Answer
$PV = nRT$; at STP, one mole of any ideal gas occupies approximately $22.4 \text{ L}$.
Explanation
$P$ is pressure, $V$ is volume, $n$ is amount in moles, $R \approx 8.314 \text{ J}/(\text{mol}\cdot\text{K})$ is the universal gas constant, and $T$ is absolute temperature in kelvins.