Chapter 3

What is an atomic mass unit? Why is it necessary to introduce such a unit?

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 List of the Elements with Their Symbols and Atomic Masses?

Explain clearly what is meant by the statement, "The atomic mass of gold is 197.0 amu."

What information would you need to calculate the average atomic mass of an element?

The atomic masses of \({ }_{17}^{35} \mathrm{Cl}(75.53\) percent) and \({ }_{17}^{37} \mathrm{Cl}(24.47\) percent) are 34.968 amu and 36.956 amu, respectively. Calculate the average atomic mass of chlorine. The percentages in parentheses denote the relative abundances.

The atomic masses of \({ }_{3}^{6} \mathrm{Li}\) and \({ }_{3}^{7} \mathrm{Li}\) are \(6.0151 \mathrm{amu}\) and 7.0160 amu, respectively. Calculate the natural abundances of these two isotopes. The average atomic mass of \(\mathrm{Li}\) is \(6.941 \mathrm{amu}\).

Define the term "mole." What is the unit for mole in calculations? What does the mole have in common with the pair, the dozen, and the gross? What does Avogadro's number represent?

Earth's population is about 7.2 billion. Suppose that every person on Earth participates in a process of counting identical particles at the rate of two particles per second. How many years would it take to count \(6.0 \times 10^{23}\) particles? Assume that there are 365 days in a year.

How many atoms are there in 5.10 moles of sulfur (S)?

How many moles of cobalt (Co) atoms are there in \(6.00 \times 10^{9}\) (6 billion) Co atoms?

How many moles of calcium (Ca) atoms are in \(77.4 \mathrm{~g}\) of \(\mathrm{Ca} ?\)

How many grams of gold (Au) are there in 15.3 moles of Au?

What is the mass in grams of a single atom of each of the following elements? (a) \(\mathrm{Hg}\), (b) Ne.

What is the mass in grams of a single atom of each of the following elements? (a) As, (b) Ni.

What is the mass in grams of \(1.00 \times 10^{12}\) lead \((\mathrm{Pb})\) atoms?

A modern penny weighs \(2.5 \mathrm{~g}\) but contains only \(0.063 \mathrm{~g}\) of copper (Cu). How many copper atoms are present in a modern penny?

Which of the following has more atoms: \(1.10 \mathrm{~g}\) of hydrogen atoms or \(14.7 \mathrm{~g}\) of chromium atoms?

Which of the following has a greater mass: 2 atoms of lead or \(5.1 \times 10^{-23}\) mole of helium.

Calculate the molecular mass or formula mass (in amu) of each of the following substances: (a) \(\mathrm{CH}_{4}\) (b) \(\mathrm{NO}_{2},(\mathrm{c}) \mathrm{SO}_{3},(\mathrm{~d}) \mathrm{C}_{6} \mathrm{H}_{6}\) (e) NaI, (f) \(\mathrm{K}_{2} \mathrm{SO}_{4}\) \((\mathrm{g}) \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}\)

Calculate the molar mass of the following substances: (a) \(\mathrm{Li}_{2} \mathrm{CO}_{3},\) (b) \(\mathrm{CS}_{2},\) (c) \(\mathrm{CHCl}_{3}\) (chloro-form), (d) \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6}\) (ascorbic acid, or vitamin \(\mathrm{C}\) ), (e) \(\mathrm{KNO}_{3},\) (f) \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\).

Calculate the molar mass of a compound if 0.372 mole of it has a mass of \(152 \mathrm{~g}\).

How many molecules of ethane \(\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)\) are present in \(0.334 \mathrm{~g}\) of \(\mathrm{C}_{2} \mathrm{H}_{6} ?\)

Calculate the number of \(\mathrm{C}, \mathrm{H},\) and \(\mathrm{O}\) atoms in \(1.50 \mathrm{~g}\) of glucose \(\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right),\) a sugar.

