Chemists use the mole to count atoms, molecules, ions, and formula units.

Section 1: Measuring Matter

KWhat I Know

WWhat I Want to Find Out

LWhat I Learned

• 8(A) Define and use the concept of a mole.• 8(B) Use the mole concept to calculate the number of atoms,

ions, or molecules in a sample of material.• 2(G) Express and manipulate chemical quantities using

scientific conventions and mathematical procedures, including dimensional analysis, scientific notation, and significant figures.

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Essential Questions

• How is a mole used to indirectly count the number of particles of matter?

• What is a common everyday counting unit to which the mole can be related?

• How can moles be converted to number of representative particles and vice versa?

Measuring MatterCopyright © McGraw-Hill Education

Review• molecule

New• mole• Avogadro’s number

Measuring MatterCopyright © McGraw-Hill Education

Vocabulary

Counting Particles

Chemists need a convenient method for accurately counting the number of atoms, molecules, or formula units of a substance. The mole is the SI base unit used to measure the amount of a substance. 1 mole is the amount of atoms in 12 g of pure carbon-12, or 6.02 ×1023 representative particles, which is any kind of particle – an atom, a molecule, a formula unit, an electron, an ion, etc. The number is called Avogadro’s number.

Measuring MatterCopyright © McGraw-Hill Education

Converting Between Moles and Particles

Conversion factors must be used.Moles to particles:

• Number of molecules in 3.50 mol of sucrose:

Measuring MatterCopyright © McGraw-Hill Education

Converting Between Moles and Particles

Particles to molesUse the inverse of Avogadro’s number as the conversion factor.

Measuring MatterCopyright © McGraw-Hill Education

Measuring MatterCopyright © McGraw-Hill Education

PARTICLES-TO-MOLES CONVERSION

Use with Example Problem 1.

Problem Zinc (Zn) is used as a corrosion-resistant coating on iron and steel. It is also an essential trace element in your diet. Calculate the number of moles of zinc that contain 4.50 × 1024 atoms.

ResponseANALYZE THE PROBLEMYou are given the number of atoms of zinc and must find the equivalent number of moles. If you compare 4.50 × 1024 atoms Zn with 6.02 × 1023, the number of atoms in 1 mol, you can predict that the answer should be less than 10 mol.

KNOWN

Number of atoms = 4.50 × 1024 atoms Zn

1 mol Zn = 6.02 × 1023 atoms Zn

UNKNOWN

Moles Zn = ? mol

SOLVE FOR THE UNKNOWN

Use a conversion factor—the inverse of Avogadro’s number—that relates moles to atoms.

• Apply the conversion factor.

Measuring MatterCopyright © McGraw-Hill Education

PARTICLES-TO-MOLES CONVERSION

EVALUATE THE ANSWER

Both the number of Zn atoms and Avogadro’s number have three significant figures. Therefore, the answer is expressed correctly with three digits. The answer is less than 10 mol, as predicted, and has the correct unit, moles.

SOLVE FOR THE UNKNOWN

• Substitute the number of Zn atoms = 4.50 × 1024. Multiply and divide numbers and units.

Measuring MatterCopyright © McGraw-Hill Education

Review

Essential Questions

• How is a mole used to indirectly count the number of particles of matter?

• What is a common everyday counting unit to which the mole can be related?

• How can moles be converted to number of representative particles and vice versa?

Vocabulary•mole•Avogadro’s number