Stoichiometry

We use stoichiometry when given the KNOWN quantity of a compound or element and asked to find the UNKNOWN quantity of another compound or element in a balanced chemical equation. 

Stoichiometric Reactions

Concept: Defining Stoichiometry

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Video Transcript

In this example, we have 2 H2 gas reacting with 1 02 gas to give us 2 H2O gas. We're going to say that these numbers that are in bold are called our coefficients.
We're going to say that these coefficients basically tell us the number of moles of each compound that’s reacting within our balanced equation. So what it's really saying is for every two moles of H2 gas we have one mole of O2 gas that reacts with it. Together they produce two moles of H2O gas as product. We're going to say the numerical relationship that each of these compounds have with each other in a balanced equation is called stoichiometry. 

Concept: The Stoichiometric Chart

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Video Transcript

Since we're talking about stoichiometry, it becomes important to plan some type of attack when it comes to any type of stoichiometric question. To do this and to help us, we're going to use this stoichiometric chart.
Now what we should realize is the stoichiometric chart is broken up into two basic types. We have our given information and that will be information they tell you in the very beginning of the chemical equation, chemical question. We're going to say that the given information will be in grams, moles or maybe entities. When I say entities—entities means atoms, molecules or particles.
Now the other part of this stoichiometric chart deals with unknown information, basically what we're asked to find. We going to say this unknown information, we could be asked to find moles, grams or entities.
Now, the way this works is, our main goal is to go from moles of given and jump over to our unknown information. So, as we're making the transition from moles of given to moles of unknown, we're basically taking a leap of faith because we're going from an area where we know information to an area where we know nothing at all. So we're going to say that this transition is called 'The Jump', so as we go from moles of given to moles of unknown, that's our jump.
We're going to say when we make this transition, we have to do what called a mole to mole comparison. Now what that means is—that means that we have to use the coefficients in the balanced equation. So, this is basically the only time we're going to look at the coefficients in our balanced equation. So use the coefficients in the balanced equation.
Now when do we use this chart? We use this chart anytime they give us chemical equation, a balanced chemical equation with the known quantity of a compound or element and they ask us to find the unknown quantity of another compound or element. 
 

Example: How many grams of H2O are produced when 12.3 g Hreacts?

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Remember whenever you have to find the unknown quantity of a compound or element just start with your given quantity and use the stoichiometric chart for guidance. 

Stoichiometric Reaction Calculations

The word “excess” means that a compound isn’t important in our calculations and can be ignored. 

Example: The oxidation of chromium solid is represented by the following equation:  

a)   How many moles of chromium (III) oxide are produced when 34.69 g Cr reacts with excess oxygen gas?

b)   How many grams of O2 were needed to produce 4.28 x 103 molecules Cr2O3

14m

What happens when we mix stoichiometry and the old concept of density? Let’s see. 

Example: If the density of ethanol, CH3CH2OH, is 0.789 g/mL, how many milliliters of ethanol are needed to produce 4.8 g of H2O in the following reaction? 

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For those up for a challenge you maybe given the compounds in a reaction and asked to write it out and balance it, followed by doing the stoichiometric problem. 

Problem: Dinitrogen monoxide gas decomposes to form nitrogen gas and oxygen gas. How many molecules of oxygen are formed when 8.00 g of dinitrogen monoxide decomposes?

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Limiting Reactant or Reagent

The limiting reactant or limiting reagent is the compound that determines the maximum amount of product obtained in a chemical reaction. 

Concept: Understanding the limiting reactant or reagent

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Video Transcript

We’re going to say in a chemical reaction, the reactant that’s consumed or used up when a reaction occurs and determines the maximum amount of product formed, is called the limiting reactant. You might also hear your professor say limiting reagent. Reagent, reactant, they both mean the same exact thing.
Now, we’re going to say the amount of product the limiting reactant forms is called the theoretical yield, and we’re going to say the reactant that remains after the completion of the chemical reaction is called the excess reactant. So these are three terms that sometimes we all may get confused about what exactly do they mean, but just realize that they’re each distinct from each other.  

In a limiting reagent or reactant question we are given information on more than one compound and asked to find the unknown quantity of another compound (usually the product). 

