How many of the following ions act as strong oxidising agents in aque…

The correct answer is Option (1) → 3

An ion that is easily reduced to a more stable lower state is a strong oxidising agent; an ion that is easily oxidised to a more stable higher state is a strong reducing agent.

Strong oxidising agents:

• Mn³⁺ (reduced to Mn²⁺, stable half-filled d⁵)

• Co³⁺ (with Mn³⁺, one of the two strongest oxidising 3d ions in water)

• Ce⁴⁺ (reverts to the common +3 state; E° Ce⁴⁺/Ce³⁺ = +1.74 V)

Strong reducing agents (not counted):

• Ti²⁺ — oxidised to Ti³⁺; liberates hydrogen from a dilute acid.

• V²⁺ — oxidised to V³⁺.

• Cr²⁺ — liberates hydrogen from a dilute acid:

      \( \displaystyle \ce{2Cr^2+(aq) + 2H+(aq) -> 2Cr^3+(aq) + H2(g)} \)

• Eu²⁺ — changes to the common +3 state.

Strong oxidising agents: Mn³⁺, Co³⁺ and Ce⁴⁺ = 3.

NCERT (2026-27 edition):
4.3.8 Chemical Reactivity and E° Values – “An examination of the E^o values for the redox couple M³⁺/M²⁺ … shows that Mn³⁺ and Co³⁺ ions are the strongest oxidising agents in aqueous solutions. The ions Ti²⁺, V²⁺ and Cr²⁺ are strong reducing agents and will liberate hydrogen from a dilute acid, e.g., 2 Cr²⁺(aq) + 2 H⁺(aq) → 2 Cr³⁺(aq) + H₂(g)”

4.5.3 Oxidation States – “…Thus, the formation of Ce^IV is favoured by its noble gas configuration, but it is a strong oxidant reverting to the common +3 state. The E^o value for Ce⁴⁺/ Ce³⁺ is + 1.74 V which suggests that it can oxidise water.” … “However, Eu²⁺ is a strong reducing agent changing to the common +3 state.”