4.2: Limiting and Excess Reagents

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Limiting Reagent

Exercise \(\PageIndex{1}\)

The box below shows a group of nitrogen and hydrogen molecules that will react to produce ammonia, NH₃. What is the limiting reagent?

Answer: Nitrogen is the limiting reagent.

Exercise \(\PageIndex{2}\)

Given the statement “20.0 g of methane is burned in excess oxygen,” is it obvious which reactant is the limiting reagent?

Answer: Yes; methane is the limiting reagent.

Calculations with Limiting Reagent

Exercise \(\PageIndex{3}\)

Acetylene (C₂H₂) is formed by reacting 7.08 g of C and 4.92 g of H₂.

2C(s) + H₂(g) → C₂H₂(g)

What is the limiting reagent? How much of the other reactant is in excess?

Answer: C is the limiting reagent; 4.33 g of H₂ are left over.

Exercise \(\PageIndex{4}\)

To form the precipitate PbCl₂, 2.88 g of NaCl and 7.21 g of Pb(NO₃)₂ are mixed in solution. How much precipitate is formed? How much of which reactant is in excess?

Answer: 6.06 g of PbCl₂ are formed; 0.33 g of NaCl is left over.

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