Example: Chromium (III) oxide reacts with hydrogen sulfide (H2S) gas to form chromium (III) sulfide and water:

What is the mass of chromium (III) sulfide formed when 14.20 g Cr2O3 reacts with 12.80 g H2S? 

8m

The limiting reactant is the one that shows a smaller product yield, with that yield representing the theoretical yield. The excess reactant is that one that shows a greater product yield.

Example: Identify the limiting reactant, excess reactant and theoretical yield

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Percent Yield

The percent yield determines how efficiently your reaction was in producing products. The higher the percent yield then the more successful the chemical reaction

Concept: Understanding Percent Yield

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Video Transcript

We're going to say the percent yield of a reaction is basically a way of us determining how efficient or effective our chemical reaction is in producing the products that we want. We're going to say a high percent yield would mean that your chemical reaction is highly successful.
Basically, when you do any type of chemical reaction, it's not only important that you create the products that you need, but it's also important that you create a great amount of product, a lot of product. Now, you'll notice that in lab, your professors are always asking you, "What is the percent yield?" that's because they not only want to test to see if you can form those products, they want to see if you can isolate a great amount of that product. 

The amount of product we isolate when a lab experiment is done is called the actual yield. Relating that with our theoretical yield helps us determine the percent yield of our reaction. 

Example: A scientist performs an experiment in the laboratory and obtains 13.27 g Cr2S3. If his calculations on scratch paper give him a theoretical yield of 18.23 g what is the percent yield?

3m

By combining your knowledge of stoichiometry to find the theoretical yield we can now use the actual yield to find the percent yield of a chemical reaction. 

Problem: Consider the following balanced chemical reaction: 2 C6H6 (l) + 15 O2 (g) ----> 12 CO2 (g) + 6 H2O (l) a. If a 2.6 g sample of C6H6 reacted with excess O2 to produce 1.25 g of water, what is the percent yield of water?

6m

Problem: Consider the following balanced chemical reaction: 2 C6H6 (l) + 15 O2 (g) ----> 12 CO2 (g) + 6 H2O (l) b. If the above reaction only went to 75% completion, how many moles of CO2 would be produced if 1.57 x 10-5 molecules of C6H6 were reacted with excess oxygen?

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Stoichiometry Additional Practice Problems

Iron metal reacts with bromine as follows: Fe(s) + Br2(g) → FeBr3(s)

How many moles of FeBr3 are obtained when 4.6 mol of Brreacts with excess Fe?

(A) 2.30 mol

(B) 3.07 mol

(C) 4.60 mol

(D) 6.90 mol

(E) 9.2 mol

Watch Solution

How many moles of NH 3 are needed to react completely with 30.0 mol of F 2?

2 NH3 + 5 F2 → N2F4 + 6 HF

(A) 2 mol

(B) 12 mol

(C) 5 mol

(D) 30 mol

(E) 75 mol

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How many moles of oxygen are formed when 58.6 g of KNO 3 decomposes according to the following reaction? The molar mass of KNO3 is 101.11 g/mol.

4 KNO3(s) → 2 K2O(s) + 2 N2(g) + 5 O2(g)

A. 0.290 mol O2

B. 0.580 mol O2

C. 18.5 mol O2

D. 0.724 mol O2

E. 1.73 mol O2

Watch Solution

What volume of 3.05 M HCl(aq) will react with 25.0 g Zn(s)?

 Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)

a. 0.251 L

b. 4.01 L

c. 0.125 L

d. 0.0627 L

e. 2.33 L

 

 

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Aluminum reacts with hydrochloric acid to produce aluminum chloride and hydrogen gas.

        2 Al(s) + 6 HCl(aq) → 2 AlCl3(aq) + 3 H2(g)

What mass of H2(g) is produced from the reaction of 0.75 g Al(s) with excess hydrochloric acid?

a. 0.056 g

b. 0.084 g

c. 0.17 g

d. 1.1 g

e. 2.3 g

 

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Potassium reacts with chlorine gas to produce potassium chloride. How many moles of K will react with 0.40 moles of Cl2?

a. 0.20 mol

b. 0.32 mol

c. 0.80 mol

d. 0.60 mol

e. 0.40 mol

 

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Calculate the oxygen atoms in 25.0g of CaCO3

 

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Each mole of Al(NO)3 contains how many moles of oxygen atoms?