Pheromones are a special type of compound secreted by the females of many insect species to attract the males for mating. One pheromone has the molecular formula \(\mathrm{C}_{19} \mathrm{H}_{38} \mathrm{O}\). Normally, the amount of this pheromone secreted by a female insect is about \(1.0 \times 10^{-12} \mathrm{~g} .\) How many molecules are there in this quantity?

Describe the operation of a mass spectrometer.

Describe how you would determine the isotopic abundance of an element from its mass spectrum.

Carbon has two stable isotopes, \({ }_{6}^{12} \mathrm{C}\) and \({ }_{6}^{13} \mathrm{C},\) and fluorine has only one stable isotope, \({ }_{9}^{19} \mathrm{~F}\). How many peaks would you observe in the mass spectrum of the positive ion of \(\mathrm{CF}_{4}^{+} ?\) Assume that the ion does not break up into smaller fragments.

Hydrogen has two stable isotopes, \({ }_{1}^{1} \mathrm{H}\) and \({ }_{1}^{2} \mathrm{H},\) and sulfur has four stable isotopes, \({ }_{16}^{32} \mathrm{~S},{ }_{16}^{33} \mathrm{~S},{ }_{16}^{34} \mathrm{~S},\) and \({ }_{16}^{36} \mathrm{~S}\). How many peaks would you observe in the mass spectrum of the positive ion of hydrogen sulfide, \(\mathrm{H}_{2} \mathrm{~S}^{+} ?\) Assume no decomposition of the ion into smaller fragments.

Use ammonia \(\left(\mathrm{NH}_{3}\right)\) to explain what is meant by the percent composition by mass of a compound.

Describe how the knowledge of the percent composition by mass of an unknown compound can help us identify the compound.

Tin (Sn) exists in Earth's crust as \(\mathrm{SnO}_{2}\). Calculate the percent composition by mass of \(\mathrm{Sn}\) and \(\mathrm{O}\) in \(\mathrm{SnO}_{2}\)

For many years chloroform \(\left(\mathrm{CHCl}_{3}\right)\) was used as an inhalation anesthetic in spite of the fact that it is also a toxic substance that may cause severe liver, kidney, and heart damage. Calculate the percent composition by mass of this compound.

Cinnamic alcohol is used mainly in perfumery, particularly in soaps and cosmetics. Its molecular for\(\begin{array}{lll}\text { mula is } & \text { C }_{9} \text { H }_{10} \text { O. }\end{array}\) (a) Calculate the percent composition by mass of \(\mathrm{C}, \mathrm{H},\) and \(\mathrm{O}\) in cinnamic alcohol. (b) How many molecules of cinnamic alcohol are contained in a sample of mass \(0.469 \mathrm{~g} ?\)

The formula for rust can be represented by \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) How many moles of Fe are present in \(24.6 \mathrm{~g}\) of the compound?

Tin(II) fluoride \(\left(\mathrm{SnF}_{2}\right)\) is often added to toothpaste as an ingredient to prevent tooth decay. What is the mass of \(\mathrm{F}\) in grams in \(24.6 \mathrm{~g}\) of the compound?

What are the empirical formulas of the compounds with the following compositions? (a) 2.1 percent \(\mathrm{H}\), 65.3 percent \(\mathrm{O}, 32.6\) percent \(\mathrm{S} ;\) (b) 20.2 percent \(\mathrm{Al},\) 79.8 percent Cl.

What are the empirical formulas of the compounds with the following compositions? (a) 40.1 percent C, 6.6 percent \(H, 53.3\) percent \(O ;\) (b) 18.4 percent \(C\), 21.5 percent \(\mathrm{N}, 60.1\) percent \(\mathrm{K}\)

The anticaking agent added to Morton salt is calcium silicate, CaSiO \(_{3}\). This compound can absorb up to 2.5 times its mass of water and still remain a free-flowing powder. Calculate the percent composition of \(\mathrm{CaSiO}_{3}\).

