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Limiting Reagents

If you have worked out the MOLES of two reactants then the limiting reagent is the one with less moles. Sorted?

Well, it depends. If the reactants are in a 1:1 ratio then the above is true. But if the ratio is not equal then I'm afraid it's a bit more complicated.

Let's say reactants A and B have a ratio of 2:1 in the balanced equation.

If we have 0.020 moles of A and 0.015 moles of B which is the limiting reagent?

If we start with A we can see from the ratio that 0.02 moles of A will react with 0.01 moles of B.

As we started with 0.015 moles of B then not all of B has reacted.

Therefore B is in excess and A must be the limiting reagent.

If we start with B then we can see from the ratio that 0.015 moles of B will react with 0.030 moles of A.

As we only started with 0.02 moles A then we have insufficient moles of A. Therefore we can see that A is the limiting reagent.

So we can see that A is the limiting reagent even though we started with more moles of A than B.

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