26 past paper MCQs on Oxidation and Reduction, from the Electrochemistry unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.
1.In which of the following does oxygen show a fractional oxidation number? (MDCAT 2022)
Correct answer: (c) $\mathrm{KO_2}$
In the superoxide $\mathrm{KO_2}$ the two oxygens share a single negative charge, so each is $-\tfrac{1}{2}$.
2.Which statement describes the conversion of magnesium atoms to magnesium ions for ionic bond formation with chlorine? (MDCAT 2022)
Correct answer: (b) the change is oxidation, because there has been a loss of electrons
Mg loses two electrons to become $\mathrm{Mg^{2+}}$, and loss of electrons is oxidation.
3.The oxidation number of a free element is: (MDCAT 2023)
Correct answer: (a) zero
An uncombined atom has shared its electrons with nothing, so its oxidation number is zero.
4.The oxidation number of Mn in $\mathrm{KMnO_4}$ is: (MDCAT 2024)
Correct answer: (d) $+7$
$+1$ for K and $-2$ for each of four oxygens leaves $+7$ for manganese.
5.Which of the following best identifies the redox reaction: (UHS MDCAT 2025)
Correct answer: (b) transfer of electron
Oxidation is loss of electrons and reduction is gain, so a redox reaction is an electron transfer.
6.In an electrolytic cell when the current passes through a solution, the anode is: (KMU MDCAT 2025)
Correct answer: (a) a positive electrode where oxidation occurs
Oxidation always happens at the anode, and in an electrolytic cell the anode is the positive electrode.
7.The oxidation number of Cl in HClO2 is: (NUMS MDCAT 2025)
Correct answer: (b) +3
Hydrogen is $+1$ and each oxygen $-2$, so chlorine must be $+3$ for the molecule to be neutral.
8.In the given reaction CH3OH -> HCOOH, oxidation state of carbon changes from: (SZABMU MDCAT 2025)
Correct answer: (b) -2 to +2
Carbon is $-2$ in methanol and $+2$ in formic acid, so it loses four electrons.
9.Which species contain an element with an oxidation number of +4? (SZABMU MDCAT 2025)
Correct answer: (d) Na2CO3
Each oxygen is $-2$ and each sodium $+1$, so the carbon of the carbonate is $+4$.
10.Oxidation number of 'Mn' in KMnO4 is (UHS MDCAT 2024)
Correct answer: (d) +7
Potassium is $+1$ and the four oxygens are $-8$, so manganese must be $+7$.
11.Which of the following is NOT a correct feature of electrolytic cells? (UHS MDCAT 2024)
Correct answer: (c) Alternating current source is connected to electrodes
An electrolytic cell is driven by direct current; alternating current would reverse the electrodes every half cycle.
12.Q.91 In which of the following, oxygen shows fractional oxidation number? (UHS MDCAT 2023)
Correct answer: (c) KO2
In the superoxide ion the charge is shared between two oxygens, so each is at $-\tfrac12$.
13.Q.94 Which statement describes the conversion of magnesium atoms to magnesium ions for ionic bond formation with chlorine? (UHS MDCAT 2023)
Correct answer: (b) The change is oxidation, because there has been a loss of electrons
Magnesium gives up two electrons, and loss of electrons is oxidation.
14.Oxidation number of an element in free state is: (SZABMU MDCAT 2023)
Correct answer: (c) Zero
An uncombined element has an oxidation number of zero.
15.During electrolysis, reduction always occurs at the: (NUMS MDCAT 2023)
Correct answer: (b) Cathode
Reduction is gain of electrons, and the electrons enter the cell at the cathode.
16.0xidation number of free magnesium is? (UHS MDCAT 2023)
Correct answer: (a) 0
An uncombined element has an oxidation number of zero.
17.Oxidation state of Cr in K2Cr207 is: (PMC MDCAT 2022)
Correct answer: (d) +6
Two potassium at $+1$ and seven oxygen at $-2$ leave $+12$ for two chromium atoms, so each is $+6$.
18.In the electrolysis of fused salt, where do the cations move? (PMC MDCAT 2022)
Correct answer: (b) Move towards cathode and discharge on it
Cations are attracted to the cathode, where they take electrons and are discharged.
19.Electrons flow from _____ (PMC MDCAT 2022)
Correct answer: (a) Anode to cathode
Electrons leave the electrode where oxidation happens, the anode, and travel to the cathode.
20.During electrolysis, reduction always occurs at: (NUMS MDCAT 2022)
Correct answer: (b) Cathode
Reduction is gain of electrons, and the electrons enter at the cathode.
21.Current in electrolysis is carried through? (PMC Practice 2021)
Correct answer: (d) Both B and C
In the electrolyte the current is carried by the ions, positive one way and negative the other.
22.During oxidation process, oxidation number of an element: (PMC MDCAT 2020)
Correct answer: (b) Increases
Oxidation is loss of electrons, which raises the oxidation number.
23.The oxidation state of “S’ in the (S2O3)-2 is: (PMC MDCAT 2020)
Correct answer: (d) +2
Three oxygens at $-2$ and a charge of $-2$ leave $+4$ for two sulphur atoms, that is $+2$ each.
24.The common oxidation number of halogens is: (PMC MDCAT 2020)
Correct answer: (a) -1
A halogen needs one electron to complete its octet, so it is usually $-1$.
25.Oxidation number of particular element can be directly or indirectly inferred from its: (UHS MDCAT 2019)
Correct answer: (a) Group number
The group number gives the number of outer electrons, and so the usual oxidation state.
26.For the purification, impure copper is made the: (UHS MDCAT 2017)
Correct answer: (b) Anode
The impure copper is the anode and dissolves; the pure metal is deposited on the cathode.