Chapter 2:  The Mole

 

Section 2-1:  Introduction to Avogadro’s Number and the Mole

Section 2-2: Molar Mass

Section 2-3:  Converting Between Mass and Number of Atoms or Molecules

Chapter 2 Practice Exercises and Review Quizzes

 

 

 

 

 

Section 2-1:  Introduction to Avogadro’s Number and the Mole

 

The unit mole (mol) is used to describe the amount of a substance: 

 

1 mol = 6.02 x 1023 atoms or molecules (more than one bonded atom)

 

The number 6.02 x 1023 is known as Avogadro’s number and will be given the symbol NA in calculations throughout this textbook.  The following problem will demonstrate how to convert back and forth between an amount in moles and the number of atoms or molecules using dimensional analysis:

 

Sample Exercise 2A:

 

Convert the following:

 

(a) 3.00 moles of silver to number of silver atoms

(b) 1.2 x 1021 gold atoms to moles of gold

(c) 0.04 moles of carbon dioxide to number of carbon dioxide molecules

(d) 5.42 x 1022 water molecules to moles of water

Solution:

 

 

 

 

Section 2-2:  Molar Mass

 

The mass of one mole of an element or compound in the unit grams per mole (g/mol) is known as the molar mass (M) and can be obtained from the periodic table.  Note that the molar masses found on different periodic tables may differ slightly.  Throughout this textbook, we will round all molar mass values to 4 sig. fig.s.  The following table shows some examples:

 

Element or Compound

M (g/mol)

Cu

63.55

Cl2

2(35.45) = 70.90

C6H12O6

6(12.01) + 12 (1.008) + 6(16.00) = 180.2

(NH2)2CO

2(14.01) + 4(1.008) + 12.01 + 16.00 = 60.06

 

Once we have obtained the molar mass of an element or compound, we can convert back and forth between an amount in moles and the mass using dimensional analysis as demonstrated in the following problem:

 

Sample Exercise 2B:

 

Convert the following:

 

(a) 0.03 moles of copper to grams

(b) 14.2 grams of Cl2 to moles

(c) 40. moles of C6H12O6 to grams

(d) 0.360 kilograms of (NH2)2CO to moles

 

Solution:

 

 

 

 

Section 2-3:  Converting Between Mass and Number of Atoms or Molecules

 

We can combine the methods used in the previous two sections to convert back and forth between mass and the number of atoms or molecules using dimensional analysis and one of the following strategies:

 

 

The following problem will demonstrate each of the strategies above:

 

Sample Exercise 2C:

 

Convert the following:

 

(a) 0.080 grams of I2 to number of I2 molecules

(b) 2.29 x 1024 CS2 molecules to kilograms of CS2

 

Solution:

 

(a) M of I2 = 2(126.9) = 253.8 g/mol

 

 

 

(b) M of CS2 = 12.01 + 2(32.07) = 76.15 g/mol

 

 

Note that it was not necessary to stop and actually calculate the moles along the way to finding the final answer.

 

 

If we know the number of molecules in a sample, we can calculate the number of atoms of any element present in the sample as follows:

 

Sample Exercise 2D:

 

A sample contains 8.40 x 1025 molecules C6H4(NO2)2.  How many oxygen atoms are in the sample?

 

Solution:

 

We see from the chemical formula that one molecule contains 4 oxygen atoms, so we simply multiply the number of molecules by a conversion factor representing the number of oxygen atoms per one molecule:

 

 

 

 

Chapter 2 Practice Exercises and Review Quizzes:

2-1) Convert the following:


a. 20. kilograms iron to number of iron atoms

b. 2.76 x 1017 zinc atoms to milligrams zinc

Click for Solution

2-1)


2-2) Convert 4.4 x 1021 molecules of C6H4(OH)2 to kilograms.

Click for Solution

 

2-2) M of C6H4(OH)2 = 6(12.01) + 6(1.008) + 2(16.00) = 110.1 g/mol

 

 

 

 

 

2-3) Convert 640. milligrams of (CH3)3N to molecules.  How many hydrogen atoms are in this sample?

Click for Solution

 

2-3) M of (CH3)3N = 3(12.01) + 9(1.008) + 14.01 = 59.11 g/mol

 

 

 

 

 

Click for Review Quiz 1

Click for Review Quiz 1 Answers