1. 1 mol

2. 6 mol

3. 3 mol

4. 9 mol 

5. 12 mol

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Calculate the answer in GRAMS.

How much copper is in 5.268 mol of copper(II) chloride?

1. 350.981

2. 334.776

3. 297.473

4. 258.454

5. 314.059

6. 305.48

7. 325.244

8. 356.701

9. 320.922

10. 265.953

Watch Solution

If the reaction of 1.00 mole NH3 (g) and 1.00 mole O(g)

4 NH(g) + 5 O(g) → 4 NO(g) + 6 HO(l)

is carried out to completion,

1. all of the O2 is consumed and 1.5 mol of NO(g) is produced.

2. all of the O2 is consumed and 4.0 mol of NO(g) is produced.

3. all of the O2 is consumed and 1.5 mol of HO is produced.

4. all of the NH3 is consumed.

5. 1.5 mol of HO is produced.

6. None of the other answers is correct.

7. all of the NH3 is consumed and 4.0 mol of NO(g) is produced.

8. all of the NH3 is consumed and 1.5 mol of HO is produced.

9. 4.0 mol of NO(g) is produced and 1.5 mol of H2O is produced.

10. all of the O2 is consumed.

Watch Solution

How many grams of P are contained in one mole of Mg  3 (PO4 )2

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What is the mass of 0.1 moles of ammonium nitrate? _________________

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How many grams of oxygen are in 12.4 moles Al 2(SO4)3?

a. 2.38 x 103 g O

b. 9.30 g O

c. 12 g O

d. 192 g O

Watch Solution

How many moles of sulfur are in 12.4 moles Al 2(SO4)3?

a. 3

b. 12.4

c. 37.2

d. 32

Watch Solution

Aqueous solutions of sodium hypochlorite (NaOCl), also known as household bleach, are prepared by reaction of sodium hydroxide with chlorine:

                2 NaOH (aq) + Cl 2 (g)  →  NaOCl (aq) + NaCl (aq) + H 2O (l)

 

The grams of NaOH needed to react with 25.0 g of Cl 2 are:

           

  1. 35.2 g
  2. 55.0g
  3. 28.2 g
  4. 45.3 g
  5. 22.7 g
Watch Solution

How many molecules of H2S are required to form 79.0 g of sulfur according to the following reaction? Assume excess SO2

2 H2S(g) + SO2(g) → 3 S(s) + 2 H2O(l)

A) 1.48 × 1024 molecules H2S

B) 2.44 × 1023 molecules H2S

C) 5.06 × 1025 molecules H2S

D) 3.17 × 1025 molecules H2S

E) 9.89 × 1023 molecules H2S

Watch Solution

How many molecules of HCl are formed when 50.0 g of water reacts according to the following balanced reaction? Assume excess ICl3

2 ICl3 + 3 H2O → ICl + HIO3 + 5 HCl

A) 6.09 × 1025 molecules HCl

B) 2.78 × 1024 molecules HCl

C) 3.65 × 1024 molecules HCl

D) 1.67 × 1024 molecules HCl

E) 5.02 × 1025 molecules HCl

Watch Solution

How many grams of oxygen are formed when 6.21 moles of KOH are formed? 

4 KO (s) + 2 H2O (l) → 4 KOH (s) + O(g)

A) 11.7 g O2

B) 27.9 g O2

C) 3.59 g O2

D) 19.9 g O2

E) 49.7 g O2

Watch Solution

Consider the following balanced reaction. What mass (in g) of CO 2 can be formed from 288 mg of O2? Assume that there is excess C3H7SH present. C3H7SH (l) + 6 O(g) → 3 CO(g) + SO(g) + 4 H2O (g)

A) 0.396 g CO2

B) 0.209 g CO2

C) 0.126 g CO2

D) 0.198 g CO2

E) 0.792 g CO2

Watch Solution

Consider the following balanced reaction. How many grams of water are required to form 75.9 g of HNO3? Assume that there is excess NO2 present. The molar masses are as follows: H2O = 18.02 g/mol, HNO3 = 63.02 g/mol. 

