Question

2 fe2O3 + 5CO =
Two moles of fe2O3 reacts with 5 moles of carbon monoxide
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Answer 1

1 mole of Fe2CO3 reacts with 3 moles of CO.

Suppose we take 5 moles each of Fe2CO3 and CO.(mentioned in the question)

For one Fe2CO3 we need 3 moles of CO, then for 5 moles of Fe2CO3, we need 15 moles of CO.

But we have only 5 moles of CO.

That means that the entire 5 moles of Fe2CO3 doesn't take part in the reaction.( only 5/3 moles of Fe2CO3 gets reacted) But CO will be used up completely.

In the beginning of the reaction there is enough CO to react with the Fe2CO3. But as the reaction proceeds, the CO gets depleted and the Fe2CO3 doesnt have anymore CO to react with.

Here the amount of CO is deciding how far the reaction can occur. Therefore it is a limiting reactant.