jee main chemistry important questions | d And f-Block Elements jee main questions FREE

Q 21 :

Consider the following reactions

$K_{2} {Cr}_{2} O_{7} \to_{- H_{2} O}^{KOH} [A] \to_{- H_{2} O}^{H_{2} {SO}_{4}} [B] + K_{2} {SO}_{4}$

The products [A] and [B], respectively are: [2025]

$K_{2} {CrO}_{4} and K_{2} {Cr}_{2} O_{7}$
$K_{2} {CrO}_{4} and CrO$
$K_{2} {CrO}_{4} and {Cr}_{2} O_{3}$
$K_{2} Cr {(OH)}_{6} and {Cr}_{2} O_{3}$

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(1)

Chromate $({CrO}_{4}^{2 -})$ and dichromate $({Cr}_{2} O_{7}^{2 -})$ are inter-convertible into each other depending upon the pH of the solution. In acidic medium, ${Cr}_{2} O_{7}^{2 -}$ is more stable and in basic medium, ${CrO}_{4}^{2 -}$ is more stable.

$K_{2} {Cr}_{2} O_{7} \to_{- H_{2} O}^{KOH} \underset{[A]}{K_{2} {CrO}_{4}} \to_{- H_{2} O}^{H_{2} {SO}_{4}} \underset{[B]}{K_{2} {Cr}_{2} O_{7}} + K_{2} {SO}_{4}$

Q 22 :

Preparation of potassium permanganate from ${MnO}_{2}$ involves a two-step process in which the first step is a reaction with $KOH$ and ${KNO}_{3}$ to produce [2025]

$K_{2} {MnO}_{4}$
$K_{3} {MnO}_{4}$
${KMnO}_{4}$
$K_{4} [Mn {(OH)}_{6}]$

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(1)

Potassium permanganate is prepared by fusion of ${MnO}_{2}$ (pyrolusite) with an alkali metal hydroxide and an oxidising agent. This produces the dark green $K_{2} {MnO}_{4}$ , which disproportionates in a neutral or acidic solution to give permanganate.

${MnO}_{2} + 2 KOH + {KNO}_{3} \to K_{2} {MnO}_{4} + {KNO}_{2} + H_{2} O$

$3 {MnO}_{4}^{2 -} + 4 H^{+} \to 2 {MnO}_{4}^{-} + {MnO}_{2} + 2 H_{2} O$

Q 23 :

Which of the following oxidation reactions are carried out by both $K_{2} {Cr}_{2} O_{7}$ and ${KMnO}_{4}$ in acidic medium? [2025]

(A)   $I^{-} \to I_{2}$

(B)    $S^{2 -} \to S$

(C)   ${Fe}^{2 +} \to {Fe}^{3 +}$

(D)   $I^{-} \to {IO}_{3}^{-}$

(E)   $S_{2} O_{3}^{2 -} \to {SO}_{4}^{2 -}$

Choose the correct answer from the options given below:

(C), (D) and (E) only
(A), (B) and (C) only
(B), (C) and (D) only
(A), (D) and (E) only

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(2)

Reactant	Product with acidic ${KMnO}_{4}$	Product with faintly alkaline ${KMnO}_{4}$	Product with acidic $K_{2} {Cr}_{2} O_{7}$
$I^{-}$	$I_{2}$	${IO}_{3}^{-}$	$I_{2}$
$S^{2 -}$	$S$	$S$	$S$
${Fe}^{2 +}$	${Fe}^{3 +}$	${Fe}^{3 +}$	${Fe}^{3 +}$
$S_{2} O_{3}^{2 -}$	${SO}_{4}^{2 -} + S$	${SO}_{4}^{2 -}$	${SO}_{4}^{2 -} + S$

Q 24 :

The amphoteric oxide among $V_{2} O_{3}, V_{2} O_{4}$ and $V_{2} O_{5}$ , upon reaction with alkali leads to formation of an oxide anion. The oxidation state of V in the oxide anion is: [2025]

+4
+7
+3
+5

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(4)

$V_{2} O_{3}$ – basic, $V_{2} O_{4}$ – less basic, $V_{2} O_{5}$ – amphoteric. $V_{2} O_{5}$ reacts with alkalies as well as acids to give ${VO}_{4}^{3 -}$ and ${VO}_{4}^{+}$ respectively. Oxidation number of V in ${VO}_{4}^{3 -}$ is +5.

Q 25 :

The molar mass of the water insoluble product formed from the fusion of chromite ore $({FeCr}_{2} O_{4})$ with ${Na}_{2} {CO}_{3}$ in presence of $O_{2}$ is ______ g ${mol}^{- 1}$ . [2025]

0%

(160)

$4 {FeCr}_{2} O_{4} + 8 {Na}_{2} {CO}_{3} + 7 O_{2} \to 8 {Na}_{2} {CrO}_{4} + 2 {Fe}_{2} O_{3} + 8 {CO}_{2}$

${Fe}_{2} O_{3}$ is insoluble in water. Its molar mass $= [2 \times 56 + 3 \times 16]$ $g/mol$

$= 160 g/mol$

Q 26 :

$K_{2} {Cr}_{2} O_{7}$ paper acidified with dilute $H_{2} {SO}_{4}$ turns green when exposed to [2023]

Hydrogen sulphide
Sulphur dioxide
Carbon dioxide
Sulphur trioxide

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(2)

${Cr}_{2} O_{7}^{2 -} + 2 H^{+} + 3 {SO}_{2} \to 2 {Cr}^{3 +} (green) + 3 {SO}_{4}^{2 -} + H_{2} O$

