Chemistry / Unit 8: Stoichiometry
8.6 Stoichiometry with Gases at STP
Notes
For gases at STP, 1 mol occupies 22.4 L, so you can add a liters step to any stoichiometry problem.
- Grams A $\rightarrow$ moles A $\rightarrow$ moles B $\rightarrow$ liters B ($\times$ 22.4 L/mol).
- For two gases at the same temperature and pressure, the volume ratio equals the mole ratio: in $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$, 2 L of $\text{H}_2$ reacts with 1 L of $\text{O}_2$.
- Example: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$. 6.54 g Zn (65.38 g/mol) is 0.100 mol, which makes 0.100 mol $\text{H}_2 = 2.24$ L at STP.
Practice
In $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$, what volume of $\text{CO}_2$ at STP forms from 25.0 g of $\text{CaCO}_3$ (100.09 g/mol)?
$\displaystyle \frac{25.0}{100.09} = 0.250$ mol $\text{CaCO}_3 \rightarrow 0.250$ mol $\text{CO}_2$, and $0.250 \times 22.4 = 5.60$ L.
In $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$, what volume of $\text{O}_2$ reacts with 4.48 L of $\text{H}_2$ at the same temperature and pressure?
Volume ratio equals mole ratio: $\displaystyle 4.48 \times \frac{1}{2} = 2.24$ L.
In $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$, how many grams of Mg (24.31 g/mol) react with 1.12 L of $\text{O}_2$ at STP?
$\displaystyle \frac{1.12}{22.4} = 0.0500$ mol $\text{O}_2$. $0.0500 \times 2 = 0.100$ mol Mg, and $0.100 \times 24.31 = 2.43$ g.
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