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94 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
9. A tank of compressed helium contains 50.0 g helium. How
many moles of helium are in the tank?
10. A nickel coin is 25.0% nickel and 75.0% copper. If a nickel has
a mass of 5.00 g, how many moles of Ni are in a nickel coin?
Now that you can relate mass to moles, you can also relate mass to
number of particles by first converting mass to moles and then using
the following relationship.
1 mol of a substance ϭ 6.02 ϫ 10 23 particles of the substance
Example Problem 10-5
Converting Mass to Number of Particles
Calculate the number of atoms in 4.77 g lead.
To find the number of atoms in the sample, you must first determine
how many moles are in 4.77 g lead. According to data from the
periodic table, the molar mass of lead is 207.2 g/mol. Apply a
conversion factor to convert mass to moles.
moles of Pb ϭ grams Pb ϫ
moles of Pb ϭ 4.77 g Pb ϫ
ϭ0.0230 mol Pb
Now use a second conversion factor to convert moles to number of
particles.
atoms of Pb ϭ mol Pb ϫ
atoms of Pb ϭ 0.0230 mol Pb ϫ
ϭ
1.38 ϫ 10 22 atom Pb
You can also convert from number of particles to mass by reversing
the procedure above and dividing the number of particles by
Avogadro’s number to determine the number of moles present.
6.02 ϫ 10 23 atom Pb
ᎏᎏᎏ
1 mol Pb
6.02 ϫ 10 23 atom Pb
ᎏᎏᎏ
1 mol Pb
1 mol Pb
ᎏᎏ
207.2 g Pb
1 mol Pb
ᎏᎏ
grams Pb