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Question

A metal on combustion in air vigorously forms a superoxide X . Name the metal.

A
Li
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B
Na
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C
None of the above
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D
Rb
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Solution

The correct option is D Rb
The lithium metal in the air form Lithium monoxide.
The equation for the formation of the peroxide is:
4Li+O22Li2O
The sodium metal in the excess air form sodium peroxide.
The equation for the formation of the peroxide is:
2Na+O2Na2O2

The Rubidium metal in the air form rubidium superoxide.
The equation for the formation of the superoxide is:
Rb+O2RbO2
Hence, correct options is c.

Theory
Reactivity towards air:
Alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture
to form hydroxides.
They burn vigorously in oxygen forming oxides:
Lithium monoxide
4Li+O22Li2O
Sodium peroxide
2Na+O2Na2O2
Other superoxide:
M+O2MO2
Where M = K, Rb, Cs
The superoxide O2 ion is stable only in the presence of larger cations such as K, Rb, Cs.

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