Q.Identify the following :

Showing the 12 most recent of 17 on this concept.

1. KEAM 2026Set eng-2026-04214 marksMCQ
   Q.Which of the following compound of manganese is a mixed oxide? (A) $MnO$ (B) $Mn_3O_4$ (C) $Mn_2O_3$ (D) $MnO_2$ (E) $Mn_2O_7$

   ›Reveal solutionSolution

   $Mn_3O_4$ (= $MnO\cdot Mn_2O_3$) is the mixed oxide with Mn in $+2$ and $+3$ states.

   A mixed oxide contains a metal in more than one oxidation state. $Mn_3O_4$ is composed of $MnO$ (Mn in $+2$) and $Mn_2O_3$ (Mn in $+3$), i.e. $MnO\cdot Mn_2O_3$ — the manganese analogue of $Fe_3O_4$. The others ($MnO$, $Mn_2O_3$, …

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2. KEAM 2026Set eng-2026-04224 marksMCQ
   Q.The highest possible oxidation states of uranium and plutonium, respectively, are (A) 7 and 6 (B) 6 and 4 (C) 6 and 7 (D) 7 and 5 (E) 4 and 6

   ›Reveal solutionSolution

   Among the actinoids, uranium reaches a maximum oxidation state of $+6$ while plutonium can reach $+7$. …

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3. KEAM 2026Set pha-2026-0418F4 marksMCQ
   Q.The first transition series metal that exhibits only +2,+3,+4 and +6 oxidation states is (A) Cr (B) Mn (C) Fe (D) Co (E) Ni

   ›Reveal solutionSolution

   [!TLDR]

   Chromium is the 3d metal that displays +2, +3, +4 and +6 without rising to +7, so it fits the given set.

   Concept

   Across the first transition series the range of oxidation states peaks at manganese (+7). Chromium is well known for +3 (most stable), +6 (chromate/dichromate), together with +2 and +4 — an NCERT/CBSE d-block staple.

   Solution

   • Mn (B): shows +2 through +7 — the +7 (permanganate) rules it out of an "only up to +6" set.
   • Co (D), Ni (E): their highest common states are only about +4; they do not reach +6.
   • Fe (C): dominated by +2 and +3; its +6 (ferrate) and +4 are far less characteristic than chromium's. …

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4. KEAM 2026Set pha-2026-0420F4 marksMCQ
   Q.The oxidation number of oxygen in $O_2$, $O_2F_2$ and $RbO_2$ are respectively (A) 0, +1, +2 (B) 0, +2, -1/2 (C) 0, +1, -1/2 (D) 0, 0, +1/2 (E) 0, -1, -1/2

   ›Reveal solutionSolution

   O is 0 in $O_2$, +1 in $O_2F_2$, and $-\tfrac{1}{2}$ in the superoxide $RbO_2$.

   • $O_2$: elemental form, oxidation number of O $= 0$.
   • $O_2F_2$: fluorine is the most electronegative element and is always $-1$; with two F ($-2$ total) shared over two O, each O is $+1$. …

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5. KEAM 2025Set eng-2025-04234 marksMCQ
   Q.The most common oxidation states of chromium are (A) +2, +7 (B) +3, +6 (C) +2, +4 (D) +2, +5 (E) +3, +5

   ›Reveal solutionSolution

   The most common oxidation states of chromium are +3 and +6.

   Concept and Intuition

   Chromium (3d^5 4s^1) shows several oxidation states, but +3 (as in Cr2O3, CrCl3, chrome alum) is the most stable, and +6 (as in dichromate/chromate, CrO3) is the important higher state. These two dominate its chemistry.

   Step-by-Step Solution

   1. Recall stable Cr species: Cr^3+ salts and Cr2O7^2-/CrO4^2- (Cr in +6). …

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6. KEAM 2025Set eng-2025-04234 marksMCQ
   Q.In which of the following compound, Mn has +7 oxidation state? (A) $MnOF$ (B) $MnO_2F$ (C) $MnO_3F_2$ (D) $MnOF_2$ (E) $MnO_3F$

   ›Reveal solutionSolution

   Mn is in the +7 state in MnO3F.

   Concept and Intuition

   Assign O = -2 and F = -1, then solve for Mn so the neutral molecule sums to zero. Only one option gives +7.

   Step-by-Step Solution

   1. MnO3F: Mn + 3(-2) + (-1) = 0 → Mn - 7 = 0 → Mn = +7.
   2. Check others: MnOF → Mn = +3; MnO2F → +3; MnO3F2 → +8 (not real); MnOF2 → +4.
   3. So MnO3F gives +7. …

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7. KEAM 2025Set eng-2025-04254 marksMCQ
   Q.Which of the following transition metal does not exhibit variable oxidation state? (A) Copper (B) Scandium (C) Vanadium (D) Nickel (E) Cobalt

   ›Reveal solutionSolution

   Variable oxidation states in transition metals arise from involvement of both $(n-1)d$ and $ns$ electrons. Scandium has configuration $[Ar]3d^1 4s^2$ and loses all three to give the stable $d^0$ $Sc^{3+}$, so it shows essentially only the $+3$ state.

