Atomic Mass, Molar Mass, and the Mole
Tuesday
July 19, 2016
Atomic Mass
• Atoms are very small
• Atoms are very light: • Carbon atom = 1.99 x 10–23 g
• New unit: atomic mass unit (amu)
• 1 amu = 1.66 x 10–24 g
Atomic Mass
• How does it work?
• 1 12C atom = 1.99 x 10–23 g
• 1 amu = 1.66 x 10–24 g
• 12.01 x (1.66 x 10–24 g) = 1.99 x 10–23 g
• All average atomic masses are measured and given on periodic table!
Atomic Mass
Atomic number (Z)
Element symbol
Average atomic mass
Problem
Calculate the number of Na atoms in a sample with a mass of 1172.49 amu.
Problem
Calculate the number of Na atoms in a sample with a mass of 1172.49 amu.
1172.49 amu×
1 Na atom
= 51.00 Na atoms
22.99 amu
The Mole
• We want a more convenient unit of mass: grams
• Mole (mol): number of atoms in a sample equal to average atomic mass (in grams)
• 1 mol = 6.022 x 1023 atoms (Avogadro’s #)
The Mole
• In practice: 1 mole = Mass Number in grams
E.g.) 1 mol Al = E.g.) 1 mol N = E.g.) 1 mol K =
The Mole
• In practice: 1 mole = Mass Number in grams
E.g.) 1 mol Al = 26.98 g
E.g.) 1 mol N = 14.01 g
E.g.) 1 mol K = 39.10 g
Example
Calculate the number of moles in a sample with 5.00 x 1020 atoms of Cr.
Calculate the mass of this sample.
Example
Calculate the # of moles in 5.00 x 1020 atoms of Cr.
5.00×1034 atoms Cr×
1 mol Cr
= 8.30×10;< mol Cr
34
6.022×10 atoms
Calculate the mass of this sample.
8.30×10;< mol Cr×
52.00 g Cr
= 4.32×10;3 g Cr
1 mol Cr
Problem
How many Silicon (Si) atoms are in a 5.68 mg sample?
Problem
How many Silicon (Si) atoms are in a 5.68 mg sample?
1 g Si
1 mol Si 6.022×1023 atoms Si
5.68 mg Si×
×
×
= 1.22×103@ atoms Si
1000 mg Si 28.09 g Si
1 mol Si
Molar Mass
• We can also measure compounds using moles
• For a compound, the mass (in grams) of 1 mol of substance is equal to the sum of components’ masses
• 1 mol of H2O = 2 mol H + 1 mol O
Molar Mass Example
Calculate the molar mass of Sodium Sulfate.
Molar Mass Example
Calculate the molar mass of Sodium Sulfate.
Formula: Na2SO4
Molar Mass Example
Calculate the molar mass of Sodium Sulfate.
Formula: Na2SO4
2
1
1
4
Na+
→ 2 (22.99 g) →
SO42–
S
→ 1 (32.07 g) →
O
→ 4 (16.00 g) →
45.98 g
32.07 g
64.00 g
142.05 g
Molar Mass Example
Calculate the molar mass of Sodium Sulfate.
Formula: Na2SO4
2
1
1
4
Na+
→ 2 (22.99 g) →
SO42–
S
→ 1 (32.07 g) →
O
→ 4 (16.00 g) →
45.98 g
32.07 g
64.00 g
142.05 g
Molar Mass Example
Calculate the molar mass of Sodium Sulfate.
Formula: Na2SO4
2
1
1
4
Na+
→ 2 (22.99 g) →
SO42–
S
→ 1 (32.07 g) →
O
→ 4 (16.00 g) →
45.98 g
32.07 g
64.00 g
142.05 g
Molar Mass Example
Calculate the molar mass of Sodium Sulfate.
Formula: Na2SO4
2
1
1
4
Na+
→ 2 (22.99 g) →
SO42–
S
→ 1 (32.07 g) →
O
→ 4 (16.00 g) →
45.98 g
32.07 g
64.00 g
142.05 g
Molar Mass Example
Calculate the molar mass of Sodium Sulfate.
Formula: Na2SO4
2
1
1
4
Na+
→ 2 (22.99 g) →
SO42–
S
→ 1 (32.07 g) →
O
→ 4 (16.00 g) →
45.98 g
32.07 g
64.00 g
142.05 g
Problem
Calculate the mass of 2.00 moles of Na2SO4.
Calculate the # of molecules in this sample.
Problem
Calculate the mass of 2.00 moles of Na2SO4.
2.00 mol Na2SO4×
142. 05 g Na2SO4
= 284 mol Na2SO4
1 mol Na2SO4
Calculate the # of molecules in this sample.
6.022×1023 molecules
2.00 mol Na2SO4×
= 1.20×1024 molecules Na2SO4
1 mol Na2SO4