Q 27 :

Potassium dichromate acts as a strong oxidizing agent in acidic solution. During this process, the oxidation state changes from [2023]

+6 to +3
+6 to +2
+3 to +1
+2 to +1

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(1)

${\overset{+ 6}{Cr}}_{2} O_{7}^{- 2} \overset{H^{+}}{\to} {Cr}^{+ 3}$

Q 28 :

${KMnO}_{4}$ oxidises $I^{-}$ in acidic and neutral/faintly alkaline solution, respectively, to [2023]

${IO}_{3}^{-}$ and ${IO}_{3}^{-}$
$I_{2}$ and ${IO}_{3}^{-}$
$I_{2}$ and $I_{2}$
${IO}_{3}^{-}$ and $I_{2}$

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(2)

In acidic medium

$2 {MnO}_{4}^{-} + 10 I^{-} + 16 H^{+} ⟶ 2 {Mn}^{2 +} + 8 H_{2} O + 5 I_{2}$

In neutral or faintly alkaline medium

$2 {MnO}_{4}^{-} + I^{-} + H_{2} O ⟶ 2 {MnO}_{2} + 2 {OH}^{-} + {IO}_{3}^{-}$

Q 29 :

When ${Cu}^{2 +}$ ion is treated with $KI$ , a white precipitate, X appears in solution. The solution is titrated with sodium thiosulphate, the compound Y is formed. X and Y respectively are: [2023]

$X = {CuI}_{2}$ $Y = {Na}_{2} S_{2} O_{3}$
$X = {CuI}_{2}$ $Y = {Na}_{2} S_{4} O_{6}$
$X = {Cu}_{2} I_{2}$ $Y = {Na}_{2} S_{4} O_{6}$
$X = {Cu}_{2} I_{2}$ $Y = {Na}_{2} S_{4} O_{5}$

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(3)

${Cu}^{2 +} + 4 I^{-} ⟶ \underset{\underset{' X'}{White}}{{Cu}_{2} I_{2}} + I_{2}$

$I_{2} + 2 {Na}_{2} S_{2} O_{3} ⟶ \underset{' Y'}{2 {Na}_{2} S_{4} O_{6}} + 2 NaI$

$X = {Cu}_{2} I_{2}$ and $Y = {Na}_{2} S_{4} O_{6}$

Q 30 :

A solution of ${FeCl}_{3}$ , when treated with $K_{4} [Fe {(CN)}_{6}]$ gives a Prussian blue precipitate due to the formation of [2023]

$Fe [Fe {(CN)}_{6}]$
$K [{Fe}_{2} {(CN)}_{6}]$
${Fe}_{4} {[Fe {(CN)}_{6}]}_{3}$
${Fe}_{3} {[Fe {(CN)}_{6}]}_{2}$

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(3)

$4 {Fe}^{3 +} + 3 {[Fe {(CN)}_{6}]}^{4 -} ⟶ {Fe}_{4} {[Fe {(CN)}_{6}]}_{3} ↓$ Prussian blue precipitate.

Topic Question Set

Q 21 :

Consider the following reactions

$K_{2} {Cr}_{2} O_{7} \to_{- H_{2} O}^{KOH} [A] \to_{- H_{2} O}^{H_{2} {SO}_{4}} [B] + K_{2} {SO}_{4}$

The products [A] and [B], respectively are: [2025]

Q 22 :

Preparation of potassium permanganate from ${MnO}_{2}$ involves a two-step process in which the first step is a reaction with $KOH$ and ${KNO}_{3}$ to produce [2025]

Q 24 :

The amphoteric oxide among $V_{2} O_{3}, V_{2} O_{4}$ and $V_{2} O_{5}$ , upon reaction with alkali leads to formation of an oxide anion. The oxidation state of V in the oxide anion is: [2025]

Q 25 :

The molar mass of the water insoluble product formed from the fusion of chromite ore $({FeCr}_{2} O_{4})$ with ${Na}_{2} {CO}_{3}$ in presence of $O_{2}$ is ______ g ${mol}^{- 1}$ . [2025]

0%

Q 26 :

$K_{2} {Cr}_{2} O_{7}$ paper acidified with dilute $H_{2} {SO}_{4}$ turns green when exposed to [2023]

Q 27 :

Potassium dichromate acts as a strong oxidizing agent in acidic solution. During this process, the oxidation state changes from [2023]

Q 28 :

${KMnO}_{4}$ oxidises $I^{-}$ in acidic and neutral/faintly alkaline solution, respectively, to [2023]

Q 29 :

When ${Cu}^{2 +}$ ion is treated with $KI$ , a white precipitate, X appears in solution. The solution is titrated with sodium thiosulphate, the compound Y is formed. X and Y respectively are: [2023]

Q 30 :

A solution of ${FeCl}_{3}$ , when treated with $K_{4} [Fe {(CN)}_{6}]$ gives a Prussian blue precipitate due to the formation of [2023]

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Topic Question Set

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Q 26 : K2Cr2O7 paper acidified with dilute H2SO4 turns green when exposed to [2023] .question-row { display: flex; align-items: flex-start; gap: 8px; } .q-no { font-size: 17px; white-space: nowrap; margin-left: -15px; } .q-text { font-size: 18px; flex: 1; } .q-no, .q-text p { line-height: 30px; }

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Q 26 :

$K_{2} {Cr}_{2} O_{7}$ paper acidified with dilute $H_{2} {SO}_{4}$ turns green when exposed to [2023]