CHEMICAL FOUNDATIONS

CHAPTER 1

CHEMICAL FOUNDATIONS

Questions 11.A law summarizes what happens, e.g., law of conservation of mass in a chemical reaction or the ideal gas law, PV = nRT. A theory (model) is an attempt to explain why something happens. Dalton’s atomic theory explains why mass is conserved in a chemical reaction. The kinetic molecular theory explains why pressure and volume are inversely related at constant temperature and moles of gas present, as well as explaining the other mathematical relationships summarized in PV = nRT.

12.a. At 8 a.m., approximately 57 cars pass through the intersection per hour.

• At 12 a.m. (midnight), only 1 or 2 cars pass through the intersection per hour.

c.Traffic at the intersection is limited to less than 10 cars per hour from 8 p.m. to 5 a.m.Starting at 6 a.m., there is a steady increase in traffic through the intersection, peaking at

• a.m. when approximately 57 cars pass per hour. Past 8 a.m. traffic moderates to about

40 cars through the intersection per hour until noon, and then decreases to 21 cars per hour by 3 p.m. Past 3 p.m. traffic steadily increases to a peak of 52 cars per hour at 5 p.m., and then steadily decreases to the overnight level of less than 10 cars through the intersection per hour.

• The traffic pattern through the intersection is directly related to the work schedules of the

general population as well as to the store hours of the businesses in downtown.e.Run the same experiment on a Sunday, when most of the general population doesn’t work and when a significant number of downtown stores are closed in the morning.

13.The fundamental steps are (1)making observations; (2) formulating hypotheses; (3)performing experiments to test the hypotheses.The key to the scientific method is performing experiments to test hypotheses. If after the test of time the hypotheses seem to account satisfactorily for some aspect of natural behavior, then the set of tested hypotheses turns into a theory (model). However, scientists continue to perform experiments to refine or replace existing theories. Hence, science is a dynamic or active process, not a static one.

14.A c ompound will always contain the same numbers (and types) of atoms. A given amount of hydrogen will react only with a specific amount of oxygen. Any excess oxygen will remainunreacted.

15.La w of conservation of mass: Mass is neither created nor destroyed. The total mass before a chemical reaction always equals the total mass after a chemical reaction.Chemistry An Atoms First Approach, 3e Steven Zumdahl, Susan Zumdahl, Donald DeCo ste (Solutions Manual, All Chapters. 100% Original Verified, A+ Grade)

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CHAPTER 1 CHEMICAL FOUNDATIONS

Law of definite proportion: A given compound always contains exactly the same proportion of elements by mass. For example, water is always 1 g H for every 8 g oxygen.Law of multiple proportions: When two elements form a series of compounds, the ratios of the mass of the second element that combine with 1 g of the first element always can be reduced to small whole numbers. For CO

• and CO discussed in Section 1.4, the mass ratios

of oxygen that react with 1 g carbon in each compound are in a 2 : 1 ratio.

• Yes, 1.0 g H would react with 37.0 g

37 Cl, and 1.0 g H would react with 35.0 g 35 Cl.No, the mass ratio of H/Cl would always be 1 g H/37 g Cl for 37 Cl and 1 g H/35 g Cl for 35 Cl.As long as we had pure 37 Cl or pure 35 Cl, the ratios will always hold. If we have a mixture (such as the natural abundance of chlorine), the ratio will also be constant as long as the composition of the mixture of the two isotopes does not change.

17.Natural niacin and commercially produced niacin have the exact same formula of C6H5NO2.Therefore, both sources produce niacin having an identical nutritional value. There may be other compounds present in natural niacin that would increase the nutritional value, but thenutritional value due to just niacin is identical to the commercially produced niacin.

18.a. Th e smaller parts are electrons and the nucleus. The nucleus is broken down into protons and neutrons, which can be broken down into quarks. For our purpose, electrons, neutrons, and protons are the key smaller parts of an atom.b.All atoms of hydrogen have 1 proton in the nucleus. Different isotopes of hydrogen have 0, 1, or 2 neutrons in the nucleus. Because we are talking about atoms, this implies a neutral charge, which dictates 1 electron present for all hydrogen atoms. If charged ionswere included, then different ions/atoms of H could have different numbers of electrons.c.Hydrogen atoms always have 1 proton in the nucleus, and helium atoms always have 2 protons in the nucleus. The number of neutrons can be the same for a hydrogen atom and a helium atom. Tritium ( 3

• and

4 He both have 2 neutrons. Assuming neutral atoms, then the number of electrons will be 1 for hydrogen and 2 for helium.d.Water (H 2O) is always 1 g hydrogen for every 8 g of O present, whereas H2O2 is always 1 g hydrogen for every 16 g of O present. These are distinctly different compounds, each with its own unique relative number and types of atoms present.e.A chemical equation involves a reorganization of the atoms. Bonds are broken between atoms in the reactants, and new bonds are formed in the products. The number and types of atoms between reactants and products do not change. Because atoms are conserved in a chemical reaction, mass is also conserved.

