Page 121 - Theory and Problems of BEGINNING CHEMISTRY

110 FORMULA CALCULATIONS [CHAP. 7

THE MOLE
7.9. How many H atoms are there in 1 molecule of H 2 O 2 ? How many moles of H atoms are there in 1 mol of
H 2 O 2 ?
Ans. There are two H atoms per molecule of H 2 O 2 , and 2 mol H atoms per 1 mol H 2 O 2 . The chemical formula
provides both these ratios.

7.10. How many atoms of K are there in 1.00 mol K? What is the mass of 1.00 mol K?
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Ans. There are 6.02×10 atoms in 1.00 mol K (Avogadro’s number). There are 39.1 g of K in 1.00 mol K (equal
to the atomic mass in grams). This problem requires use of two of the most important conversion factors
involving moles. Note which one is used with masses and which one is used with numbers of atoms (or
molecules or formula units). With numbers of atoms, molecules, or formula units, use Avogadro’s number;
with mass, use the formula mass.

7.11. What mass in grams is there in 1.000 mol of Al 2 (SO 3 ) 3 ?
Ans. The molar mass has the same value in grams that the formula mass has in amu (Problem 7.7). Thus 1.000
mol represents 294.2 g of aluminum sulfite.

7.12. (a) Which contains more pieces of fruit, a dozen grapes or a dozen watermelons? Which weighs more?
(b) Which contains more atoms, 1 mol of lithium or 1 mol of lead? Which has a greater mass?
Ans. (a) Both have the same number of fruits (12), but since each watermelon weighs more than a grape, the dozen
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watermelons weigh more than the dozen grapes. (b) Both have the same number of atoms (6.02 × 10 ),
but since lead has a greater atomic mass (see the periodic table), 1 mol of lead has a greater mass.

Problems 7.13 through 7.16 are easier to do when both parts are worked together. Note the differences
among the parts labeled (a) and also among the parts labeled (b). On examinations, you are likely to
be asked only one such problem at a time, so you must read the problems carefully and recognize the
difference between similar-sounding problems.
7.13. (a) How many socks are there in 10 dozen socks? (b) How many hydrogen atoms are there in 10.0 mol
of hydrogen atoms?
12 socks

Ans. (a) 10 dozen socks = 120 socks
1 dozen socks
6.02 × 10 H
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(b) 10.0 mol H = 6.02 × 10 H atoms
1 mol H
7.14. (a) How many pairs of socks are there in 10 dozen pairs of socks? (b) How many hydrogen molecules
are there in 10.0 mol of hydrogen molecules?
12 pair socks

Ans. (a) 10 dozen pairs socks = 120 pairs socks
1 dozen pair socks
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6.02 × 10 H 2 24
(b) 10.0 mol H 2 = 6.02 × 10 H 2 molecules
1 mol H 2
7.15. (a) How many pairs of socks can be made with 10 dozen (identical) socks? (b) How many hydrogen
molecules can be made with 10.0 mol of hydrogen atoms?
1 dozen pairs socks 12 pairs socks

Ans. (a) 10 dozen socks = 60 pairs socks
2 dozen socks 1 dozen pairs socks
(from definition (from definition
of a pair) of a dozen)
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1 mol H 2 6.02 × 10 H 2
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(b) 10.0 mol H = 3.01 × 10 H 2 molecules
2 mol H 1 mol H 2
(from definition (from definition
of a molecule) of a mole)