### molar masses

```Rice Lab
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
Mole
– the mole = 6.022 x 1023
– 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 24.3 g/mol
– Chlorine
35.5 g/mol
– Magnesium chloride
• Write formula
• Find the atomic mass of each element
• Multiply atomic mass by the number of atoms for each
element, then total the masses
• MgCl2
• 24.3 g/mol + (2 x 35.5 g/mol) = 95.3 g/mol
Episode 701
Try it!
• Find the molar mass of aluminum oxide
(2x 27 g/mol) + (3 x16g/mol)
102 g/mol
Episode 701
Remember Rice Lab Calculations…
• 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 1023 atoms
4.0 g/mol
1 mol He
Episode 701
Formula Units… No Problem
• What would the mass of 5.3 x 1022
formula units of calcium iodide?
CaI2
Formula
Molar Mass 40.1 g/mol + (2 x 126.9 g/mol)
293.9 g/mol
?g CaI2 = 5.3 x1022 fu
1 mol CaI2
6.022 x 1023 fu
=
293.9 g CaI2
1 mol CaI2
26 g CaI2
Episode 701
Problem Set 1
• How many atoms are in 3.5 grams of
silicon?
? atoms = 3.5g Si
6.022 x 1023 atoms = 7.5 x 1023 atoms Si
1 mol Si
1mol Si
28.1g Si
• How many formula units are in 32.6
grams of potassium oxide?
? fu = 32.6 g K20
1mol K2O
6.022 x 1023 fu
94.2g K2O
1 mol K2O
= 2.1 x 1023 fu K2O
• How many molecules are in 0.25 grams
of dinitrogen pentoxide?
? molec = 0.25 g N2O5
1mol N2O5
6.022 x 1023 molec
108 g N2O5
1 mol N2O5
= 1.4 x 1021 mole N2O5
Episode 701
```