Calculate the number of moles of the gas having volume $2.5 \ L$ at $300 \ K$ and $4.5 \ atm$? $(R=0.0821 \ atm \ dm^3 \ K^{-1} \ mol^{-1})$

The gas equation $PV = nRT$ is obeyed by:

$A$ vacuum vessel of volume $1 \, dm^3$ is evacuated at a constant temperature of $-23 \, ^\circ C$ until the pressure drops to $7.6 \times 10^{-5} \, torr$. What is the number of molecules remaining in the vessel?

How many times the volume of a diatomic gas should be increased reversibly and adiabatically in order to reduce its $RMS$ velocity to half of its initial value?

The gram molecular volume of an ideal gas at $STP$ is $22400 \ cm^{3}$. However,for oxygen gas,the question asks for the volume. Given the options provided,the correct value for $1 \ \text{mole}$ of any ideal gas at $STP$ is $22400 \ cm^{3}$. Please note that the provided solution in the prompt was incorrect. (in $cm^{3}$)

Density of a gas is found to be $5.46 \, g / dm^3$ at $27^{\circ} C$ at $2 \, bar$ pressure. What will be density at $STP$?