When metallic copper comes in contact with moisture,a green powdery/pasty coating can be seen over it. This is chemically known as

Mole fraction of $O_2$ in an $O_2$ and $O_3$ mixture is $0.25$. Hence,the percentage concentration $\left( \frac{w}{W}\% \right)$ of $O_2$ in the mixture is:

Which of the following has the minimum number of oxygen atoms?

Rearrange the following ($I$ to $IV$) in the order of increasing masses and choose the correct answer (Atomic mass: $N=14, O=16, Cu=63$).
$I.$ $1$ molecule of oxygen
$II.$ $1$ atom of nitrogen
$III.$ $1 \times 10^{-10} \ g$ molecular weight of oxygen
$IV.$ $1 \times 10^{-10} \ g$ atomic weight of copper

Iron sulphide is heated in air to form $A$, an oxide of sulphur. $A$ is dissolved in water to give an acid. The basicity of this acid is:

If one liter of air is repeatedly passed over heated copper and magnesium until no further reduction in volume occurs,the final volume of the air will be approximately ......... $mL$.