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Oxidation number rules and unusual cases

55 questions on Oxidation number rules and unusual cases from the Redox Reactions chapter: 11 NEET level, 22 JEE Main, 22 JEE Advanced. Each one opens with its answer and solution.

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What is the oxidation number of chromium in K2Cr2O7? NEET level
What is the oxidation number of nitrogen in the ammonium ion, NH4+? NEET level
In hydrogen peroxide, H2O2, the oxidation number of oxygen is NEET level
The oxidation number of chlorine in HClO4 is NEET level
The oxidation number of iron in K4[Fe(CN)6] is NEET level
In sodium hydride, NaH, the oxidation number of hydrogen is NEET level
The oxidation number of oxygen in OF2 is NEET level
The average oxidation number of sulfur in sodium thiosulfate, Na2S2O3, is NEET level
Nitrogen has its highest oxidation number among the following in NEET level
The oxidation number of carbon in chloroform, CHCl3, is NEET level
The oxidation number of phosphorus in phosphorous acid, H3PO3, is NEET level
Treat the average oxidation number as asked. In Br3O8 the average oxidation number of bromine is JEE Main level
Assertion (A): The oxidation number of oxygen in OF2 is positive. Reason (R): Fluorine is more electronegative than oxygen. Choose the correct option. JEE Main level
Match the species in List I with the oxidation number of the underlined (first-named) element in List II, using the electronegativity order F > Cl > Br > I. Lis... JEE Main level
Magnetite, Fe3O4, is a mixed oxide. Using the INDIVIDUAL (not average) oxidation numbers, the three iron atoms in one formula unit are JEE Main level
In Zeise's salt, K[PtCl3(C2H4)], ethene is a neutral ligand. The oxidation number of platinum is JEE Main level
Hydrogen has oxidation number -1 in BOTH species of the pair JEE Main level
Using bond-counting by electronegativity (O > C > H, C-C bonds count zero), the oxidation numbers of the methyl carbon and the carboxyl carbon in acetic acid, C... JEE Main level
Chromium pentoxide, CrO5, has chromium in its highest oxidation state, +6, and contains peroxide linkages. The number of oxygen atoms with oxidation number -1 i... JEE Main level
The oxidation number of sulfur in peroxodisulfuric acid, H2S2O8, which has one O-O peroxide bond, is JEE Main level
In the thiosulfate ion S2O3^2- (structure S-SO3^2-), the two sulfur atoms have different individual oxidation numbers. Use the common textbook convention in whi... JEE Main level
Find the average oxidation number of manganese in Mn3O4. Give the answer to two decimal places. JEE Main level
Find the sum of the oxidation numbers of the metals in [Cu(NH3)4]SO4, K3[Fe(CN)6] and [Ni(CO)4] (NH3 and CO are neutral ligands). Give an integer. JEE Main level
Count the oxygen atoms that have oxidation number -1 in one formula unit each of H2SO5 (one O-O bond, S is +6), Na2O2 and CrO5 (Cr is +6). Give the total number... JEE Main level
Find the sum of the oxidation numbers of manganese in KMnO4, K2MnO4, MnO2 and Mn2O3. Give an integer. JEE Main level
Using bond-counting (O > C > H; C-C bonds count zero), find the algebraic sum of the oxidation numbers of the two carbon atoms in ethanol, CH3CH2OH. Give an int... JEE Main level
Find the average oxidation number of iodine in the triiodide ion, I3-. Give the answer to two decimal places (include the sign). JEE Main level
In which of the following is the oxidation number of sulfur +6? (For S2O8^2- take the peroxide linkage into account.) Select all correct options. JEE Main level
In which compounds is the oxidation number of oxygen equal to -1/2? Select all correct options. JEE Main level
In which species does the underlined element (first-named, central element) have oxidation number +3? Species: (A) N in HNO2, (B) Cl in ClF3, (C) P in H3PO3, (D... JEE Main level