3 NO(g) + H2O (l) → 2 HNO(aq) + NO (g)

A) 38.0 g H2O

B) 26.5 g H2O

C) 10.9 g H2O

D) 21.7 g H2O

E) 43.4 g H2O

Watch Solution

A Freon leak in the air-conditioning system of an old car releases 25 g of CF 2Cl2. How many molecules of Freon are emitted?

a. 1.5 x 1025

b. 0.211

c. 1.2 x 1023

d. 1.8 x 1027

Watch Solution

What is the limiting reactant when 13.9 g of oxygen react with 33.7 g of calcium to produce calcium oxide, and how many grams of calcium oxide are produced?

A)  Calcium is the limiting reactant and 47.1 g of calcium oxide are produced.

B)  Oxygen is the limiting reactant and 24.4 g of calcium oxide are produced.

C)  Calcium is the limiting reactant and 47.1 g of calcium dioxide are produced.

D)  Oxygen is the limiting reactant and 48.7 g of calcium oxide are produced.

E)  Calcium is the limiting reactant and 23.6 g of calcium oxide are produced.

Watch Solution

Calculate the mass of excess reagent remaining at the end of the reaction in which 90.0 g of SO2 are mixed with 100.0 g of O 2.                

2 SO2 + O2 → 2 SO3

A)  11.5 g            

B)  22.5 g            

C)  67.5 g            

D)  77.5 g            

E)  400 g

Watch Solution

Hydrogen peroxide (H2O2) decomposes into water and oxygen gas. How many moles of oxygen gas are formed from the decomposition of 3 moles of hydrogen peroxide?

1. 1

2. 1.5

3. 3

4. 6

5. 2

6. 0

Watch Solution

Determine the percent yield of a reaction that produces 28.65 g of Fe when 50.00 g of Fe2O3 react with excess Al according to the following reaction. (Molar masses: Fe = 55.85 g/mol, O = 16.00 g/mol, Fe2O3 = 159.70 g/mol)

Fe2O3 (s) + 2 Al (s)  →  Al 2O(s) + 2 Fe (s)

A. 61.03 %

B. 28.65 %

C. 57.30 %

D. 20.02 %

E. 81.93 %

Watch Solution

How many grams of nitrogen gas are formed when 58.6 g of KNO  3 decomposes according to the following reaction? (Molar masses: KNO3 = 101.11 g/mol, N2 = 28.02 g/mol)

4 KNO3(s)  →  2 K2O(s) + 2 N2(g)  + 5 O 2(g)

A. 8.12 g

B. 16.3 g

C. 20.3 g

D. 48.5 g

E. 518. g

Watch Solution

Photosynthesis occurs by the following chemical reaction. If a plant consumes 28.9 g of CO2 in one week, what mass of glucose (g) can the plant produce? Molar mass: CO2 = 44.01 g/mol; C6H12O6 = 180.2 g/mol

6CO2(g) + 6H2O → 6O2(g) + C6H12O6(aq)

A. 42.3 g

B. 19.7 g

C. 118 g

D. 60.5 g

Watch Solution

How many grams of P is contained in one mole of Mg 3(PO4)2?

 

 

 

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How many moles of N 2O3 contain 2.55 × 10 24 oxygen atoms?

A) 1.41 moles N2O3

B) 4.23 moles N2O3

C) 12.7 moles N2O3

D) 7.87 moles N2O3

E) 2.82 moles N2O3

Watch Solution

What mass of oxygen is needed for the complete combustion of 155 mL of C6H6 (molar mass = 78.1 g/mol) to produce carbon dioxide and water? Density of C 6H6 = 0.88 g/L.

2 C6H(l)   +   15 O 2 (g)   →   12 CO 2 (g)   +   6 H 2O (l)

 

A. 55.9 g          

B. 419 g          

C. 240 g

D. 330 g          

E. 476 g

Watch Solution

What mass of H 2 is produced when 60.0 g of aluminum reacts with excess HCl?

  Al(s)    +    HCl(aq)    →    AlCl 3(aq)    +    H 2(g)

 

A. 6.73 g          

B. 12.1 g          

C. 4.49 g

D. 2.22 g          

E. 2.99 g

Watch Solution

Diborane, B2H6, can be prepared according to the following equation:

3 NaBH4 (aq) + 4 BF3 → 3 NaBF4 (aq) + 2 B26

If 10.5 moles of NaBF4 are produced in this reaction, how many hydrogen atoms in B 2H6 are also produced?

