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Stoichiometry 26: How many moles of metal X react with 20 g of oxygen in forming X₂O₃?
Imagine this:
You’re handed a compound where an unknown metal, simply called “X,” reacts with oxygen to form X₂O₃. Your job?
Figure out how many moles of the metal reacted if you know that 20 grams of oxygen were involved in the process.
At first glance, it might look like a simple plug-in-the-values question.
But there’s a key principle here — and it lies in stoichiometric mole ratios drawn from the balanced equation.
Let’s walk through it step by step.
Step 1: Start with the balanced chemical formula
We are told the metal forms X₂O₃, which is a typical oxide for metals like aluminum or iron(III).
This formula gives us a mole ratio of 2 moles of metal X to 3 moles of oxygen atoms.
That ratio is critical.
Because it helps us connect the mass of oxygen used to the moles of metal that reacted.
Step 2: Find the number of moles of oxygen atoms in 20 g
To move forward, we need the molar mass of oxygen atoms.
The atomic mass of oxygen (O) is 16 g/mol, but here’s where many students get confused:
We’re working with oxygen atoms as they appear in the compound X₂O₃ — not with O₂ molecules.
So we stick with 16 g/mol, not 32.
Now let’s calculate:
Moles of O = 20 g ÷ 16 g/mol = 1.25 moles
So we have 1.25 moles of oxygen atoms in this compound.
Step 3: Use the mole ratio to determine moles of metal X
The formula X₂O₃ tells us that:
3 moles of oxygen atoms react with 2 moles of metal atoms.
We now set up the conversion using that ratio:
(2 moles of X / 3 moles of O) × 1.25 moles of O = 0.833 moles of X
Let’s round that to three significant figures:
Answer: 0.83 moles of metal X reacted
Now you see how mole ratios are at the heart of every stoichiometric problem.
Step 4: Why this isn’t a trick question — it’s a ratio puzzle
A lot of chemistry learners panic when they see formulas like X₂O₃ or Fe₂O₃.
But these formulas actually give you all the clues you need.
X₂O₃ means 2 metal atoms react with 3 oxygen atoms — and that’s your mole ratio right there.
You didn’t need the atomic mass of the metal because the question only asked about moles, not grams.
Sometimes the absence of data is a clue that the problem stops at moles.
Step 5: Where this shows up in the real world
Metals forming oxides is a huge deal in industrial chemistry.
From rust formation to metal refining, corrosion, welding, and even battery manufacturing, understanding how metals combine with oxygen is fundamental.
Let’s take aluminum as an example.
When aluminum forms Al₂O₃, it’s undergoing a similar 2:3 ratio with oxygen.
And in massive production environments, chemists need to calculate exactly how much aluminum to feed into the reaction to yield the right amount of oxide — or predict how much has oxidized based on how much oxygen was consumed.
Step 6: Don’t fall into these common traps
Here’s where many students make errors:
They use 32 g/mol instead of 16 g/mol for oxygen — forgetting that oxygen in compounds isn’t O₂ gas.
Or they assume the ratio is 1:1, which only works for very specific cases.
Others forget to simplify the mole ratio from the chemical formula and just guess.
And sometimes, people calculate moles correctly but forget to round properly or give an answer in moles when the question asked for grams — or vice versa.
Details matter.
Step 7: Let’s zoom out and connect the dots
You took 20 g of oxygen, figured out it represented 1.25 moles of O atoms, then used the 2:3 mole ratio from the compound X₂O₃ to calculate the moles of metal X.
From that, you correctly calculated that 0.83 moles of the metal must have reacted.
No fluff. No guesswork. Just systematic problem solving.
Application Spotlight: X₂O₃ in Materials Science
Let’s say your metal X is iron, and you’re forming Fe₂O₃, also known as iron(III) oxide.
That’s the red-brown rust you see on exposed steel surfaces.
Predicting how much metal is lost to rust helps engineers plan corrosion control and protective coating schedules.
In spacecraft, lightweight metals like aluminum form Al₂O₃, which is actually a protective coating that stops further oxidation — unlike rust.
That’s why stoichiometry matters even in space.
Chemists and engineers must know how many atoms are involved, in what ratio, and how much product or reactant is formed — right down to the last gram or mole.
Conclusion
When you are given a mass of oxygen and need to find out how many moles of a metal reacted to form X₂O₃, the solution becomes much clearer once you convert grams to moles using the atomic mass of oxygen.
In this problem, converting 20 grams of oxygen gave you 1.25 moles of oxygen atoms.
By applying the 2:3 mole ratio found in the formula X₂O₃, you can determine how many moles of metal X were involved.
After working through the ratio, you see that 0.83 moles of metal X were needed to react with 1.25 moles of oxygen atoms.
This simple calculation relies on a clear understanding of stoichiometric ratios and careful conversion between units.
Mastering this approach is the key to solving hundreds of similar questions, both in the lab and on exams.
If you want more practice and step-by-step help, visit www.copychemistry.com.
With each question you solve, you’ll build stronger skills and gain more confidence in stoichiometry.
