6.1: The Mole

Learning Objectives

• To define the mole unit.

Figure \(\PageIndex{1}\) shows that we need 2 hydrogen atoms and 1 oxygen atom to make 1 water molecule. If we want to make 2 water molecules, we will need 4 hydrogen atoms and 2 oxygen atoms. If we want to make 5 molecules of water, we need 10 hydrogen atoms and 5 oxygen atoms. The ratio of atoms we will need to make any number of water molecules is the same: 2 hydrogen atoms to 1 oxygen atom.

Figure \(\PageIndex{1}\) Water Molecules. The ratio of hydrogen atoms to oxygen atoms used to make water molecules is always 2:1, no matter how many water molecules are being made.

One problem we have, however, is that it is extremely difficult, if not impossible, to organize atoms one at a time. We deal with billions of atoms at a time. How can we keep track of so many atoms (and molecules) at a time? Wouldn't it be easier to do it by using mass rather than by counting individual atoms? Mass is something you can do in the lab pretty easily, unlike counting billions of atoms.

Johann Loschmidt, working in Germany in the mid-1800's, worked out a way to move between mass and number of atoms very easily. We already have the mass numbers of the elements in the periodic table, such as 1 u for hydrogen, or 12 u for carbon. Because protons and neutrons are essentially equivalent in mass, if we have 1.0 grams of hydrogen in one container, and 12.0 grams of carbon in another, we have the same number of atoms in both. So the number of grams of a substance is directly related to the mass number. 1.0 grams of hydrogen is called one mole of hydrogen. 12.0 grams of C is one mole of carbon. Through further work of Loschmidt and others, the number of atoms in a mole was calculated and found to be really very large, and very exact: 6.022 × 10²³

Chemists use the term mole to represent a large number of atoms or molecules. Just as a dozen implies 12 things, a mole (abbreviated 'mol') represents 6.022 × 10²³ things. The number 6.022 × 10²³, called Avogadro’s Number after the 19th-century chemist Amedeo Avogadro, is the number we use in chemistry to represent macroscopic amounts of atoms and molecules. Thus, if we have 6.022 × 10²³ O atoms, we say we have 1 mol of O atoms. If we have 2 mol of Na atoms, we have 2 × (6.022 × 10²³) Na atoms, or 1.2044 × 10²⁴ Na atoms. Similarly, if we have 0.5 mol of benzene (C₆H₆) molecules, we have 0.5 × (6.022 × 10²³) C₆H₆ molecules, or 3.011 × 10²³ C₆H₆ molecules.

A mole represents a very large number! If 1 mol of quarters were stacked in a column, it could stretch back and forth between Earth and the sun 6.8 billion times.

Notice that we are applying the mole unit to different types of chemical entities. In these examples, we cited moles of atoms and moles of molecules. The word mole represents a number of things—6.022 × 10²³ of them—but does not by itself specify what “they” are. They can be atoms, formula units (of ionic compounds), or molecules. That information still needs to be specified.

Because 1 H₂ molecule contains 2 H atoms, 1 mol of H₂ molecules (6.022 × 10²³ molecules) has 2 mol of H atoms. Using formulas to indicate how many atoms of each element we have in a substance, we can relate the number of moles of molecules to the number of moles of atoms. For example, in 1 mol of ethanol (C₂H₆O), we can construct the following relationships (Table \(\PageIndex{1}\)):

Table \(\PageIndex{1}\): Molecular Relationships

1 Molecule of \(C_2H_6O\) Has	1 Mol of \(C_2H_6O\) Has	Molecular Relationships
2 C atoms	2 mol of C atoms	\(\mathrm{\dfrac{2\: mol\: C\: atoms}{1\: mol\: C_2H_6O\: molecules}}\) or \(\mathrm{\dfrac{1\: mol\: C_2H_6O\: molecules}{2\: mol\: C\: atoms}}\)
6 H atoms	6 mol of H atoms	\(\mathrm{\dfrac{6\: mol\: H\: atoms}{1\: mol\: C_2H_6O\: molecules}}\) or \(\mathrm{\dfrac{1\: mol\: C_2H_6O\: molecules}{6\: mol\: H\: atoms}}\)
1 O atom	1 mol of O atoms	\(\mathrm{\dfrac{1\: mol\: O\: atoms}{1\: mol\: C_2H_6O\: molecules}}\) or \(\mathrm{\dfrac{1\: mol\: C_2H_6O\: molecules}{1\: mol\: O\: atoms}}\)

The following example illustrates how we can use these relationships as conversion factors.

