1. Count out 1 dozen grains of rice and place in container.
2. Repeat until you have 3 containers each holding 12 grains of rice.
3. Mass each dozen and calculate the average mass of 1 dozen grains of rice. (Don’t forget to minus out the mass of container.)
4. Calculate the number of rice grains in 1.94 gram of rice.
5. Mass out rice grains until you reach a mass equal to the mass you calculated above.
6. Count the grains of rice in the sample.
7. Calculate the percent error between the calculated number of rice grains versus the actual number of rice grains.
Episode 701

• Avogadro’s number
– the mole = 6.022 x 10 23
– 602,200,000,000,000,000,000,000
Molar Mass
The mass of one mole of any pure substance
Episode 701

• Molar mass of an element (g/mol)
– Equals the atomic mass of the element in grams
• Molar mass of a compound
– Equals the sum of the molar masses of the elements making up the compound
• Find the molar masses of-
– Magnesium g/mol
– Chlorine 35.5
– Magnesium chloride g/mol
• Write formula
• Find the atomic mass of each element
• Multiply atomic mass by the number of atoms for each element, then total the masses
• MgCl
2
• 24.3 g/mol + (2 x 35.5 g/mol) = 95.3 g/mol
Episode 701

• Find the molar mass of aluminum oxide
Episode 701

• How many moles of atoms are there in
8.0 grams of Mg?
? moles = 8.0g Mg 1 mol Mg
24.3 g/mol
• How many atoms are in 0.7 grams of
He?
? atoms = 0.7g He 1 mol He 6.022 x 10 23 atoms
4.0 g/mol 1 mol He
Episode 701

• What would the mass of 5.3 x 10 22 formula units of calcium iodide?
Formula
Molar Mass
CaI
2
40.1 g/mol + (2 x 126.9 g/mol) =
293.9 g/mol
?g CaI
2
= 5.3 x10 22 fu 1 mol CaI
2
6.022 x 10 23 fu
26 g CaI
2
293.9 g CaI
2
1 mol CaI
2
Episode 701

• How many atoms are in 3.5 grams of silicon?
? atoms = 3.5g Si 1mol Si 6.022 x 10 23 atoms
= 7.5 x 10 23 atoms Si
28.1g Si 1 mol Si
• How many formula units are in 32.6 grams of potassium oxide?
? fu = 32.6 g K
2
0 1mol K
2
O 6.022 x 10 23 fu
= 2.1 x 10 23 fu K
2
O
94.2g K
2
O 1 mol K
2
O
• How many molecules are in 0.25 grams of dinitrogen pentoxide?
? molec = 0.25 g N
2
O
5
1mol N
2
O
5
6.022 x 10 23 molec
= 1.4 x 10 21 mole N
2
O
5
108 g N
2
O
5
1 mol N
2
O
5
Episode 701