Q: If a reaction has a theoretical yield of 100 g but only 80 g of product is obtained, what is the percent yield?

$$\frac{80}{100} \times 100 = 80\%$$

Q: A reaction produces 12.0 g of product, but the theoretical yield is 15.0 g. What is the percent yield?

$$\frac{12.0}{15.0} \times 100 = 80\%$$

Question 1

Which of the following best describes the limiting reagent in a chemical reaction?

Accepted Answer

The reactant that is completely consumed and determines the maximum amount of product formed.

Question 2

In the reaction $$2 \mathrm{Al} + 3 \mathrm{Fe}_2\mathrm{O}_3 \rightarrow 2 \mathrm{Al}_2\mathrm{O}_3 + 6 \mathrm{Fe}$$, if 150 g of Al and 432 g of Fe$$_2O$_3$$ are reacted, which is the limiting reagent?

Accepted Answer

Aluminum (Al)

Question 3

What is the theoretical yield in a chemical reaction?

Accepted Answer

The maximum amount of product that can be formed from the limiting reagent.

Question 4

Given the reaction $$2 \mathrm{C}_2\mathrm{H}_4 + 2 \mathrm{NH}_3 + 3 \mathrm{O}_2 \rightarrow 2 \mathrm{C}_2\mathrm{H}_3\mathrm{N} + 6 \mathrm{H}_2\mathrm{O}$$, if you start with 12.0 g C$$_2H$_4$$, 10.0 g NH$$_3$$, and 5.0 g O$$_2$$, which reactant is the limiting reagent?

Accepted Answer

O$$_2$$

Question 5

In a mole-to-mole comparison, what is the key value used from the balanced chemical equation?

Accepted Answer

The coefficients in the balanced equation.

Question 6

If 12.80 g of H$$_2S $$reacts with excess Cr$$_2O$_3$$ according to the equation $$Cr_2O_3 + 3 H_2S \rightarrow 2 CrS + 3 H_2O$$, what is the mass of CrS produced? (Molar masses: H$$_2S = 34.08 g/$$mol, CrS = 84.07 g/mol)

Accepted Answer

18.90 g

Question 7

Which step comes first when solving a limiting reagent problem?

Accepted Answer

Convert grams of each reactant to moles.

Question 8

A student mixes 10.0 g of NH$$_3$$ and 12.0 g of C$$_2H$_4$$ for the reaction $$2 \mathrm{C}_2\mathrm{H}_4 + 2 \mathrm{NH}_3 + 3 \mathrm{O}_2 \rightarrow 2 \mathrm{C}_2\mathrm{H}_3\mathrm{N} + 6 \mathrm{H}_2\mathrm{O}. $$What is the first step to determine the mass of acrylonitrile (C$$_2H$_3N) $$produced?

Accepted Answer

Convert grams of each reactant to moles.

Question 9

In the reaction $$2 \mathrm{Al} + 3 \mathrm{Fe}_2\mathrm{O}_3 \rightarrow 2 \mathrm{Al}_2\mathrm{O}_3 + 6 \mathrm{Fe}$$, if Al is the limiting reagent, what is the correct sequence to find the mass of Fe produced?

Accepted Answer

Convert grams of Al to moles, use the mole ratio to find moles of Fe, then convert moles of Fe to grams.

Question 10

Why is it important to identify the limiting reagent in a chemical reaction?

Accepted Answer

To calculate the maximum amount of product that can be formed.

Question 11

If a reaction has a theoretical yield of 100 g but only 80 g of product is obtained, what is the percent yield?

Accepted Answer

$$\frac{80}{100} 	imes 100 = 80\%$$

Question 12

A reaction produces 12.0 g of product, but the theoretical yield is 15.0 g. What is the percent yield?

Accepted Answer

$$\frac{12.0}{15.0} 	imes 100 = 80\%$$

Question 13

Which of the following is NOT a step in solving a stoichiometry problem involving limiting reagents?

Accepted Answer

Add the masses of all reactants to get the mass of product.

Question 14

In a reaction, if one reactant is present in excess, what happens to it after the reaction is complete?

Accepted Answer

It remains unreacted.

Question 15

Which equation is used to convert moles of a given reactant to moles of an unknown product in a balanced chemical reaction?

Accepted Answer

$$	ext{moles of unknown} = 	ext{moles of given} 	imes \frac{	ext{coefficient of unknown}}{	ext{coefficient of given}}$$

Question 16

A chemist wants to produce the maximum amount of water from the reaction $$2 \mathrm{H}_2 + O_2$ \rightarrow 2 \mathrm{H}_2O. If $$she has 10.0 g of H$$_2$$ and 80.0 g of O$$_2$$, which reactant will limit the amount of water produced?

Accepted Answer

Hydrogen (H$$_2)$$

Question 17

In the reaction $$Cr_2O_3 + 3 H_2S \rightarrow 2 CrS + 3 H_2O$$, what is the mole ratio of H$$_2S to $$CrS?

Accepted Answer

$$3:2$$

Question 18

A student calculates the mass of product using the excess reagent instead of the limiting reagent. What is the likely result?

Accepted Answer

The calculated mass will be too high.

Question 19

Which of the following is the correct sequence for solving a multi-step stoichiometry problem?

Accepted Answer

Convert grams to moles, use mole ratio, convert moles to grams.

Limiting Reagent and Mole-to-Mole Comparisons in Chemical Reactions

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