19.J. J . Thomson’s study of cathode-ray tubes led him to postulate the existence of negatively charged particles that we now call electrons. Ernest Rutherford and his alpha bombardment of metal foil experiments led him to postulate the nuclear atom−an atom with a tiny dense center of positive charge (the nucleus) with electrons moving about the nucleus at relatively large distances away; the distance is so large that an atom is mostly empty space. 2 / 4

CHAPTER 1 CHEMICAL FOUNDATIONS

20.The atom is composed of a tiny dense nucleus containing most of the mass of the atom. The nucleus itself is composed of neutrons and protons. Neutrons have a mass slightly larger tha n that of a proton and have no charge. Protons, on the other hand, have a 1+ relative charge as compared to the 1− charged electrons; the electrons move about the nucleus at relatively large distances. The volume of space that the electrons move about is so large, as compared to the nucleus, that we say an atom is mostly empty space.

21.T he number and arrangement of electrons in an atom determine how the atom will react with other atoms. The electrons determine the chemical properties of an atom. The number of neutrons present determines the isotope identity.

22.Density = mass/volume; if the volumes are assumed equal, then the much more massive proton would have a much larger density than the relatively light electron.

23.For lighter, stable isotopes, the number of protons in the nucleus is about equal to the number of neutrons. When the number of protons and neutrons is equal to each other, the mas s number (protons + neutrons) will be twice the atomic number (protons). Therefore, for lighter isotopes, the ratio of the mass number to the atomic number is close to 2. Fo r example, consider 28 Si, which has 14 protons and (28 – 14 =) 14 neutrons. Here, the mass number to atomic number ratio is 28/14 = 2.0. For heavier isotopes, there are more neutrons than protons in the nucleus. Therefore, the ratio of the mass number to the atomic number increases steadily upward from 2 as the isotopes get heavier and heavier. For example, 238 U has 92 protons and (238 – 92 =) 146 neutrons. The ratio of the mass number to the atomic number for 238 U is 238/92 = 2.6.

24.S ome elements exist as molecular substances. That is, hydrogen normally exists as H 2 molecules, not single hydrogen atoms. The same is true for N2, O2, F2, Cl2, Br2, and I2.Exercises Development of the Atomic Theory 25.a. The composition of a substance depends on the numbers of atoms of each element making up the compound (depends on the formula of the compound) and not on the composition of the mixture from which it was formed.b.Avogadro’s hypothesis (law) implies that volume ratios are equal to molecule ratios at constant temperature and pressure. H 2(g) + Cl2(g) → 2 HCl(g). From the balanced equation, the volume of HCl produced will be twice the volume of H

• (or Cl2) reacted.26.Avogadro’s hypothesis (law) implies that volume ratios are equal to molecule ratios at

constant temperature and pressure. Here, 1 volume of N

• reacts with 3 volumes of H2 to

produce 2 volumes of the gaseous product or in terms of molecule ratios:

• N2 + 3 H2 → 2 product

In order for the equation to be balanced, the product must be NH3. 3 / 4

CHAPTER 1 CHEMICAL FOUNDATIONS

• From the law of definite proportions, a given compound always contains exactly the same

proportion of elements by mass. The first sample of chloroform has a total mass of 12.0 g C + 106.4 g Cl + 1.01 g H = 119.41 g (carrying extra significant figures). The mass percent of

carbon in this sample of chloroform is:

12.0 g C 119.41 g total × 100 = 10.05% C by mass From the law of definite proportions, the second sample of chloroform must also contain

10.05% C by mass. Let x = mass of chloroform in the second sample:

30.0 g C x

× 100 = 10.05, x = 299 g chloroform 28.A compound will always have a constant composition by mass. From the initial data given, the mass ratio of H : S : O in sulfuric acid (H

2SO4) is:

2.02 2.02

:

32.07 2.02

:

64.00

= 1 : 15.9 : 31.7

2.02 If we have 7.27 g H, then we will have 7.27 × 15.9 = 116 g S and 7.27 × 31.7 = 230. g O in the second sample of H 2SO4.

29.Mass is conserved in a chemical reaction because atoms are conserved. Chemical reactions involve the reorganization of atoms, so formulas change in a chemical reaction, but the number and types of atoms do not change. Because the atoms do not change in a chemical reaction, mass must not change. In this equation we have two oxygen atoms and four hydrogen atoms both before and after the reaction occurs.

30.Mass is conserved in a chemical reaction.ethanol + oxygen → water + carbon dioxide

Mass: 46.0 g 96.0 g 54.0 g ?

Mass of reactants = 46.0 + 96.0 = 142.0 g = mass of products 142.0 g = 54.0 g + mass of CO 2, mass of CO2 = 142.0 – 54.0 = 88.0 g 31.To get the atomic mass of H to be 1.00, we divide the mass of hydrogen that reacts with 1.00 the same division.

0.126 0.126 g of oxygen by 0.126; that is,

= 1.00. To get Na, Mg, and O on the same scale, we do 0.126

= 22.8; Mg:

1.500

0.126 0.126

Na:

2.875

= 11.9; O:

1.00

= 7.94

H O Na Mg 1.00 7.94 22.8 11.9 Relative value Accepted value 1.008 16.00 22.99 24.31 For your information, the atomic masses of O and Mg are incorrect. The atomic masses of H and Na are close to the values given in the periodic table. Something must be wrong about the assumed formulas of the compounds. It turns out the correct formulas are H 2O, Na2O, and MgO. The smaller discrepancies result from the error in the assumed atomic mass of H.

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