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Chemistry · Stoichiometry and Yield

Limiting Reagent and Leftover Reactants

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Wrap-up

What you learned

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Read next: Percent Yield and Reaction ExtentLower actual output below the limiting cap.

Key takeaway

Each reactant supply has its own recipe-supported batch cap.
The smaller cap is the limiting reagent and sets the theoretical output.
Leftover packets belong to the excess side because the limiting side is spent first.
Target-leftover questions can be built by choosing a batch cap, spending the recipe, and adding the desired excess.

Common misconception

Do not pick the limiting reagent from raw packet counts or from the larger coefficient alone; compare possible complete batches from every supply.

Limiting reagent depends on the recipe as well as the starting supplies.

From batch cap to product and leftovers

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Lower the actual outputStoichiometry and Yield

Percent Yield and Reaction Extent

Compare actual output with the same theoretical recipe cap so percent yield stays visual and honest on one shared bench.

Carry amount bookkeeping into solutionsSolutions and pH

Concentration and Dilution

Use one beaker to separate how concentration changes when you add solvent from how it changes when you add more solute.

Next in Recipe batchesStoichiometry and Yield

Stoichiometric Ratios and Recipe Batches

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Equation snapshotLimiting-reagent snapshot · Smaller batch cap sets the outputShow 3 equations

Convert both supplies into possible recipe batches first; the smaller cap sets theoretical output and decides which side can have leftovers.

Limiting-reagent snapshot
$b_{A} = \frac{A}{a}, b_{B} = \frac{B}{b}$
Divide each supply by its recipe requirement to see how many full batches that reactant could support on its own.
Smaller batch cap sets the output
$b_{theory} = min (b_{A}, b_{B})$
The theoretical output is set by the lower of the two supported batch counts.
Leftover on the excess side
$L_{excess} = N_{excess} - b_{theory} n_{recipe}$
After the limiting number of full batches, the excess reactant keeps whatever packets were not used.

Why it behaves this way

Explanation

Do not start by comparing raw packet counts. First ask how many complete recipe batches each supply can support. This bench keeps the supply trays, recipe card, batch cap, and leftover readout together so you can see which side runs out first.

For each reactant, divide the available packets by the packets needed per batch. The smaller supported batch count is the limiting reagent and sets the theoretical output. The other side is excess, so it is the side that can have packets left over.

Key ideas

01Use the recipe card to convert each supply into the number of full batches it can support.

02The smaller supported batch count caps the product, even if that reactant does not have the smaller raw packet count.

03After the last complete batch, the limiting side is used up and leftovers remain only on the excess side.

Worked examples

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Frozen walkthrough

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Frozen walkthrough

Use the live recipe bench to compare supported batches before you think about leftovers.

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Frozen valuesUsing frozen parameters

In the 8 A, 15 B run with a 2:3 recipe, which reactant limits the reaction and what is left over?

A

Reactant A packets

B

Reactant B packets

b

B per batch

1. Turn each supply into complete batches

A can support 8 / 2 = 4 full batches, while B can support 15 / 3 = 5.

2. Choose the smaller batch cap

Because 4 batches is the smaller cap, A is the limiting reagent.

3. Use that batch count to find leftovers

After the full-yield run, 3 packets of B remain while A is fully used.

Limiting reagent

A limits; 3 B packets remain.

A limits because it supports fewer full recipe batches than B.

Common misconception

Use this only when you want to pressure-test a mistaken intuition.

The reactant with the smaller raw packet count must always be the limiting reagent.

Limiting reagent depends on the recipe as well as the starting supplies.

A reactant can start with more packets and still be limiting if each batch uses that reactant more quickly.

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A limits first

Open a run where A supports fewer full batches than B, so A runs out first and B is left over.

B limits first

Open a run where B supports fewer full batches than A, so B runs out first and A is left over.

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Stoichiometry and Yield

Limiting Reagent and Leftover Reactants appears later in this track, so it is cleaner to start from the beginning first.

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