For which of the following is the AVERAGE oxidation number of the metal or non-metal element a non-integer? Select all correct options. JEE Main level
In which of the following does carbon have oxidation number zero? Use C-H as -1 for carbon, C-O and C-Cl as positive. Select all correct options. JEE Main level
In which of the following does the metal have oxidation number +2? (NH3 and en are neutral.) Select all correct options. JEE Main level
Assertion (A): In the tetrathionate ion, S4O6^2-, all four sulfur atoms have oxidation number +2.5. Reason (R): The average oxidation number of sulfur in S4O6^2... JEE Advanced level
Match the nitrogen species in List I with the oxidation number of N in List II (for HN3 use the average). List I: (P) N in NH2OH, (Q) N in N2H4, (R) N in HN3, (... JEE Advanced level
Arrange the following in decreasing order of the average oxidation number of oxygen (potassium is +1, ozonide is O3-): K2O, K2O2, KO2, KO3. JEE Advanced level
In the peroxo complex K3[Cr(O2)4] each O2 unit is a peroxide ligand (O2^2-). The oxidation number of chromium is JEE Advanced level
Arrange the sulfur oxoanions in increasing order of the AVERAGE oxidation number of sulfur: thiosulfate S2O3^2-, tetrathionate S4O6^2-, dithionite S2O4^2-, dith... JEE Advanced level
Use bond-counting (O > C > H; C-C bonds count zero). When ethanal, CH3CHO, is oxidised to ethanoic acid, CH3COOH, the change in oxidation number of the function... JEE Advanced level
The brown-ring complex is [Fe(H2O)5(NO)]^2+. Taking NO as the nitrosonium ligand NO+ and water as neutral, the oxidation number of iron is JEE Advanced level
Calcium hydride reacts with water: CaH2 + 2H2O -> Ca(OH)2 + 2H2. Which statement is correct? JEE Advanced level
Which one of the following reactions is NOT a redox reaction? JEE Advanced level
Carbon suboxide has the structure O=C=C=C=O. Using electronegativity (O > C), the individual oxidation numbers of the terminal carbons and the central carbon ar... JEE Advanced level
Wustite has the non-stoichiometric composition Fe_0.93O, with iron present only as Fe^2+ and Fe^3+. Find the percentage of iron atoms present as Fe^3+. Give the... JEE Advanced level
Find the sum of the INDIVIDUAL oxidation numbers of all sulfur atoms in one formula unit of H2S2O8 (one O-O bond) plus all sulfur atoms in the S4O6^2- ion (O3S-... JEE Advanced level
Magnetite, Fe3O4 (treat as one Fe^2+ and two Fe^3+), is oxidised completely to Fe2O3. How many moles of electrons are lost by 69.6 g of Fe3O4? (Fe 56, O 16.) Gi... JEE Advanced level
Sodium nitroprusside is Na2[Fe(CN)5NO]. Taking NO as the nitrosonium ligand NO+, find the oxidation number of iron. Give an integer. JEE Advanced level
Peroxodisulfuric acid H2S2O8 has one O-O bond. How many moles of oxygen atoms with oxidation number -1 are present in 19.4 g of it? (H 1, S 32, O 16.) Give two ... JEE Advanced level
Ascorbic acid has the molecular formula C6H8O6. Find the average oxidation number of carbon (H is +1, O is -2). Give two decimal places (include the sign). JEE Advanced level
Sodium dithionite, Na2S2O4, is oxidised completely to sulfate, SO4^2-. What mass of dithionite loses 0.060 mol of electrons? (Na 23, S 32, O 16.) Give two decim... JEE Advanced level
Which of the following contain a peroxide (O-O) linkage? Select all correct options. JEE Advanced level
Considering magnetite, Fe3O4, as FeO.Fe2O3, select the correct statements. JEE Advanced level
Select the correct statements about thiosulfate S2O3^2- (S-SO3 skeleton) and tetrathionate S4O6^2- (O3S-S-S-SO3). JEE Advanced level
Use the electronegativity order F > Cl > Br > I. In which compounds does the first-named halogen have oxidation number +1? Select all correct options. JEE Advanced level
Use bond-counting (O > C > H; C-C bonds count zero) for the sequence ethanol, ethanal, ethanoic acid. Select the correct statements. JEE Advanced level