Chapter 2 Suggested Homework Answers

 

Problems 8, 10, 16, 20, 22, 24, 30, 36, 42, 44, 48, 50, 54, 56, 60, 64, 66, 68

 

8. (a) If 6.500 g of hydrogen sulfide contains 0.384 g of hydrogen, then the remaining mass must be the mass of sulfur:

Mass of sulfur = 6.500 g – 0.384 g = 6.116 g

(b) This experiment demonstrates the law of conservation of mass.

(c) In Dalton's atomic theory, atoms cannot be created nor destroyed in chemical reactions. Since atoms cannot be changed, the total mass in a chemical reaction must remain the same.

 

10.  (a) According the the law of multiple proportions, if two elements, such as iodine and fluorine, combine to form more than one compound, the masses of fluorine that combines with one gram of iodine are in the ratio of small whole numbers. In compound 1, the mass of fluorine per gram of iodine is:

For compound 2, the mass of fluorine per gram of iodine is:

For compound 3, the mass of fluorine per gram of iodine is:

To find the smallest whole number ratio between these quotients, divide each quotient by the smallest quotient in the list then multiply each number by a factor which converts all the quotients to whole numbers:

(b) According to Dalton's atomic theory, a compound forms when atoms of more than one element combine in a particular ratio. Different compounds between the same two elements will have different relative amounts of each type of atom. However, since the basic building blocks are atoms, the mass of fluorine that combines with a fixed amount of iodine will be some whole number multiple of the mass of a fluorine atom.

 

16.  (a) First divide the diameter by two to obtain the radius:

To perform the conversions, the following conversion factors were used: 1 Å = 10^-10 m; 100 cm = 1 m

Therefore, the radius of a tin atom is 1.4 × 10^-10 m or 1.4 Å.

(b)  Convert the diameter of a Sn atom into mm and divide it into the distance of 6.0 mm. Conversion factor: 1 mm = 10^-6 m:

(c)   Calculate the volume of a Sn atom using its radius (in units of meters) and the formula for the volume of a sphere:

20 (a)  Elements are identified by their atomic number.  The atomic number in the chemical symbol notation is the subscripted number in front of the chemical symbol.  Therefore, are isotopes of the same element. 

(b)  The identity of these two isotopes is the element with an atomic number of 16.  Looking at the periodic table, this element has a chemical symbol of S, which stands for sulfur.

 

22.  (a)  Phosphorus-32 has 15 protons because the atomic number of P is 15.  It has 17 neutrons (32 – 15 = 17).

(b)  Chromium-51 has 24 protons because the atomic number of Cr is 24.  It has 27 neutrons (51 – 24 = 27).

(c)  Cobalt-60 has 27 protons because the atomic number of Co is 27.  It has 33 neutrons (60 – 27 = 33).

(d)  Technetium-99 has 43 protons because the atomic number of Tc is 43.  It has 56 neutrons (99 – 43 = 56).

(e)  Iodine-131 has 53 protons because the atomic number of I is 53.  It has 78 neutrons (131 – 53 = 78).

(f)  Thallium-201 has 81 protons because the atomic number of Tl is 81.  It has 120 neutrons (201 – 81 = 120).

 

24.   The filled-in gaps are italicized.  The atomic number is equal to the number of protons.  The mass number (mass no.) is equal to the number of protons plus the number of neutrons.  For atoms, the number of electrons is equal to the number of protons.  The symbolic notation has the mass number superscripted in front of the chemical symbol.

Symbol	121Sb	103Rh	88Sr	127Te	239Pu
Protons	51	45	38	52	94
Neutrons	70(=121-51)	58	50	75(=127–52)	145(=239–94)
Electrons	51	45	38	52	94
Mass no.	121	103(=45+58)	88(=50+38)	127	239

 

26.  The atomic weight is the weighted average of the four naturally occurring isotopes of lead:

36.  There is a stepwise line starting to the left of the element B in the periodic table and ending up between the elements Bi and Po in the periodic table.  Elements that lie to the left and/or below this stepwise line are metals.  Elements that lie to the right and/or above this stepwise line are nonmetals.  Elements that border the stepwise line are called metalloids with the exception of Al which is a metal.

(a)  Na, sodium,  is a metal.

(b)  Ti, titanium, is a metal.

(c)  Ga, gallium, is a metal.

(d)  U, uranium, is a metal.

(e)  Pd, palladium, is a metal.

(f)  Se, selenium, is a metalloid.

(g)  Kr, krypton, is a nonmetal.

 

42.  (a)  A molecule of benzene has six carbon atoms and six hydrogen atoms.  Its molecular formula is C₆H₆.  Divide both coefficients by “6” to obtain the empirical formula, CH.

(b)  A molecule of silicon tetrachloride has one silicon atom and four chlorine atoms.  Its molecular formula is SiCl₄.  This is already the lowest whole number ratio so the empirical formula is the same as the molecular formula.

(c)  Diborane has two boron atoms and six hydrogen atoms.  Its molecular formula is B₂H₆.  Divide both coefficients by “2” to obtain the empirical formula, BH₃.

(d)  Glucose has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.  Its molecular formula is C₆H₁₂O₆.  Divide all coefficients by “6” to obtain the empirical formula, CH₂O.

 

44.  (a)  There are 4 carbon atoms in C₂H₅COOCH₃.

