Chapter 3
Homework Questions
Show all work for calculation problems
1) What is the mass (in amu) of a carbon-12 atom? Why is the atomic mass of carbon listed
as 12.01 amu in the table on the inside front cover of this book?
2) The atomic masses of 35Cl (75.53 percent) and
36.956 amu, respectively.
37
Cl (24.47 percent) are 34.968 amu and
Calculate the average atomic mass of chlorine.
3) How many moles of calcium (Ca) atoms are in 77.4 g of Ca?
4) How many grams of gold (Au) are there in 15.3 moles of Au?
5) How many atoms are there in 5.10 moles of sulfur (S)?
6) How many moles of cobalt (Co) atoms are there in 6.00 x 109 Co atoms?
7) How many atoms are present in 3.14 g of copper (Cu)?
8) Calculate the molar mass of each of the following substances:
(Adapted from Chemistry by Chang)
Attach Scratch paper as needed.
C2H4
K2SO4
Mg(OH)2
C6H6N
Ca3(PO4)2
9) How many molecules of ethane (C2H6 ) are present in 0.334 g of C2H6 ?
10) Calculate the number of H and O atoms in 1.50 g of glucose (C6H12O6 ), a sugar.
11) Tin (Sn) exists in Earth’s crust as SnO2 . Calculate the percent composition by mass of
Sn and O in SnO2.
12) Cinnamic alcohol is used mainly in perfumery, particularly in soaps and cosmetics. Its
molecular formula is C9H10O.
(a) Calculate the percent composition by mass of C, H, and O in cinnamic alcohol
(b) Determine how many grams of C, H, and O each can be found in a sample of 0.469 g
13) What are the empirical formulas of the compounds with the following compositions?
(Adapted from Chemistry by Chang)
Attach Scratch paper as needed.
(a)
2.1 % H
65.3 % O
32.6 %S
(b)
20.2 % Al
79.8 % Cl.
14) The empirical formula of a compound is CH. If the molar mass of this compound is
about 78 g, what is its molecular formula?
15) Allicin is the compound responsible for the characteristic smell of garlic. An analysis of
the compound gives the following percent composition by mass: C: 44.4 percent;
H: 6.21 percent; S: 39.5 percent; O: 9.86 percent.
(a) Calculate its empirical formula.
(b) What is its molecular formula given that its molar mass is about 162 g?
16) Use the formation of water from hydrogen and oxygen to explain the following terms:
chemical reaction, reactant, product.
17) Why must a chemical equation be balanced? What law is obeyed by a balanced chemical
equation?
18) Balance the following chemical reactions:
(Adapted from Chemistry by Chang)
Attach Scratch paper as needed.
N2 + H2 → NH3
NaOH +
S8 + O2 → SO2
C4H8O2 + O2
H2SO4
→ Na2SO4 + H2O
→
CO2
+
H2O
19) Balance the following chemical reactions:
Fe
+
O2 →
Fe3O4
C4H10 + O2 →
CO2 + H2O
20) Silicon tetrachloride (SiCl4) can be prepared by heating Si in chlorine gas:
Si + 2Cl2 → SiCl4
In one reaction, 0.507 mole of SiCl4 is produced.
How many moles chlorine gas were used in the reaction?
21)
2KClO3 → 2KCl + 3O2
a) How many grams of KCl are produced by the decomposition of 10.0 grams KClO3?
b) How many grams of O2 are produced by the decomposition of 10.0 grams KClO3?
c) Show that the Law of Conservation of Mass is maintained by this reaction.
22)
(Adapted from Chemistry by Chang)
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23) Define limiting reagent and excess reagent. What is the significance of the limiting
reagent in predicting the amount of the product obtained in a reaction?
24) Give an everyday example that illustrates the limiting reagent concept.
25) What is the theoretical yield of Al2O3 that can be formed when 65.3 grams of O2 react
with 51.4 grams of Al? State the limiting and excess reagent.
4Al + 3O2 → 2Al2O3
26) Balance the chemical equation:
____H3PO4 + ____KOH → ____H2O + ____ K3PO4
The reaction of 55.0 g of H3PO4 with 75.0 g of KOH actually yielded 53.3 g of K3PO4
product. 1st determine the theoretical yield and then calculate the percent yield for the reaction?
(Adapted from Chemistry by Chang)
Attach Scratch paper as needed.