Page 129 - Theory and Problems of BEGINNING CHEMISTRY

118 FORMULA CALCULATIONS [CHAP. 7

(b) The empirical formula mass is 14.0 amu. There are
70.0 g/mol 5 mol empirical formula units
=
14.0 g/mol empirical formula unit 1 mol
The molecular formula is C 5 H 10 .

Supplementary Problems

7.60. Define or identify each of the following: molecule, ion, formula unit, formula mass, mole, molecular mass,
Avogadro’s number, percent, empirical formula, molecular formula, molar mass, empirical formula mass, molecular
weight.
Ans. See the text.

7.61. How is molecular mass related to formula mass?
Ans. They are the same for compounds which form molecules.

7.62. Does the term atomic mass refer to uncombined atoms, atoms bonded in compounds, or both?
Ans. Both

7.63. A certain fertilizer is advertised to contain 10.5% K 2 O. What percentage of the fertilizer is potassium?

78.2gK
Ans. 10.5gK 2 O
94.2gK O
2
100 g sample = 8.72% K
7.64. A compound consists of 92.26% C and 7.74% H. Its molecular mass is 65.0 amu. Calculate its molecular formula.
Ans. C 5 H 5 . This problem is exactly the same as Problem 7.58. The steps are the same even though they are not
specified in the statement of this problem.

7.65. (a) Calculate the percent composition of C 3 H 6 .(b) Calculate the percent composition of C 4 H 8 .(c) Compare the
results and explain the reason for these results.
Ans. (a, b) There is 85.7% C and 14.3% H in each. (c) They are the same because they have the same ratio of
moles of elements.

7.66. Name and calculate the empirical formula of each of the following compounds.
(a) 36.77% Fe 21.10% S 42.13% O (d) 71.05% Co 28.95% O
(b) 27.93% Fe 24.05% S 48.01% O (e) 72.7% O 27.3% C
(c) 63.20% Mn 36.8% O ( f ) 36.0% Al 64.0% S
Ans. (a) FeSO 4 (b)Fe 2 S 3 O 12 (c) MnO 2 (d)Co 2 O 3 (e)CO 2 ( f )Al 2 S 3
The names are (a) iron(II) sulfate, (b) iron(III) sulfate [Fe 2 (SO 4 ) 3 ], (c) manganese(IV) oxide, (d) cobalt(III)
oxide, (e) carbon dioxide, and ( f ) aluminum sulfide.

7.67. What mass of oxygen is contained in 42.8 g CaCO 3 ?

1 mol CaCO 3 3 mol O 16.0gO
Ans. 42.8 g CaCO 3 = 20.5gO
100 g CaCO 1 mol O
3 1 mol CaCO 3
You can also do this problem by using percent composition (Sec. 7.5).
7.68. Determine the molecular formula of a compound with molar mass between 105 and 115 g/mol which contains
88.8% C and 11.2% H.