The calculation of chemical quantities in chemical reactions
(What you need to remember to succeed in this chapter)

Balance Chemical Equations

Mole conversions (Moles  molecules, Moles  grams, Moles  liters (@ STP)
Chemists often times want to relate the amount of product made in a chemical reaction to the amount of reactants that they started off with. An example:
Let’s apply this to a chemical system:
5
12(g)
2(g)

2(g)
2
(g)
We can look at the above reaction in terms of atoms, molecules, mass and volume, but first we have to balance the chemical equation.

Mole-Mole Calculations:
Chemical equations provide more information than we have had before. We can now relate moles of product to the moles of reactant (or any species in a chemical equation, really) in mathematical equations.
Mole ratio: A conversion factor derived from the coefficients of a balanced equation

This means you must ALWAYS check to see if the equation given to you is balanced

Used when you want to relate different chemicals to each other (moles of A to moles of B)
Let’s write some mole ratios for the reaction on the previous page:
___C
5
H
12(g)
+ ___O
2(g)

___CO
2(g)
+ ___H
2
O
(g)
Example 1: How many moles of water are produced when 4 moles of oxygen are reacted?
Example 2: How many moles of pentane (C
5
H
12
) are needed to produce 12 moles of CO
2
?
Example 3:
___Al
(s)
+ ___O
2(g)

___Al
2
O
3(s)
Write the six mole ratios that can be derived from the equation.
How many moles of Al are needed to form 3.7 moles of Al
2
O
3
?
How many moles of O
2
are required to react with 14.8 moles of Al?
How many moles ofAl
2
O
3
are formed when 0.78 moles of O
2
react with excess Al?

(so draw it again here)
In a lab chemists don’t look at moles directly….
they look at mass (unit = grams), volumes (unit = L or mL), and sometimes representative particles (atoms, formula units, and molecules).

These problems are like the ones we have already done (i.e. you will always use the mole ratio), but we add one or two more conversion factors to the problem.
1.
Convert the given quantity in the problem to moles. Already in moles? Skip this step.
2.
Use the mole ratio. Hint: You will never skip this step.
3.
Convert the value to the unit asked for in the problem. If it asks for moles…then you are done

1 C
5
H
12(g)
+ 8 O
2(g)

5 CO
2(g)
+ 6 H
2
O
(g)
Example 4: How many moles of carbon dioxide will be produced when 20.5 grams of pentane react with excess oxygen gas?
Example 5: How many grams of oxygen gas are needed to react if 12.8 moles of water are formed?

(In chemistry these are the most common)
Example 6: How many grams of water will be formed when 552.3 grams of C
5
H
12
are reacted in the presence of oxygen gas?
This is about as hard as I can make them….unless I decide to give you mass in kilograms rather than grams….or maybe instead of grams I give you molecules or liters! Let’s practice.
We use the same conversion factors that we used in chapter 7 (6.20x10
23
rp = 1 mole and 22.4 L = 1 mole). The goal: Get whatever number is given to you in moles before you use the mole ratio.
Example 7: How many molecules of O2 are produced by the decomposition of 6.54 grams of KClO
3
?
___KClO
3

___KCl + ___O
2
Example 8: How many liters of O2 are required to burn 3.86 liters of CO?
___CO
(g)
+ ___O
2(g)

___CO
2(g)
You will get more practice on all of these problems before the test

. Practice makes proficient. To ace the test make sure you do all of the homework (go back to problems you don’t understand) and participate actively in class.

The only way to get good at stoichiometry problems is to practice. I’d like to tell you that book osmosis works, or even copying your neighbor’s homework, but come test day, how much you have practiced will determine your success on the test. This series of worksheets will work you though solving the tough problems with
(hopefully) no problem at all.
Answer all questions on a separate piece of paper. Transfer your numerical answer to this worksheet to make it easier to check your answers in class. No work = no credit.
5 F
2
+ 2 NH
3

N
2
F4 + 6 HF
1. 5.0 moles NH
3 moles HF moles NH
3 moles F
2 moles of F
2 moles N
2
F
4
2. 2.13 moles F
2
3. 0.52 moles N
2
F
4
4. 8.2 moles HF
5. 0.112 moles HF
6. 0.92 moles N
2
F
4
7. 1.52 moles F
2
8. 7.3 moles NH
3
9. 0.056 moles N
2
F
4
10. 4.5 moles HF
11. 49 g F
2
12. 19.6 g HF
13. 238 g N
2
F
4
14. 521 g NH
3
15. 88 g NH
3
16. 42.9 g HF
17. 8.22 g N
2
F
4
18. 0.958 g NH
3
19. 16000 g N
2
F
4
20 1562433 g HF g HF g NH
3 g F
2 g NH
3 g N
2
F
4 moles HF moles N
2
F
4 moles F
2 moles F
2 moles N2F
4 g F
2 g NH
3 g HF g F
2 g HF

