Temperature remaining constant,the pressure of a gas is decreased by $20 \%$. What is the percentage change in volume?

The volume of a gas becomes four times if:

$A$ cylinder contains hydrogen gas at a pressure of $249\; kPa$ and temperature $27^{\circ} C$. Its density is $............. kg/m^{3}$ $(R = 8.3\; J\; mol^{-1} K^{-1})$.

$2 \, g$ of ${O_2}$ gas is taken at $27^{\circ}C$ and a pressure of $76 \, cm$ of $Hg$. Find the volume of the gas (in liters).

$10 \ g$ of a gas at $STP$ occupies a volume of $2 \ L$. At what temperature will the volume be doubled,if pressure and amount of the gas remain constant?

The floor area of a room is $20 \, m^2$ and its walls are $3 \, m$ high. If the pressure of air in the room is $1 \, atm$ and the temperature is $27 \, ^\circ C$,find the mass of the air in the room. (Take the molar mass of air as $29 \, g/mol$). Given: $1 \, atm = 1.01 \times 10^5 \, N \, m^{-2}$,$R = 8.31 \, J \, mol^{-1} K^{-1}$.