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

Air contains $23\% \, O_2$ and $77\% \, N_2$ by weight. What is the percentage of $O_2$ by volume?

Write formulas for the following compounds:
$(1)$ Mercury $(II)$ chloride
$(2)$ Nickel $(II)$ sulphate
$(3)$ Tin $(IV)$ oxide
$(4)$ Thallium $(I)$ sulphate

If the density of liquid water is $1.0 \, g \cdot cm^{-3}$ and the density of water vapor is $0.0006 \, g \cdot cm^{-3}$ at $100 \, ^\circ C$ and $1 \, atm$ pressure,then what is the volume occupied by water molecules in $1 \, L$ of steam at the same temperature in $cm^3$?

At $300 \ K$ temperature,$20 \ g \ H_2$,$220 \ g \ CO_2$,and $140 \ g \ N_2$ are filled in a vessel having volume $2 \ L$. Find the total pressure in $bar$ unit and which gas is removed from the vessel so that pressure can be reduced by $50 \%$. $(R = 8.34 \times 10^{-2} \ L \ bar \ K^{-1} \ mol^{-1})$

Which of the following should be done in order to prepare $0.40 \ M \ NaCl$ starting with $100 \ mL$ of $0.30 \ M \ NaCl$ (mol.wt. of $NaCl = 58.5$)?