### JEE Mains Previous Years Questions with Solutions

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1

### AIEEE 2006

According to Bohr's theory, the angular momentum of an electron in 5th orbit is
A
10 $h/\pi$
B
2.5 $h/\pi$
C
25 $h/\pi$
D
1.0 $h/\pi$

## Explanation

Formula of angular momentum of an electron,

mvr = ${{nh} \over {2\pi }}$

here n = 5

$\therefore$ mvr = ${{5h} \over {2\pi }}$ = 2.5${{h} \over {\pi }}$
2

### AIEEE 2005

Pick out the isoelectronic structure from the following :

\eqalign{ & \left( i \right)\,\,\,\,\,\,C{H_3}^ + \cr & \left( {ii} \right)\,\,\,\,{H_3}{O^ + } \cr & \left( {iii} \right)\,\,\,N{H_3} \cr & \left( {iv} \right)\,\,\,\,C{H_3}^ - \cr}
A
$(i)$ and $(ii)$
B
$(iii)$ and $(iv)$
C
$(i)$ and $(iii)$
D
$(ii), (iii)$ and $(iv)$

## Explanation

Ions No. of electrons
CH3+ 8
H3O+ 10
NH3 10
CH3- 10

$\therefore$ $\,\,\,$ ${H_3}{O^ + },N{H_3}$ and $C{H_3}^ -$ are isoelectronic.
3

### AIEEE 2005

In a multi-electron atom, which of the following orbitals described by the three quantum members will have the same energy in the absence of magnetic and electric fields?
(A) n = 1, l = 0, m = 0
(B) n = 2, l = 0, m = 0
(C) n = 2, l = 1, m = 1
(D) n = 3, l = 2, m = 1
(E) n = 3, l = 2, m = 0
A
(D) and (E)
B
(C) and (D)
C
(B) and (C)
D
(A) and (B)

## Explanation

As here is no electric and magnetic field so ignore m to calculate the energy of orbital.

As here atom is multi-electron so ( n + l ) rule is applicable here. This rule says those orbitals which have more value of ( n + l ) will have more energy.

In (A), n + l = 1 + 0 = 1

In (B), n + l = 2 + 0 = 2

In (C), n + l = 2 + 1 = 3

In (D), n + l = 3 + 2 = 5

In (E), n + l = 3 + 2 = 5

So (D) and (E) will be of equal energy.
4

### AIEEE 2005

Of the following sets which one does NOT contain isoelectronic species?
A
$BO_3^{3 - }$, $CO_3^{2 - }$, $NO_3^{- }$
B
$SO_3^{2 - }$, $CO_3^{2 - }$, $NO_3^{-}$
C
$CN^{- }$, $N_2$, $C_2^{2 - }$
D
$PO_4^{3 - }$, $SO_4^{2 - }$, $ClO_4^{ - }$

## Explanation

In $SO_3^{2 - }$ no of electrons = 16 + 24 + 2 = 42

In $CO_3^{2 - }$ no of electrons = 6 + 24 + 2 = 32

In $NO_3^{-}$ no of electrons = 7 + 24 + 1 = 32

So they are not isoelectronic.