Quantities in Chemistry

The Mole and Molar Mass

Mole Review• A Mole is a unit of measurement in chemistry. It

represents 6.02 x 1023 of an entity. One mole of sodium is 6.02 x 1023 atoms of sodium. The symbol of mole is “mol”.

• 6.02 x 1023 is also known as Avogadro’s constant.

Molar Mass

• Molar Mass (M) is the mass (in grams) of one mole of a chemical entity.

• The formula for molar mass is:• Molar Mass (M) = mass (g) / mole (mol)• The molar mass of an element can be determined

from the periodic table. It is the same number as the atomic mass.

• For example Hydrogen’s molar mass is 1.01 g/mol

Molar Mass of Compounds

• The molar mass of a compound is equal to the sum of the molar masses of the atoms in the molecules or ions in the compound.

• For example, the molar mass of water is:

• MH2O = 2(MH) + (MO)

= 2(1.01g/mol) + 1(16.0g/mol)

=18.02 g/mol

Example: Find the molar mass of H2CO2

• M H2CO2 = 2(MH) + (MC) +2(MO)

= 2(1.01g/mol) + (12.00g/mol) + 2(16.0g/mol)

=46.02 g/mol

Calculations Involving Atoms, Moles, Mass & Molar Mass

• It is often more convenient and practical to determine the number of atoms using mass instead of counting.

• It is easier to determine mass by using number of moles

Atoms, Moles, Mass & Molar Mass Calculations

• Example 1: Calculate the molar mass of magnesium if 0.330mol has a mass of 8.02g

M=m/n

= 8.02g/0.330mol

= 24.3g/mol

Atoms, Moles, Mass & Molar Mass Calculations

• Example 2: Calculate the mass, in grams, of 2.00 mol of calcium atoms.

M=m/n

m = M x n

= 40.08g/mol x 2.0 mol

= 80.2g

Atoms, Moles, Mass & Molar Mass Calculations

• Example 3: How many moles of gold are in 275.8g of pure gold?

M=m/n

n = m/M

= 275.8g / 197.0 g/mol

= 1.400 mol

Calculating Number of Atoms

• If you have a mass of the number of moles of an item, you can determine the number of molecules in an amount of an element using Avogadro’s number.

• Avogadro’s number = 6.02 x 1023 atoms/mol

Atom Calculations• Example 4: How many atoms of gold are in a

232.8g of pure gold?• Step 1: Determine the number of moles

– n = m/M = 232.8g / 197.0g/mol = 1.182 mol

• Step 2: Using Avogadro’s number and the number of moles, determine the number of atoms.– N= n x NA = 1.182 mol x 6.02 x 1023 atoms/mol = 7.12 x 1023 atoms

Atom Calculations• Example 5: How many atoms of sulfur are in a

230.0g of pure sulfur?• Step 1: Determine the number of moles

– n = m/M = 230.0g / 32.06g/mol = 7.174 mol

• Step 2: Using Avogadro’s number and the number of moles, determine the number of atoms.– N= n x NA = 7.174 mol x 6.02 x 1023 atoms/mol = 4.319 x 1024 atoms