4^{+}$ and $SiCl_4$
C
PF5 and BrF5
D
$AlF_6^-$ and $SF_6$

Explanation

PF5 $\Rightarrow$ sp3d hybridized and shape is trigonal bipyramidal.

BrF5 $\Rightarrow$ 5 bond pair and 1 lone pair present so sp3d2 hybridized and shape is square pyramidal.
4

AIEEE 2012

Ortho–Nitrophenol is less soluble in water than p– and m– Nitrophenols because :
A
o–Nitrophenol is more volatile in steam than those of m – and p–isomers
B
o–Nitrophenol shows Intramolecular H–bonding
C
o–Nitrophenol shows Intermolecular H–bonding
D
Melting point of o–Nitrophenol is lower than those of m–and p–isomers.

Explanation

As in ortho-Nitrophenol H of -OH is already in Intra-moleculer H Bond and become stable so it will not perticipate in H bonding with H2O by breaking this H bonding. That is why solublity of Ortho–Nitrophenol is less in water. para– and meta– Nitrophenols both shows inter-moleculer H bonding with H2O. That is why both are more soluble in water.  