Limiting and Excess Reagents: Fact or Fiction?

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In summary, the limiting reagent in a chemical reaction is determined by comparing the ratio of moles of reactants to the ratio of moles needed to react completely. The excess reagent is the one that is left over after the reaction is complete. A balanced equation is necessary to determine the limiting reagent. The reactant with the smallest given mass may not always be the limiting reagent due to the importance of molar mass in determining the ratio of moles.
  • #1
ProblemSets
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Which of the following is NOT true about limiting and excess reagents?

a.)The amount of product obtained is determined by the limiting reagent.
b.)Some of the excess reagent is left over after the reaction is complete.
c.)A balanced equation is necessary to determine which reactant is the limiting reagent.
d.)The reactant that has the smallest given mass is the limiting reagent



I know that A is true and that B is true for sure. I am not sure about C. To me it seems that I always balance an equation, but maybe that is unneccesary work. My guess though is that because of molar mass, D is not true.

Am i correct?
 
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  • #2
Why would molar mass make D untrue?
 
  • #3
well yes molar mass is important.

example:

if you have 40 grams of hydrogen gas and 50 grams of oxygen gas and you're making water, the balanced equation is 2H2 + O2 -> 2H2O . to find limiting reagent, you need to compare the ratio of the moles you HAVE of reactants to the ratio of moles you NEED to react as far as possible. so to get moles you use molar mass and end up with approximately 20 mol of H2 and 1.6 mol of O2.

you answer's in these questions:
-which will run out first?
-does the fact that there's a total smaller mass of hydrogen gas than oxygen gas?
-did your equation need to be balanced to find the ratio?
 
  • #4
I was kinda hoping that Problemsets would answer that one...
 

What is a limiting reagent?

A limiting reagent is a substance that is completely used up in a chemical reaction, limiting the amount of product that can be formed.

What is an excess reagent?

An excess reagent is a substance that is left over after a reaction has reached completion, meaning there is still some of it left even though the reaction has stopped.

How do you determine the limiting reagent?

The limiting reagent can be determined by calculating the moles of each reactant and comparing them to the mole ratio in the balanced chemical equation. The reactant with the lower moles is the limiting reagent.

What happens if there is an excess of the limiting reagent?

If there is an excess of the limiting reagent, it will not affect the amount of product that can be formed. The excess reagent will simply be left over after the reaction is complete.

How does the presence of a limiting or excess reagent affect the yield of a reaction?

The limiting reagent determines the maximum amount of product that can be formed in a reaction, so having an excess of it will not increase the yield. However, an excess reagent may affect the purity of the product and may need to be removed through purification methods.

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