Chapter 3
Stoichiometry
Section 3.1
Counting by Weighing
EXERCISE!
A pile of marbles weigh 394.80 g. 10 marbles weigh 37.60 g. How many marbles are in the pile?
Avg. Mass of 1 Marble =
37.60 g
= 3.76 g / marble
10 marbles
394.80 g = 105 marbles
3.76 g
Copyright © Cengage Learning. All rights reserved
2
1
Section 3.2
Atomic Masses
EXERCISE!
An element consists of 62.60% of an isotope with mass 186.956 u and 37.40% of an isotope with mass 184.953 u.
 Calculate the average atomic mass and identify the element.
186.2 u
Rhenium (Re)
Copyright © Cengage Learning. All rights reserved
3
Section 3.3
The Mole
EXERCISE!
Calculate the number of iron atoms in a 4.48 mole sample of iron.
2.70×1024 Fe atoms
Copyright © Cengage Learning. All rights reserved
4
2
Section 3.4
Molar Mass
EXERCISE!
Consider separate 100.0 gram samples of each of the following:
H2O, N2O, C3H6O2, CO2
 Rank them from greatest to least number of oxygen atoms.
H2O, CO2, C3H6O2, N2O
Copyright © Cengage Learning. All rights reserved
5
Section 3.6
Percent Composition of Compounds
EXERCISE!
Consider separate 100.0 gram samples of each of the following:
H2O, N2O, C3H6O2, CO2
 Rank them from highest to lowest percent oxygen by mass. H2O, CO2, C3H6O2, N2O
Copyright © Cengage Learning. All rights reserved
6
3
Section 3.7
Determining the Formula of a Compound
EXERCISE!
The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about 146 g/mol.
 What is the empirical formula?
C3H5O2
 What is the molecular formula?
C6H10O4
Copyright © Cengage Learning. All rights reserved
7
Section 3.9
Balancing Chemical Equations
EXERCISE!
Which of the following correctly balances the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it.
CaO + C CaC2 + CO2
I. II.
III.
IV.
CaO2 + 3C CaC2 + CO2
2CaO + 5C 2CaC2 + CO2
CaO + (2.5)C CaC2 + (0.5)CO2
4CaO + 10C 4CaC2 + 2CO2
Copyright © Cengage Learning. All rights reserved
4
Section 3.10
Stoichiometric Calculations: Amounts of Reactants and Products
EXERCISE!
Consider the following reaction:
P4(s) + 5 O2(g)  2 P2O5(s)
If 6.25 g of phosphorus is burned, what mass of oxygen does it combine with?
8.07 g O2
Copyright © Cengage Learning. All rights reserved
9
Section 3.10
Stoichiometric Calculations: Amounts of Reactants and Products
EXERCISE!
(Part I)
Methane (CH4) reacts with the oxygen in the air to produce carbon dioxide and water.
Ammonia (NH3) reacts with the oxygen in the air to produce nitrogen monoxide and water.
 Write balanced equations for each of these reactions.
Copyright © Cengage Learning. All rights reserved
10
5
Section 3.10
Stoichiometric Calculations: Amounts of Reactants and Products
EXERCISE!
(Part II)
Methane (CH4) reacts with the oxygen in the air to produce carbon dioxide and water.
Ammonia (NH3) reacts with the oxygen in the air to produce nitrogen monoxide and water.
 What mass of ammonia would produce the same amount of water as 1.00 g of methane reacting with excess oxygen?
Copyright © Cengage Learning. All rights reserved
11
Section 3.11
The Concept of Limiting Reactant
EXERCISE!
You react 10.0 g of A with 10.0 g of B. What mass of product will be produced given that the molar mass of A is 10.0 g/mol, B is 20.0 g/mol, and C is 25.0 g/mol? They react according to the equation:
A + 3B 2C
8.33 g of C is produced
Copyright © Cengage Learning. All rights reserved
12
6
Section 3.11
The Concept of Limiting Reactant
EXERCISE!
Consider the following reaction:
P4(s) + 6F2(g) 4PF3(g)
 What mass of P4 is needed to produce 85.0 g of PF3 if the reaction has a 64.9% yield?
46.1 g P4
Copyright © Cengage Learning. All rights reserved
13
7