A. 1.58 x 1024

B. 5.69 x 1025

C. 2.03 x 1023

D. 2.53 x 1025

Watch Solution

When 1 mole of glucose (C6H12O6) is completely burned into CO2, how many moles of H2O are produced? The unbalanced equation for this reaction is as follows:

C6H12O6 + O2 → CO2 + H2O

 

a. 1

b. 2

c. 3

d. 6

e. 12

Watch Solution

Lithium metal is the only member of Group 1 that reacts directly with nitrogen gas to produce a nitride according to:

6 Li (s)+ N2 (g)      →       2 Li 3N(s) 

What mass of lithium nitride can form 48.0 g of Lithium and 7.50 x 10  23 molecules of N 2?

a) 56.8

b) 80.3

c) 96.1

d) 101

e) 115

Watch Solution

What is the minimum grams of air needed for the complete combustion of 8.00 g of heptane (C7H16)? Air is 21% O2 by mass.

 

a) 28

b) 5.9

c) 130

d) 65

e) 18

Watch Solution

For the following acid-base reaction, calculate the mass (in grams) of the acid necessary to completely react with and neutralize 4.85 g of the base.

2 HNO3 (aq) + Ca(OH)2 (aq)      →       2 H 2O (l) + Ca(NO3)2 (aq)

Watch Solution

Sulfuric acid dissolves aluminum metal according to the reaction:

2 Al (s) + 3 H2SO4 (aq)      →       Al 2(SO4)3 (aq) + 3 H2 (g)

Suppose you want to dissolve an aluminum block with a mass of 15.2 g.

a) What minimum mass of H 2SO4 (in grams) do you need?

 

 

 

 

 

 

 

 

 

b) What mass of H 2 gas (in grams) does the complete reaction of the aluminum block produce? 

Watch Solution

For the next four question consider a gaseous hydrocarbon X which contains only carbon and hydrogen. It has a relative molar mass 2.25 times greater than molecular oxygen. The balanced combustion reaction of one mole of hydrogen X is:

1 X + 8 O2 → 5 CO2 + 6H2O

What is the molecular formula for hydrocarbon X?

a. CH4

b. C2H6

c. C3H8

d. C4H10

e. C5H12

 

What is the minimum mass (grams) of hydrocarbon X required to completely react with 4.0 g oxygen as shown (this can be determined without the previous result)?

a. 0.24

b. 0.5

c. 1.1

d. 5.0

e. 8.6

 

Which is true when 2.0 moles of hydrocarbon X react with 14.0 moles of oxygen?

a. All the oxygen is consumed.

b. All the hydrocarbon is consumed.

c. No reagents remain.

d. An equal mass of each reagent remains.

e. None of these.

 

Which is the mass spectrum for the products of the combustion of hydrocarbon X?

 

Watch Solution

A 54 g sample of aluminum reacts completely with 48.0 g of oxygen gas. Which is the formula of the oxide?

a. Al2O3

b. AlO

c. AlO2

d. Al6O5

e. Al3O5

Watch Solution

Calculate the mass of hydrogen and the mass of oxygen required to make 250 L of water assuming the reaction goes to completion (i.e. 100% yield). The molar mass of water is 18.013 g/mol.

2 H2 (g) + O2 (g)      →       2 H 2O (/)

Watch Solution

Taxol is a remarkable new weapon in the war against cancer. Isolated from the bark of the Pacific Yew, it has been shown to be highly effective against refractive ovarian and breast cancers. Due to the extremely low concentrations of it found in nature, a great deal of effort has gone into alternative ways of obtaining it. The Taxol nucleus has the molecular formula C32H40O11. If a 115 mg sample of Taxol were subjected to combustion, calculate both the amount of carbon dioxide and water (in mg) expected. 

Watch Solution

How many grams of Ca(NO 3)2 can be produced by reacting excess HNO3 with 7.40 g of Ca(OH)2?

a) 10.2 g

b) 16.4 g

c) 32.8 g

d) 8.22 g

e) 7.40 g

Watch Solution