Gas Stoichiometry at STP and Non-STP Worked Examples Cheat Sheet

Key Facts

• At STP, $1\ \text{mol}$ of an ideal gas occupies $22.4\ \text{L}$, so $n = \frac{V}{22.4\ \text{L/mol}}$.
• For non-STP gases, use the ideal gas law $PV = nRT$ and solve for moles with $n = \frac{PV}{RT}$.
• Use $R = 0.0821\ \frac{\text{L}\cdot\text{atm}}{\text{mol}\cdot\text{K}}$ when pressure is in atm, volume is in liters, and temperature is in kelvin.
• Convert Celsius to kelvin using $T_K = T_{^\circ\text{C}} + 273.15$ before using any gas law formula.
• Coefficients in a balanced equation give mole ratios, such as $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$ meaning $2\ \text{mol H}_2 : 1\ \text{mol O}_2 : 2\ \text{mol H}_2\text{O}$.
• For gases at the same temperature and pressure, volume ratios equal mole ratios, so $2\ \text{L H}_2$ reacts with $1\ \text{L O}_2$ in $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$.
• A common gas stoichiometry path is $\text{given gas volume} \rightarrow \text{moles given} \rightarrow \text{moles wanted} \rightarrow \text{wanted volume or mass}$.
• In limiting reactant problems, calculate product from each reactant and choose the smaller product amount as the theoretical yield.

Vocabulary

STP

STP means standard temperature and pressure, usually $0^\circ\text{C}$ or $273.15\ \text{K}$ and $1\ \text{atm}$.

Molar Volume

Molar volume is the volume occupied by $1\ \text{mol}$ of a gas, which is $22.4\ \text{L}$ for an ideal gas at STP.

Ideal Gas Law

The ideal gas law is $PV = nRT$, which relates pressure, volume, moles, and temperature for an ideal gas.

Mole Ratio

A mole ratio is a conversion factor made from coefficients in a balanced chemical equation.

Limiting Reactant

The limiting reactant is the reactant that runs out first and determines the maximum amount of product formed.

Theoretical Yield

The theoretical yield is the maximum amount of product predicted by stoichiometry from the limiting reactant.