(b)  There are 6 oxygen atoms in Ca(ClO₃)₂.  (6 = 3×2)

(c)  There are 9 hydrogen atoms in (NH₄)₂HPO₄.  (9=4×2+1)

48.  The filled-in gaps are italicized.  The atomic number is equal to the number of protons.  The mass number (mass no.) is equal to the number of protons plus the number of neutrons.  For ions, the number of electrons is equal to the number of protons minus the net charge (or the net charge is equal to the number of protons minus the number of electrons).  The symbolic notation has the mass number superscripted in front of the chemical symbol and the charge superscripted after the chemical symbol.

Symbol	75As3-	59Ni2+	127Te¯	197Au3+
Protons	33	28	53	79(=76+3)
Neutrons	42(=75–33)	31	74	118
Electrons	36(=33-(-3))	26	54(=53–(–1))	76
Net charge	3–	2+(=28–26)	1–	3+

 

50.  (a)  Sr is a metal in group 2A.  The charge of its ion is +2.

(b)  Sc is a metal in the “B” groups.  There is no simple rule to predict the charge of its ion.

(c)  P is a nonmetal in group 5A.  The charge of its ion is –3 (= 5 – 8).

(d)  I is a nonmetal in group 7A.  The charge of its ion is –1 (= 7 – 8).

(e)  Se is a nonmetal in group 6A.  The charge of its ion is –2 (= 6 – 8).

 

54.  For an ionic compound, the formula will have the lowest whole number ratio between the two ions in which the charges of the ions add up to zero.

(a)  The compound formed from Cu⁺ and S^2- is Cu₂S.

(b)  The compound formed from Fe³⁺ and O^2- is Fe₂O₃.

(c)  The compound formed from Hg₂²⁺ and CO₃^2- is Hg₂CO₃.

(d)  The compound formed from Ca²⁺ and AsO₄^3- is Ca₃(AsO₄)₂

(e)   The compound formed from NH₄⁺ and CO₃^2- is (NH₄)₂CO₃

 

56.  Compounds with only nonmetal atoms are molecular.  Compounds with a metal and a nonmetal are ionic.

(a)  PF₅ is molecular.

(b)  NaI is ionic.

(c)  SCl₂ is molecular.

(d)  Ca(NO₃)₂ is ionic.

(e)  FeCl₃ is ionic.

(f)  LaP is ionic.

(g)  CoCO₃ is ionic.

(h)  N₂H₄ is molecular.

 

60.  (a)  Li₂O is lithium oxide.

(b)  NaClO is sodium hypochlorite.  ClO¯ is called the hypochlorite ion.

(c)  Sr(CN)₂ is strontium cyanide.  CN¯ is called the cyanide ion.

(d)  Cr(OH)₃ is called chromium(III) hydroxide.  Chromium(III) ion is Cr³⁺.  The charge on the chromium ion can be determined from the hydroxide ion, OH¯.  Since there are 3 hydroxide for each chromium ion, its charge must be +3 so that the charges add up to zero.

(e)  Fe₂(CO₃)₃ is called iron(III) carbonate.  Iron(III) ion is Fe³⁺.  The charge on the iron ion can be determined from the carbonate ion, CO₃^2-.  Since the charge on 3 carbonate ions can cancel out the charge on 2 iron ions, the charge on the iron ion must be +3.

(f)  Co(NO₃)₃ is called cobalt(III) nitrate.  Cobalt(III) ion is Co³⁺.  The charge on the cobalt ion can be determined from the nitrate ion, NO₃¯.  Since it takes three nitrate ions to cancel out the charge on the cobalt ion, its charge must be +3.

(g)  (NH₄)₂SO₃ is called ammonium sulfite.  The ammonium ion is NH₄⁺.  The sulfite ion is SO₃^2-.

(h)  NaH₂PO₄ is called sodium dihydrogen phosphate.  The dihydrogen phosphate ion is H₂PO₄¯.

(i)  KMnO₄ is called potassium permanganate.  The permanganate ion is MnO₄¯.

(j)  Ag₂Cr₂O₇ is called silver dichromate.  The dichromate ion is Cr₂O₇^2-.

 

64.  (a)  Hydrobromic acid is HBr.

(b)  Hydrosulfuric acid is H₂S.

(c)  Nitrous acid is HNO₂.

(d)  H₂CO₃ is carbonic acid.

(e)  HClO₃ is chloric acid.

(f)  HC₂H₃O₂ is acetic acid.

 

66.  (a)  N₂O is called dinitrogen monoxide.

(b)  NO is nitrogen monoxide.

(c)  NO₂ is nitrogen dioxide.

(d)  N₂O₅ is dinitrogen pentoxide.

(e)  N₂O₄ is dinitrogen tetroxide.

 

68.  (a)  Sodium hydrogen carbonate is NaHCO₃.  (Sodium ion is Na⁺.  Hydrogen carbonate ion is HCO₃¯)

(b)  Calcium hypochlorite is Ca(ClO)₂.  (Calcium ion is Ca²⁺.  Hypochlorite ion is ClO¯.)

(c)  Hydrogen cyanide is HCN.

(d)  Magnesium hydroxide is Mg(OH)₂.  (Magnesium ion is Mg²⁺.  Hydroxide ion is OH¯).

(e)  Tin(II) fluoride is SnF₂.  (Tin(II) ion is Sn²⁺.  Fluoride ion is F¯.)

(f)  Calcium sulfide is CaS (Calcium ion is Ca²⁺, sulfide ion is S^2-).  Sulfuric acid is H₂SO₄.  Hydrogen sulfide is H₂S.