The observation that 20g of hydrogen gas always combines with 160g of oxygen gas to form 180g of water, even when there is more
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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,
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.