The only way to get good at stoichiometry problems is to practice. I’d like to tell you that book osmosis works, or even copying your neighbor’s homework, but come test day, how much you have practiced will determine your success on the test. This series of worksheets will work you though solving the tough problems with
(hopefully) no problem at all.
Answer all questions on a separate piece of paper. Transfer your numerical answer to this worksheet to make it easier to check your answers in class. No work = no credit.
4NH
3
+ 6NO  5 N
2
+ 6H
2
O
1. 10 L NH
3 moles N
2
2. 124 L NO
3. 16 L H
2
O
4. 5 moles NO
5. 0.627 moles NH
3
6. 15 moles NO
7. 0.7 moles N
2
8. 0.30 moles NH
3
9. 7x10 25 molecules H
2
O
10. 8.2 x 10 21 molecules NO
11. 67 g NH
3
12. 983 moles NO
13. 5 moles H
2
O
14. 72 g N
2
15. 1954 g NH
3
16. 17 L NO
17. 0.5 L N
2
18. 6.2x10
26 molecules NO
19. 2.8x10
13 molecules NH
3
20 120 L NO moles H
2
O moles NO
L H
2
O
L N
2 molecules H
2
O molecules NH
3 molecules N
2 moles N
2 moles NH
3 moles N
2 moles NH
3 g N
2 g H
2
O g NO
L NH
3
L H
2
O molecules N
2 molecules N
2 molecules H
2
O

The only way to get good at stoichiometry problems is to practice. I’d like to tell you that book osmosis works, or even copying your neighbor’s homework, but come test day, how much you have practiced will determine your success on the test. This series of worksheets will work you though solving the tough problems with
(hopefully) no problem at all.
Answer all questions on a separate piece of paper. Transfer your numerical answer to this worksheet to make it easier to check your answers in class. No work = no credit.
___C
2
H
6
+ ___O
2
 ___CO
2
+ ___H
2
O
1. 7.0 moles O
2
L CO
2
2. 1.5x10
46 molecules C
2
H
6
3. 184 g O
2
4. 1.9 moles C
2
H
6
moles H
2
O moles C
2
H
6
L CO
2
5. 120 moles CO
2
6. 15 g H
2
O
7. 0.9 L O
2
8. 7x10
22
molecules C
2
H
6
9. 20 g O
2
10. 8.0x10
25 molecules CO
2
11. 2.52 L O
2
12. 79 g CO
2
13. 0.2 L O
2
14. 8.2x10
24 molecules CO
2
15. 945 g H
2
O
16. 12g H
2
O
17. 5x10 28 molecules O
2
18. 22.4 L of CO
2
19. 9x10 20 molecules C
2
H
6
20 80 g H
2
O g C
2
H
6 molecules CO
2 molecules C
2
H
6 g CO
2
L CO
2 molecules H
2
O g CO
2 molecules H
2
O molecules C
2
H
6 molecules C
2
H
6
LO
2 g C
2
H
6
L CO
2
LO
2 g C
2
H
6
L C
2
H
6

9
PRACTICE PROBLEMS
In your notebook, solve the following problems.
Use the 3-step problem-solving approach you learned in Chapter 4.
1. An apple pie needs 10 large apples, 2 crusts (top and bottom), and 1 tablespoon of cinnamon. Write a balanced equation that fits this situation. How many apples are needed to make 25 pies?
2. Two moles of potassium chloride (KCl) and three moles of oxygen (O
2
) are produced from the decomposition of two moles of potassium chlorate, KClO
3
( s ). Write the balanced equation. How many moles of oxygen (O
2
) are produced from twelve moles of potassium chlorate?
3. Using the equation from problem 2, how many moles of oxygen are produced from 14 moles of potassium chlorate?
4. Two molecules of hydrogen (H
2
) react with one molecule of oxygen (O
2
) to produce two molecules of water. How many molecules of water are produced from 2.0 X 10
23 molecules of oxygen? How many moles of water are produced from 22.5 moles of oxygen?
1. Calculate the number of moles of hydrogen chloride (HCl) produced from 10 moles of hydrogen (H
2
).
H
2
( g ) + Cl
2
( g )

2HCl( g )
2. Calculate the number of moles of chlorine (Cl
2
) needed to form 14 moles of iron(III)chloride (FeCl
3
).
2Fe( s ) + 3Cl
2
( g )

2FeCl
3
( s )
3. Calculate the number of grams of nitrogen dioxide (NO
2
) that are produced from 4 moles of nitric oxide (NO).
2NO( g ) + O
2
( g )

2NO
2
( g )
4. Calculate the mass of oxygen (O
2
) produced from the decomposition of 75.0 g of potassium chlorate (KClO
3
).
2KClO
3
( s )

2KCl( s ) + 3O
2
( g )
5. Calculate the mass of silver needed to react with chlorine to produce 84 g of silver chloride.
Hint: Balance the equation first. Ag + Cl
2

AgCl
6. How many liters of carbon monoxide (CO) at STP are needed to react with 4.80 g of oxygen gas ( O
2
) to produce carbon dioxide?
2CO( g ) + O
2
( g )

2CO
2
( g )
7. Calculate the number of liters of oxygen gas ( O
2
) needed to produce 15.0 liters of dinitrogen trioxide (N
2
O
3
). Assume all gases are at the same conditions of temperature and pressure.
2N
2
( g ) + 3O
2
( g )

2N
2
O
3
( g )