   Examining the options:

   • Copper: $+1$, $+2$ (variable).
   • Scandium: only $+3$; after losing 3 electrons it reaches the stable $d^0$ configuration — no variable states. …

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8. KEAM 2025Set eng-2025-04264 marksMCQ
   Q.Which of the following 3d transition metal has +5 state as the more stable state? (A) Titanium (B) Vanadium (C) Manganese (D) Nickel (E) Silver

   ›Reveal solutionSolution

   Among the listed 3d metals, vanadium has the most stable +5 state.

   Vanadium (electronic configuration $[Ar]3d^3 4s^2$) can lose all five of its $3d$ and $4s$ electrons to reach the +5 state, well represented by $V_2O_5$ and vanadate ($VO_4^{3-}$/$VO_2^+$) species, which are its characteristic stable higher-oxidation-state forms. Titanium's highest common state is +4, manganese …

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9. KEAM 2025Set eng-2025-04284 marksMCQ
   Q.Which of the following outermost electronic configuration of the element shows the highest oxidation state? (A) $3d^3 4s^2$ (B) $3d^5 4s^1$ (C) $3d^5 4s^2$ (D) $3d^6 4s^2$ (E) $3d^2 4s^2$

   ›Reveal solutionSolution

   Maximum oxidation state peaks at Mn ($3d^5 4s^2$) with $+7$. …

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10. KEAM 2025Set eng-2025-04294 marksMCQ
    Q.The correct statement/s about $Cr^{2+}$ and $Mn^{3+}$ is/are [Atomic numbers of Cr = 24 and Mn = 25]

    (i) $Cr^{2+}$ is a reducing agent

    (ii) $Mn^{7+}$ is an oxidising agent in acidic medium

    (iii) Both $Cr^{2+}$ and $Mn^{3+}$ exhibit $d^4$ electronic configuration

    (iv) The highest oxide of Mn is $Mn_3O_4$.

    (v) $Cr^{2+}$ and $Mn^{3+}$ have the same magnetic moment as both have four unpaired electrons. (A) Only

    (i) (B) (i),

    (ii) and

    (iii) (C) (i),

    (iv) and

    (v) (D)

    (i) and

    (v) only (E) (i), (ii),

    (iii) and (v)

    ›Reveal solutionSolution

    $Cr^{2+}$ ($d^4$) is a reducing agent; $Mn^{7+}$ is a strong oxidiser in acid; both $Cr^{2+}$ and $Mn^{3+}$ are $d^4$ with 4 unpaired electrons and the same spin-only moment. Only (iv) is wrong (highest Mn oxide is $Mn_2O_7$). So (i), (ii), (iii), (v) are correct.

    With $Cr\,(Z=24)$ and $Mn\,(Z=25)$:

    • (i) $Cr^{2+}$ ($d^4$) readily oxidises to the more stable $Cr^{3+}$ ($d^3$, half-filled $t_{2g}$), so it is a reducing agent — true.
    • (ii) $Mn^{7+}$ (as $MnO_4^-$) is a powerful oxidising agent in acidic medium — true.
    • (iii) $Cr^{2+}$: $24-2 = 22$ electrons $\to [Ar]3d^4$; $Mn^{3+}$: $25-3 = 22 \to [Ar]3d^4$. Both are $d^4$ — true. …

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11. KEAM 2025Set pha-2025-0424F4 marksMCQ
    Q.The $3d$ metal that forms fluoride in +6 oxidation state is (A) Titanium (B) Chromium (C) Vanadium (D) Manganese (E) Cobalt

    ›Reveal solutionSolution

    Among the listed 3d metals, chromium attains the +6 oxidation state in its highest fluoride, $CrF_6$.

    The highest oxidation state a 3d metal shows in its fluorides is limited by how many electrons it can lose. Chromium (group 6, $3d^54s^1$) can use all six of its valence electrons and forms chromium hexafluoride, $CrF_6$ (Cr in the +6 state). …

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12. KEAM 2025Set pha-2025-0424A4 marksMCQ
    Q.Which of the following $3d$ metal forms only dihalide? (A) Titanium (B) Vanadium (C) Copper (D) Chromium (E) Zinc

    ›Reveal solutionSolution

    Zinc forms only dihalides.

    A metal that exhibits a single stable oxidation state forms halides in only that state. Among the choices:

    • Titanium, Vanadium, Chromium — form halides in multiple oxidation states (+2, +3, +4, etc.).
    • Copper — forms both $CuX$ (Cu$^+$) and $CuX_2$ (Cu$^{2+}$). …

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