What does the word "empirical" in empirical formula mean?

If we know the empirical formula of a compound, what additional information do we need to determine its molecular formula?

The empirical formula of a compound is CH. If the molar mass of this compound is about \(78 \mathrm{~g},\) what is its molecular formula?

The molar mass of caffeine is \(194.19 \mathrm{~g}\). Is the molecular formula of caffeine \(\mathrm{C}_{4} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{O}\) or \(\mathrm{C}_{8} \mathrm{H}_{10} \mathrm{~N}_{4} \mathrm{O}_{2} ?\)

Peroxyacylnitrate (PAN) is one of the components of smog. It is a compound of \(\mathrm{C}, \mathrm{H}, \mathrm{N},\) and \(\mathrm{O} .\) Determine the percent composition of oxygen and the empirical formula from the following percent composition by mass: 19.8 percent \(\mathrm{C}, 2.50\) percent \(\mathrm{H}, 11.6\) percent \(\mathrm{N}\). What is its molecular formula given that its molar mass is about \(120 \mathrm{~g}\) ?

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, \(\mathrm{H}: 6.21\) percent, \(\mathrm{S}: 39.5\) percent, \(\mathrm{O}:\) 9.86 percent. Calculate its empirical formula. What is its molecular formula given that its molar mass is about \(162 \mathrm{~g}\) ?

Monosodium glutamate (MSG), a food-flavor enhancer, has been blamed for "Chinese restaurant syndrome," the symptoms of which are headaches and chest pains. MSG has the following composition by mass: 35.51 percent \(\mathrm{C}, 4.77\) percent \(\mathrm{H}\), 37.85 percent \(\mathrm{O}, 8.29\) percent \(\mathrm{N},\) and 13.60 percent Na. What is its molecular formula if its molar mass is about \(169 \mathrm{~g}\) ?

Use the formation of water from hydrogen and oxygen to explain the following terms: chemical reaction, reactant, product.

What is the difference between a chemical reaction and a chemical equation?

Why must a chemical equation be balanced? What law is obeyed by a balanced chemical equation?

Write the symbols used to represent gas, liquid, solid, and the aqueous phase in chemical equations.

Balance the following equations using the method outlined in Section 3.7 : (a) \(\mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}\) (b) \(\mathrm{CO}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}\) (c) \(\mathrm{H}_{2}+\mathrm{Br}_{2} \longrightarrow \mathrm{HBr}\) (d) \(\mathrm{K}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{KOH}+\mathrm{H}_{2}\) (e) \(\mathrm{Mg}+\mathrm{O}_{2} \longrightarrow \mathrm{MgO}\) (f) \(\mathrm{O}_{3} \longrightarrow \mathrm{O}_{2}\) (g) \(\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (h) \(\mathrm{N}_{2}+\mathrm{H}_{2} \longrightarrow \mathrm{NH}_{3}\) (i) \(\mathrm{Zn}+\mathrm{AgCl} \longrightarrow \mathrm{ZnCl}_{2}+\mathrm{Ag}\) (j) \(\mathrm{S}_{8}+\mathrm{O}_{2} \longrightarrow \mathrm{SO}_{2}\) (k) \(\mathrm{NaOH}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}+\mathrm{H}_{2} \mathrm{O}\) (l) \(\mathrm{Cl}_{2}+\mathrm{NaI} \longrightarrow \mathrm{NaCl}+\mathrm{I}_{2}\) \((\mathrm{m}) \mathrm{KOH}+\mathrm{H}_{3} \mathrm{PO}_{4} \longrightarrow \mathrm{K}_{3} \mathrm{PO}_{4}+\mathrm{H}_{2} \mathrm{O}\) (n) \(\mathrm{CH}_{4}+\mathrm{Br}_{2} \longrightarrow \mathrm{CBr}_{4}+\mathrm{HBr}\)