The ideal gas

Ideal gas: ignoring collisions between molecules, $$ \frac{\normalsize P V}{\normalsize T} = k_B N $$

Boltzmann's constant: $$k_B = 1.38\times 10^{-23}\quad\frac{\mbox{J}}{\mbox{K}}$$

Avogadro's number: $$N_A = 6.022\times 10^{23}\quad\frac{\mbox{molecules}}{\mbox{mole}}$$

Universal gas constant: $$R = k_B N_A = 8.31\quad\frac{\mbox{J}}{\mbox{mol K}} = 0.0821\quad \frac{\mbox{l atm}}{\mbox{mol K}}$$

Origin of pressure: collisions of molecules with the walls of the container

Maxwell's velocity distribution:

$$ {K}_{\rm average} = \frac{1}{2} m v_{\rm rms}^2 = \frac{3}{2} k_B T\quad, \qquad v_{\rm rms} = \sqrt{\frac{\normalsize 3k_BT}{\normalsize m}} = \sqrt{\frac{\normalsize 3RT}{\normalsize M}} $$