The density of air is higher at sea level. This fact is a practical application of which law?

The following graphs are based on Boyle's Law at constant mass and temperature. Describe what each graph represents.

$A$ certain mass of a gas occupies a volume of $250 \ mL$ at $2 \ atm$ pressure. Calculate the volume of the gas if the pressure is increased to $2.5 \ atm$ at a constant temperature. (in $mL$)

At what temperature in the Celsius scale,$V$ (volume) of a certain mass of gas at $27\,^oC$ will be doubled keeping the pressure constant?

For $1 \ mol$ of an ideal gas at a constant temperature $T,$ the plot of $(\log \ P)$ against $(\log \ V)$ is a ($P :$ Pressure,$V:$ Volume)

An $LPG$ (Liquefied Petroleum Gas) cylinder weighs $15.0 \ kg$ when empty. When full, it weighs $30.0 \ kg$ and shows a pressure of $3.0 \ atm$. In the course of usage at $27^{\circ}C$, the mass of the full cylinder is reduced to $24.2 \ kg$. The volume of the used gas in cubic metre at the normal usage condition ($1 \ atm$ and $27^{\circ}C$) is (assume $LPG$ to be normal butane and it behaves ideally). (in $m^3$)