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Chemistry MCQs for NEET — Practice Questions with Answers

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Which of the following has maximum number of unpaired electrons?

Fe²⁺

Fe³⁺

Co³⁺

Co²⁺

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Explanation

Fe3+ has the maximum number of unpaired electrons (5 unpaired electrons) among the given options. The electronic configuration of Fe3+ is [Ar] 3d5, where all five 3d electrons are unpaired, aligning their spins parallel due to the strong exchange energy.

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The atomc number of potassium is 19 & that of manganese is 25. Although the colour of MnO₄^- is dark violet yet the K⁺ is colourless.This is due to the fact that

Mn is a transition element; while K⁺ is not

[MnO₄]^- is negatively charged; while K⁺ has a positive charge.

The effective atomic number of Mn in [MnO₄]^- is 26; while for K⁺ , the atomic number s 18

The Mn in a high positive oxidation state allow charge transfer transitions.

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Explanation

In [MnO₄]^-the oxidation state of Mn is +7 (d^o configuration),expected to be colourless but [MnO₄]^-is coloured due to transfer of charge from oxygen to manganese.

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Heating of Ag with dil. HNO₃ give

NO₂

N₂O

N₂O₃

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Explanation

$2 H N O_{3} (d i l .) \to 2 N O + H_{2} O + 3 O 2 A g + 2 H N O_{3} + O \to 2 A g N O_{3} + H_{2} O] \times 3 6 A g + 8 H N O_{3} \to 6 A g N O_{3} + 2 N O + 4 H_{2} O$

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Zinc dissolve in an excess of NaOH, because of the formation of

ZnO

Zn(OH)₂

NaZn(OH)₃

Na₂ZnO₂

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Explanation

It react with NaOH on moderate heating & form soluble Zincate.

$Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$

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When excess of SnCl₂ is added to HgCl₂, the substance formed is

Hg₂Cl₂

Cl₂

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Explanation

$S n C l_{2} r e d u c e s H g C l_{2} t o H g_{2} C l_{2} a n d f i n a l l y t o H g 2 H g C l_{2} + S n C l_{2} \to H g_{2} C l_{2} (w h i t e p p t) + S n C l_{4} H g_{2} C l_{2} + S n C l_{2} \to 2 H g (b l a c k) + S n C l_{4}$

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The oxidation number is changed in which of the following case

SO₂ gas is passed into $C r_{2} O_{7}^{2 -} / H^{+}$

Aqueous solution of $C r O_{4}^{2 -}$ is acidified

$C r O_{2} C l_{2}$ is dissolved in NaOH

$C r_{2} O_{7}^{2 -}$ solution is made alkaline

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Explanation

$(A) C r_{2} O_{7}^{2 -} + S O_{2} + 2 H^{+} \to 2 C r^{3 +} + 3 S O_{4}^{2 -} + H_{2} O (c h a n g e) + 6 + 4 + 3 + 6 (B) 2 C r_{2} O_{4}^{2 -} + 2 H^{+} \to C r_{2} O_{7}^{2 -} + H_{2} O (N o c h a n g e) + 6 + 6 (C) C r O_{2} C l_{2} + 2 N a O H \to N a C r O_{4} (N o c h a n g e) + 6 + 6 (D) C r_{2} O_{7}^{2 -} + 2 O H^{-} \to 2 C r O_{4}^{2 -} + H_{2} O (N o c h a n g e) + 6 + 6$

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Which of the following elements form stable dinuclear ions

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Explanation

Hg(l) compound contain mercurous ions (Hg-Hg)²⁺ & not Hg⁺.

The two atoms are bonded together using 6s-orbital.

Zn & Cd also form dinuclear ion (Zn-Zn)²⁺ & (Cd-Cd)²⁺ ; however these ions are unstable & have only been detected in melts of Zn/ZnCl₂ &Cd/CdCl₂ respectively.

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A mixture of Mn²⁺ & Zn²⁺ can be separated by using an excess of

NH₄OH

NaOH

H₂SO₄

HNO₃

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Explanation

$M n^{2 +} + 2 N a O H \to M n (O H)_{2} + 2 N a^{+} (p p t) Z n^{2 +} + 2 N a O H \to Z n (O H)_{2} + 2 N a^{+} (p p t) Z n (O H)_{2} + 2 N a O H \to N a_{2} [Z n (O H)_{2}] S o l u b l e c o m p l e x$

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The correct electronic configuration of Ti(Z=22 atom) is

$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{2}$

$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{2} 4 p^{6}$

$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{4}$

$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{2}$

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Explanation

Ti(Z=22) is $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 3 d^{2}$

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Of the ions, Zn²⁺, Ni²⁺and Cr³⁺(atomic number of Zn =30, Ni=28 and Cr=24).

Only Zn²⁺is Colourless and Ni²⁺and Cr³⁺are coloured

All these are colourless

All these are coloured

Only Ni²⁺is coloured and Zn²⁺and Cr³⁺are colourless.

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Explanation

Zn²⁺is Colourless due to absence of any unpaired electron.Ni²⁺and Cr³⁺are coloured due to 2 and 3 unpaired electrons respecively.

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