CHAPTER 9: THE MOLE CONCEPT
Problems: 1-20, 23-26, 29-46, 77-78
9.1
AVOGADRO'S NUMBER (N) = 6.02 x 1023 (3 sig figs)
How big is this?
• If 6.02 x 1023 hydrogen atoms were laid side by side, the total length
would encircle the earth about a million times.
• The mass of 6.02 x 1023 Olympic shotput balls is about equal to the mass
of the Earth.
• The volume of 6.02 ×1023 softballs is about equal to the volume of the
Earth.
1 mole (abbreviated mol) = 6.02 ×1023 items
Similar to: 1 dozen = 12 entities:
1 dozen doughnuts = 12 doughnuts
1 mole of doughnuts = 6.02 x 1023 doughnuts
9.2
MOLE CALCULATIONS I
a. How many eggs are in 3 dozen eggs?
____________
b. How many eggs are in 3 moles of eggs?
c. How many moles of C atoms are present in a sample of 1.25×1024 C atoms?
CHM 130: Chapter 9
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9.3
MOLAR MASS
Atomic masses and molar masses:
– The mass of 1 C atom (on average) is 12.01 amu
– The mass of 1 mole of C atoms is 12.01 g (or 12.01 g/mol)
→
1 mole (6.02 x 1023) is the amount of atoms of any element that has a mass
in grams equal to the mass of ONE atom in amu.
→
The atomic masses reported for each element in the Periodic Table give the
atomic mass in amu and the molar mass in g/mol.
Example. What is the mass in grams of 1 mole of each of the following?
(Use the atomic masses reported for each in the Periodic T able.)
a.
Mg:
___________
c.
Ar:
___________
b.
Si:
___________
d.
Sn:
___________
Molar mass (MM): Mass in grams of 1 mole of any element/compound
– To obtain, multiply the molar mass of each element by the number
of each present, then add up all the constituent parts.
Ex. Determine the molar mass of each of the following compounds:
a.
O2:
2 (molar mass of O) = 2 (16.00 g/mol) = 32.00 g/mol
b.
NaCl:
c.
CO2:
d.
H2SO4:
e.
Ca3(PO4)2:
CHM 130: Chapter 9
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9.3
MOLE CALCULATIONS II
Use the unit analysis method:
1. Write the units of the final answer
2. Write the given information related to the answer
3. Determine unit factors (Avogadro’s #, molar masses)
necessary to get answer from given information
a. How many moles of Hg are in 75.0 g of Hg?
b. How many Hg atoms are in 75.0 g of Hg?
c. How many moles of H2O are in 12.5 g of H2O?
d. How many H2O molecules are in 12.5 g of H2O?
e. How many H atoms are in 12.5 g of H2O?
CHM 130: Chapter 9
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9.5
MOLAR VOLUME: Volume occupied by 1 mole of any gas
Avogadro's Law:
An equal number of gas molecules at the same temperature
and pressure will occupy the same volume.
Standard temperature and pressure (STP): T=0˚C and P=1.00 atm
At STP, 1 mole of gas occupies 22.4 L!
3 sig figs
Write 2 unit factors
using this information:
9.6 Mole Calculations III
– Use molar volume at STP in calculations
Ex. 1
How many moles of ozone, O3, occupy a volume of 3.36L at STP?
Ex. 2
What mass of CO2 occupies a volume of 1.05 L at STP?
Ex. 3
What is the volume occupied by 0.125 g of H2S gas at STP?
Ex. 4
How many He atoms are present in 0.124 L of He gas at STP?
CHM 130: Chapter 9
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GAS DENSITY
– Gas densities are about 1000 times lower than densities for solids and liquids
– The densities for gases are generally reported in grams per liter (g/L).
Example: Calculate the density for the following gases at STP.
a. Helium, He (g)
b. Carbon dioxide gas, CO2 (g)
c. Acetylene gas, C2H2 (g)
9.7
PERCENTAGE COMPOSITION
– lists the mass percent of each element in the compound
Mass Percent of Element =
Mass of Element
"100%
Total Mass of Compound
Steps to determine percentage composition:
1.!Calculate the total mass of each individual element in the compound
2. Add up all the masses of each element to get the total mass of the compound
3. Divide the mass of each individual element with the total mass of compound
Ex. 1 What is the percent composition by mass of H and O in H2O?
CHM 130: Chapter 9
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Ex. 2 What is the percent composition by mass of each element in Al2S3?
Ex. 3 What is the percent composition by mass of each element in
TNT: C7H5(NO2)3?
CHM 130: Chapter 9
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