Page 124 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 7] FORMULA CALCULATIONS 113

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Ans. There are 6.02×10 molecules in 1.00 mol F 2 (Avogadro’s number). Since there are two atoms per molecule,
there are

2 atoms
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6.02 × 10 molecules = 1.20 × 10 atoms F in 1.00 mol F 2
1 molecule
The mass is that of 1.00 mol F 2 or 2.00 mol F:

2 × 19.0gF 2
1.00 mol F 2 = 38.0gF 2
1 mol F 2
19.0gF
or 2.00 mol F = 38.0gF
1 mol F
The mass is the same, no matter whether we focus on the atoms or molecules. (Compare the mass of 1 dozen
pairs of socks rolled together to that of the same socks unpaired. Would the two masses differ? If so, which
would be greater? See the prior problem.)
7.30. (a) Create one factor that will change 6.17 g of calcium carbonate to a number of formula units of calcium
carbonate. (b) Is it advisable to learn and use such a factor?
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Ans. (a) 1 mol CaCO 3 6.02 × 10 units CaCO 3 6.02 × 10 units CaCO 3
=
100 g CaCO 3 1 mol CaCO 3 100 g CaCO 3
(b) It is possible to use such a conversion factor, but it is advisable while you are learning to use the factors
involved with moles to use as few different ones as possible. That way, you have to remember fewer.
Also, in each conversion you will change either the unit (mass → moles) or the chemical (CaCO 3 →
O atoms) in a factor, but not both. Many texts do use such combined factors, however.
7.31. How many moles of Na are there in 7.20 mol Na 2 SO 4 ?
2 mol Na

Ans. 7.20 mol Na 2 SO 4 = 14.4 mol Na
1 mol Na 2 SO 4
7.32. How many moles of water can be made with 1.76 mol H atoms (plus enough O atoms)?

1 mol H 2 O

Ans. 1.76 mol H = 0.880 mol H 2 O
2 mol H
7.33. Make a table showing the number of molecules and the mass of each of the following: (a) 1.00 mol Cl 2 ,
(b) 2.00 mol Cl 2 , and (c) 0.135 mol Cl 2
Number of Molecules Mass

Ans. (a) 1.00 mol Cl 2 6.02 × 10 23 71.0 g
(b) 2.00 mol Cl 2 1.20 × 10 24 142 g
(c) 0.135 mol Cl 2 8.13 × 10 22 9.59 g
Once you know the mass or number of molecules in 1 mol, you merely have to multiply to get the mass or
number of molecules in any other given number of moles. Sometimes the calculation is easy enough to do
in your head.

7.34. How can you measure the thickness of a sheet of notebook paper with a 10-cm ruler?
Ans. One way is to measure the combined thickness of many sheets and divide that distance by the number of
sheets. For example, if 500 sheets is 5.05 cm thick, then each sheet is (5.05 cm)/500 = 0.0101 cm thick. Note
that it is impossible to measure 0.0101 cm with a centimeter ruler, but we did accomplish the same purpose
indirectly. (See also the following problem.)

7.35. How can you measure the mass of a carbon atom? Compare this problem with the prior problem.
Ans. We want to measure the mass of a large number of carbon atoms and divide the total mass by the num-
ber of atoms. However, we have the additional problem here, compared with counting sheets of paper