Page 120 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 7] FORMULA CALCULATIONS 109

Solved Problems

MOLECULES AND FORMULA UNITS
7.1. Why is the term molecular mass inappropriate for NaCl?
Ans. NaCl is an ionic compound; it does not form molecules and so does not have a molecular mass. The formula
mass of a NaCl is 58.5 amu, calculated and used in exactly the same manner as the molecular mass for a
molecular compound would be calculated and used.

7.2. Which of the following compounds occur in molecules? (a)C 6 H 6 ,(b)CH 4 O, (c)C 6 H 12 O 6 ,(d) CoCl 2 ,
(e) COCl 2 ,( f )NH 4 Cl, (g) CO, and (h) FeCl 2 .
Ans. All but (d), ( f ), and (h) form molecules; (d), ( f ), and (h) are ionic.

7.3. The simplest type of base contains OH ions. Which of the following compounds is more apt to be a
−
base, CH 3 OH or KOH?
Ans. KOH is ionic and is a base. CH 3 OH is covalently bonded and is not a base.

FORMULA MASSES
7.4. The standard for atomic masses is 12 C, at exactly 12 amu. What is the standard for formula masses of
compounds?
12
Ans. The same standard, C, is used for formula masses.
7.5. What is the difference between the atomic mass of bromine and the molecular mass of bromine?
Ans. The atomic mass of bromine is 79.90 amu, as seen in the periodic table or a table of atomic masses. The
molecular mass of bromine, corresponding to Br 2 , is twice that value, 159.8 amu.

7.6. What is the formula mass of each of the following? (a)P 4 ,(b)Na 2 SO 4 ,(c) (NH 4 ) 2 SO 3 ,(d)H 2 O 2 ,
(e)Br 2 ,( f )H 3 PO 3 ,(g)Hg 2 (ClO 3 ) 2 , and (h) (NH 4 ) 2 HPO 3 .

Ans. (a) 4 × 30.97 amu = 123.9 amu (d) 34.0 amu
(b) 2 Na: 2 × 23.0 amu = 46.0 amu (e) 159.8 amu
S: 1 × 32.0 amu = 32.0 amu ( f ) 82.0 amu
4O:4 × 16.0 amu = 64.0 amu (g) 568.1 amu
total = 142.0 amu (h) 116.0 amu
(c) 116.1 amu

7.7. What is the mass, in amu, of 1 formula unit of Al 2 (SO 3 ) 3 ?
Ans. There are 2 aluminum atoms, 3 sulfur atoms, and 9 oxygen atoms in the formula unit.
2 × atomic mass of aluminum = 54.0 amu
3 × atomic mass of sulfur = 96.2 amu
9 × atomic mass of oxygen = 144.0 amu
Formula mass = total = 294.2 amu
The formula represents 294.2 amu of aluminum sulfite.

7.8. Calculate the formula mass of each of the following compounds: (a) NaCl, (b)NH 4 NO 3 ,(c) SiF 4 ,
(d) Fe(CN) 2 , and (e) KCN.
Ans. (a) 58.44 amu (b) 80.05 amu (c) 104.1 amu (d) 107.9 amu (e) 65.12 amu