Example \(\PageIndex{1}\)

If a sample consists of 2.5 mol of ethanol (C₂H₆O), how many moles of carbon atoms, hydrogen atoms, and oxygen atoms does it have?

Solution

Using the relationships in Table \(\PageIndex{1}\), we apply the appropriate conversion factor for each element:

Note how the unit mol C₂H₆O molecules cancels algebraically. Similar equations can be constructed for determining the number of H and O atoms:

\(\mathrm{2.5\: mol\: C_2H_6O\: molecules\times\dfrac{6\: mol\: H\: atoms}{1\: mol\: C_2H_6O\: molecules}=15\: mol\: H\: atoms}\)

\(\mathrm{2.5\: mol\: C_2H_6O\: molecules\times\dfrac{1\: mol\: O\: atoms}{1\: mol\: C_2H_6O\: molecules}=2.5\: mol\: O\: atoms}\)

Exercise \(\PageIndex{1}\)

If a sample contains 6.75 mol of Na₂SO₄, how many moles of sodium atoms, sulfur atoms, and oxygen atoms does it have?

The fact that 1 mol equals 6.022 × 10²³ items can also be used as a conversion factor.

Example \(\PageIndex{2}\)

How many formula units are present in 2.34 mol of NaCl? How many ions are in 2.34 mol?

Solution

Typically in a problem like this, we start with what we are given and apply the appropriate conversion factor. Here, we are given a quantity of 2.34 mol of NaCl, to which we can apply the definition of a mole as a conversion factor:

\(\mathrm{2.34\: mol\: NaCl\times\dfrac{6.022\times10^{23}\: NaCl\: units}{1\: mol\: NaCl}=1.41\times10^{24}\: NaCl\: units}\)

Because there are two ions per formula unit, there are

\(\mathrm{1.41\times10^{24}\: NaCl\: units\times\dfrac{2\: ions}{NaCl\: units}=2.82\times10^{24}\: ions}\)

in the sample.

Exercise \(\PageIndex{2}\)

How many molecules are present in 16.02 mol of C₄H₁₀? How many atoms are in 16.02 mol?

Concept Review Exercise

1. What is a mole?

Answer

1. A mole is 6.022 × 10²³ things.

Key Takeaway

• A mole is 6.022 × 10²³ things.

Exercises

1. How many dozens are in 1 mol? Express your answer in proper scientific notation.

2. A gross is a dozen dozen, or 144 things. How many gross are in 1 mol? Express your answer in proper scientific notation.

3. How many moles of each type of atom are in 1.0 mol of C₆H₁₂O₆?

4. How many moles of each type of atom are in 1.0 mol of K₂Cr₂O₇?

5. How many moles of each type of atom are in 2.58 mol of Na₂SO₄?

6. How many moles of each type of atom are in 0.683 mol of C₃₄H₃₂FeN₄O₄? (This is the formula of heme, a component of hemoglobin.)

7. How many molecules are in 16.8 mol of H₂O?

8. How many formula units are in 0.778 mol of iron(III) nitrate?

9. A sample of gold contains 7.02 × 10²⁴ atoms. How many moles of gold is this?

10. A flask of mercury contains 3.77 × 10²² atoms. How many moles of mercury are in the flask?

11. An intravenous solution of normal saline may contain 1.72 mol of sodium chloride (NaCl). How many sodium and chlorine atoms are present in the solution?

12. A lethal dose of arsenic is 1.00 × 10²¹ atoms. How many moles of arsenic is this?

Answers

1. 5.018 × 10²² dozens

3. 6.0 mol of C atoms, 12.0 mol of H atoms, and 6.0 mol of O atoms

5. 5.16 mol of Na atoms, 2.58 mol of S atoms, and 10.32 mol of O atoms

7. 1.012 × 10²⁵ molecules

9. 11.7 mol

11. 1.04 × 10²⁴ Na atoms and 1.04 × 10²⁴ Cl atoms