Section 10-1

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 atoms.

• The number is called Avogadro’s number.

Section 10-1

Converting Between Moles and Particles

• Conversion factors must be used.

• Moles to particles

Section 10-1

Converting Between Moles and Particles (cont.)

• Particles to moles

• Use the inverse of Avogadro’s number as the conversion factor.

End of Section 10-1

Section 10-2

The Mass of a Mole

• 1 mol of copper and 1 mol of carbon have different masses.

• One copper atom has a different mass than 1 carbon atom.

• Molar mass is the mass in grams of one mole of any pure substance.

• The molar mass of any element is numerically equivalent to its atomic mass and has the units g/mol.

Section 10-2

Using Molar Mass

• Moles to mass

• Convert mass to moles with the inverse molar mass conversion factor.

• Convert moles to atoms with Avogadro’s number as the conversion factor.

End of Section 10-2

Section 10-3

Section 10.3 Moles of Compounds (cont.)

The molar mass of a compound can be calculated from its chemical formula and can be used to convert from mass to moles of that compound.

• Chemical formulas indicate the numbers and types of atoms contained in one unit of the compound.

Section 10-3

The Molar Mass of Compounds

• The molar mass of a compound equals the molar mass of each element, multiplied by the moles of that element in the chemical formula, added together.

• The molar mass of a compound demonstrates the law of conservation of mass.

Section 10-3

Converting Moles of a Compound to Mass

• For elements, the conversion factor is the molar mass of the compound.

• The procedure is the same for compounds, except that you must first calculate the molar mass of the compound.

• The conversion factor is the inverse of the molar mass of the compound.

Section 10-3

Converting the Mass of a Compound to Number of Particles

• Convert mass to moles of compound with the inverse of molar mass.

• Convert moles to particles with Avogadro’s number.

End of Section 10-3

Section 10-4

Percent Composition

• The percent by mass of any element in a compound can be found by dividing the mass of the element by the mass of the compound and multiplying by 100.

Section 10-4

Percent Composition (cont.)

• The percent by mass of each element in a compound is the percent composition of a compound.

• Percent composition of a compound can also be determined from its chemical formula.

Section 10-4

Empirical Formula

• The empirical formula for a compound is the smallest whole-number mole ratio of the elements.

• You can calculate the empirical formula from percent by mass by assuming you have 100.00 g of the compound. Then, convert the mass of each element to moles.

• The empirical formula may or may not be the same as the molecular formula.

Molecular formula of hydrogen peroxide = H2O2

Empirical formula of hydrogen peroxide = HO

Section 10-4

Molecular Formula

• The molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance.

• Molecular formula is always a whole-number multiple of the empirical formula.

Section 10-4

Molecular Formula (cont.)

End of Section 10-4

Section 10-5

Naming Hydrates

• A hydrate is a compound that has a specific number of water molecules bound to its atoms.

• The number of water molecules associated with each formula unit of the compound is written following a dot.

• Sodium carbonate decahydrate = Na2CO3 • 10H2O

Section 10-5

Analyzing a Hydrate

• When heated, water molecules are released from a hydrate leaving an anhydrous compound.

• To determine the formula of a hydrate, find the number of moles of water associated with 1 mole of hydrate.

Section 10-5

Analyzing a Hydrate (cont.)

• Weigh hydrate.

• Heat to drive off the water.

• Weigh the anhydrous compound.

• Subtract and convert the difference to moles.

• The ratio of moles of water to moles of anhydrous compound is the coefficient for water in the hydrate.

Section 10-5

Use of Hydrates

• Anhydrous forms of hydrates are often used to absorb water, particularly during shipment of electronic and optical equipment.

• In chemistry labs, anhydrous forms of hydrates are used to remove moisture from the air and keep other substances dry.