Chapter 8 Notes
What is a mole?

1 mole (mol) = ________________

6.02 x 1023 and is referred to as ___________________
Why do some elements have a larger mass per mole than other elements? Give an example.
Atoms, Molecules, Formula Units, Ions to Moles
Write Flow Chart Below
Steps in solving mole Problems
1. Identify what you know
2. Identify what you are solving for
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3. Use your flow chart to make a plan of action
4. Set up your problem only filling in units
5. Make sure all units will cancel
6. Fill in numbers
7. Multiply everything on the top and divide by everything on the bottom
8. Record your answer plus or minus 1 significant figures
9. Don’t forget to include your units in your answer!
1. Convert 4.27 x 1023 molecules of CO2 to moles
2. How many molecules of octane are there in 0.85 moles of octane, C8H18
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3. How many moles of sodium ions are there in 7.6 x 1022 Na+ ions
4. How many formula units are there in 2.78 moles of CaCl2
Moles to Grams
1 mole of an element = the average atomic mass of that element in grams, rounded to one
number after the decimal.
◦
1 mol Ca =
g Ca
1 mol P =
gP
◦
1 mol Cl =
g Cl
1 mol K =
gK
◦
1 mol S =
gS
1 mol Cu =
g Cu
◦
1 mol Br =
g Br
1 mol Si =
g Si
Write Flow Chart Below
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1. How many moles of aluminum are there in a 10.0 gram sample of Al
2. How many grams of iron are there in a 0.643 mol sample of Fe?
___________ – mass of 1 mol of a pure substance usually written in grams per mole
(g/mol)
3. Determine the molar mass for:
Fe2(SO4)3
Ni(C4H4O6)2
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4. Determine the number of moles in a 22.3 gram sample of Al2(CO3)3
5. How many grams of ammonium phosphate, (NH4)3PO4, are present in a 0.082 mole
sample of (NH4)3PO4?
6. How many grams of gold, Au, are present in a 8.21 x 1022 atom sample of Au?
7. Determine the number of formula units there are in a 47.9 gram sample of
Mg(NO3)2
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Molecular Composition
To determine percent composition
1. Determine molar mass of your compound (g/mol).
2. Divide the total mass for each element by the molar mass of the compound.
3. Multiply each element’s % by 100.
4. If you add up the percentages for all of the elements, you should get very close to
100%.
Determine the percent composition for each element in the following compounds:
NaCl
Na2SO4
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Empirical Formula - A formula that gives the simplest whole-number ratio of atoms
in a compound.
Steps for Determining an Empirical Formula
1. Start with the number of grams of each element, given in the problem.
o If percentages are given, assume that the total mass is 100 grams so
that the mass of each element = the percent given.
2. Convert the mass of each element to moles using the molar mass from the
periodic table.
3. Divide each mole value by the smallest number of moles calculated.
4. Round to the nearest whole number. This is the mole ratio of the elements
and is represented by subscripts in the empirical formula.
o If the number is too far to round (x.1 ~ x.9), then multiply each
solution by the same
factor to get the lowest whole number multiple.
1. e.g. If one solution is 1.5, then multiply each solution in the
problem by 2 to get 3.
2. e.g. If one solution is 1.25, then multiply each solution in the
problem by 4 to get 5.
A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and 0.675 g H. What is
the empirical formula of the compound?
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NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O. Calculate the empirical
formula of NutraSweet.
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Molecular Formula:
Determining Molecular Formula:
1. Determine the empirical formula for the compound.
2. Determine the molar mass of the empirical formula.
3. Divide the molar mass of the molecular formula over the molar mass of the
empirical formula.
4. Multiply the answer from #3 by the empirical formula.
Tip: To check to see if your molecular formula is correct, determine the molar
mass of your molecular formula. It should be equal to the molar mass of the
molecular formula you were given in the problem.
What is the molecular formula of a compound with an empirical formula of CH2 and
a molar mass of 56.0 g/mol?
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Determine the empirical formula for a compound with the following elemental
composition:
40.00% C, 6.72% H, 53.29% O.
If know that the molar mass of this compound is 180 g/mol, what is the molecular
formula?
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Determine the empirical and molecular formulas for a compound containing 26.1%C, 50.7%
N and 23.2% O. The molar mass of the molecular formula equals 828.0 g/mol.
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