6.6 Multi-Step Mole Problems
Notes
The mole is the center of a "mole map." Every conversion goes through moles.
- Grams $\leftrightarrow$ moles: use molar mass.
- Particles $\leftrightarrow$ moles: use $6.02 \times 10^{23}$.
- Liters of gas at STP $\leftrightarrow$ moles: use 22.4 L/mol.
- Example: mass of 5.60 L of $\text{O}_2$ at STP. $\displaystyle \frac{5.60}{22.4} = 0.250$ mol, and $0.250 \times 32.00 = 8.00$ g.
- Never go straight from grams to liters; always stop at moles first.
Practice
What is the mass of 44.8 L of $\text{CO}_2$ at STP? (molar mass 44.01 g/mol)
$\displaystyle \frac{44.8}{22.4} = 2.00$ mol, and $2.00 \times 44.01 = 88.0$ g.
How many molecules are in 11.2 L of $\text{H}_2$ at STP?
$\displaystyle \frac{11.2}{22.4} = 0.500$ mol, and $0.500 \times 6.02 \times 10^{23} = 3.01 \times 10^{23}$.
What volume at STP is occupied by 8.02 g of methane, $\text{CH}_4$? (molar mass 16.05 g/mol)
$\displaystyle \frac{8.02}{16.05} = 0.500$ mol, and $0.500 \times 22.4 = 11.2$ L.
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