11.2 Mass and the Mole
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One mole of a monatomic element consists of 6.02 X 10 atoms of that element.
The mass of a mole of any substance is called the molar mass of the substance.
For example, the molar mass of a monatomic element is numerically equal to
the atomic mass of the element, but expressed in grams.
molar mass of a substance =
grams of the substance
1 mol of the substance
This relationship can be used to convert between mass and moles.
Example Problem 11-3
Converting Mass to Moles
A roll of copper wire has a mass of 848 g. How many moles of copper
are in the roll? Use the atomic mass of copper given on the periodic table
to apply a conversion factor to the mass given in the problem.
moles of Cu = grams Cu X 1 mol Cu
grams Cu
moles of Cu = 848 g Cu X 1 mol Cu = 13.3 mol Cu
63.546 g Cu
Note that the conversion factor is arranged so that g Cu cancel,
leaving only mol Cu, the desired quantity.
Practice Problems
1. Calculate the mass of 6.89 mol antimony.
2. A chemist needs 0.0700 mol selenium for a reaction. What mass of selenium
should the chemist use?
3. A tank of compressed helium contains 50.0 g helium. How many moles of
helium are in the tank?
4. 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?
5. Calculate the number of atoms in 2.00 g of platinum.
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6. How many sulfur atoms are in a metric ton (1.00 X 10 g) of sulfur?
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7. How many °grams of mercury are in 1.19 X 10 atoms of mercury?
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8. What is the mass in grams of 3.01 X 10 atoms of iodine?
9. The EPA limit for lead in the water supply is 15 parts per billion by
mass. Calculate the number of lead ions present in 1.00 kg of
water that is at the EPA limit for lead.
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