The Ideal Gas Law

The ideal gas law ties pressure, volume, moles and temperature together:

$$P \cdot V = n \cdot R \cdot T$$

P in atm, V in L, n in mol, T in kelvin, $R = 0.082\ \mathrm{atm\,L/(mol\,K)}$.

Know three and the law gives the fourth: one mole at $2\ \mathrm{atm}$ and $300\ \mathrm{K}$ takes $12.3\ \mathrm{L}$.

P·V = n·R·T

At 1 atm and 298 K one mole takes about $24.4\ \mathrm{L}$: the 25 L rule. With the moles, $m = n \times M$ gives the mass.