Answer:
3 mol Cl₂/2 mol AlCl₃ (three over two)
Step-by-step explanation:
Start with the balanced equation"
2Al + 3Cl₂⟶ 2AlCl₃
The steps in the calculation are
mass of AlCl₃ ⟶ moles of AlCl₃⟶ moles of Cl₂ ⟶ mass of Cl₂
The critical step is the <em>conversion of moles</em>.
You multiply the moles of AlCl₃ by a <em>conversion factor</em> to get moles of Al:
Moles of AlCl₃ × conversion factor = moles of Al.
The conversion factor is <em>the molar ratio</em>, and it uses the coefficients of the formulas in the balanced equation.
It is either (2 mol AlCl₃/3 mol Cl₂) or (3 mol Cl₂/2 mol AlCl₃).
You choose the one that has the desired units of the answer in the numerator.
We choose the second option, because it has the correct units.
For example,
![\text{0.2 mol AlCl}_{3} \times \frac{3\textbf{ mol Cl}_{2} }{ 2\text{ mol AlCl}_{3} } = 0.3\textbf{ mol Cl}_{2})](https://tex.z-dn.net/?f=%5Ctext%7B0.2%20mol%20AlCl%7D_%7B3%7D%20%5Ctimes%20%5Cfrac%7B3%5Ctextbf%7B%20mol%20Cl%7D_%7B2%7D%20%7D%7B%202%5Ctext%7B%20mol%20AlCl%7D_%7B3%7D%20%7D%20%3D%200.3%5Ctextbf%7B%20mol%20Cl%7D_%7B2%7D%29)
Notice how the units "mol AlCl₃" cancel and the correct units appear in the answer.
If we had used the other conversion factor, we would have gotten the wrong units.