Sherman • Chem V20
Chapter 6
Atoms are tiny – we can’t weigh them out individually. Chemists work in moles :
1 mole of atoms = 6.022 • 10 23
Why 6.022 • 10 23 atoms?
atoms
Example: Dr. Evil has one billion dollars . How many moles of dollars is that?
Work through: How many sodium atoms are in 23 moles of sodium?
The mass on the periodic table means atomic mass if we use units of molar mass if we use units
Carbon has a molar mass of 12.01 g/mol.
So 12.01 g C = moles of C = C atoms
We can also figure out the molar mass of a compound:
Example: What’s the molar mass of carbon tetrachloride?
Work through: What’s the molar mass of water?
We can use the molar mass to convert between the mass of an element or compound and the number of atoms or molecules present.
Example: How many copper(II) sulfate molecules are present in 30.0 g?
Work through: How many atoms of lead are present in a 115 g bullet?
If we know the chemical formula for a compound, we can write a mole ratio:
Example: How many moles of oxygen atoms are in a mole of Al
2
O
3
?
Work through: How many moles of nitrogen atoms are in a mole of ammonium nitrate?

Sherman • Chem V20
From here, we can figure out how much of the mass of a compound is due to one of its elements!
Example: How many grams of hydrogen are in 600.0 grams of water?
Work through: How many grams of iron are in 70.5 grams of iron(III) chloride?
We can even make this more general and figure out the mass percent of an element within a compound, using the chemical formula and periodic table.
Mass % =
Example: Calculate the mass percent of sulfur in calcium sulfate.
Work through: Calculate the mass percent of osmium in osmium tetroxide.
We can even use the masses of elements in a compound to calculate an empirical formula :
Example: Decomposing a sample of ethane yields 7.0 g hydrogen and 28.0 g carbon. Using this data, find the empirical formula for ethane.
The empirical formula for a compound can be multiplied by a whole number to give the molecular formula :
Example continued: Ethane has a molar mass of 30.07 g/mol. Use this information, and the empirical formula we found above, to determine the molecular formula of ethane.
Suggested problems: Ch 6 # 25, 27, 45, 55, 63, 67, 75, 79, 89, 101