Gas Stoichiometry at STP

In chemistry, gases are unique because their volume is heavily influenced by environment. To compare them fairly, scientists use Standard Temperature and Pressure (STP). STP is defined as a temperature of $0^\circ\text{C}$ ($273.15 \text{ K}$) and a pressure of $101.3 \text{ kPa}$ ($1 \text{ atm}$). According to Avogadro’s Law, equal volumes of all gases at the same temperature and pressure contain the same number of molecules. At STP, exactly one mole of any ideal gas occupies a volume of **$22.4 \text{ L}$. This is known as the molar volume**. Whether you have $1 \text{ mole}$ of light Hydrogen ($H_2$) or $1 \text{ mole}$ of heavy Carbon Dioxide ($CO_2$), they both take up the same $22.4 \text{ L}$ of space at STP.

Stoichiometry is the 'accounting' of chemistry. When a reaction involves both solids and gases, we use the mole as a bridge. If you know the mass of a solid reactant, you can find the moles of a gas product using the mole ratio from a balanced equation. Once you have the moles of gas, you simply multiply by $22.4 \text{ L/mol}$ to find its volume at STP. The flow usually looks like this: $\text{Mass (A)} \rightarrow \text{Moles (A)} \rightarrow \text{Moles (B)} \rightarrow \text{Volume (B)}$. This allows us to predict gas output without ever touching a measuring cylinder.

In advanced scenarios, we often deal with non-standard ratios and limiting reactants. Consider the decomposition of Sodium Azide ($NaN_3$) used in airbags: $2NaN_3(s) \rightarrow 2Na(s) + 3N_2(g)$. Here, the ratio is $2:3$. To find the volume of Nitrogen gas ($N_2$), you must be precise with your stoichiometric coefficients. If the temperature or pressure changes away from STP, the $22.4 \text{ L}$ constant no longer applies, and we would transition to the Ideal Gas Law ($PV=nRT$). However, mastering the STP calculation is the essential first step for any chemical engineer.