CHM101, Section 5, Fall 2001
Answers to first hourly exam, Chapters 1–3
Tuesday 25 September 2001

Definitions (2 points each; 20 points total)
      1. Chemical change—A process in which one or more substances are changed into one or more different substances.
      2. Cation—An ion with a positive charge.
      3. Mixture—A type of matter whose composition and properties vary from one sample to another.
      4. Molecule—Two or more atoms held together in a definite arrangement by covalent bonds.
      5. Formula unit—The simplest combination of atoms or ions consistent wit the formula of a compound.
      6. Alkane—A saturated hydrocarbon with the general formula C_nH_2n+2.
      7. Mole—(a) The amount of a substance that contains as many elementary (chemical) units as there are atoms in 12 g of carbon-12; (b) The amount of a substance with mass in grams numerically equal to its elementary mass in amu.
      8. Stoichometric coefficient—The coefficient placed in front of a formula in a balanced chemical equation.
      9. Empirical formula—The formula that describes the ratios of the elements in a compound with the smallest whole numbers.
      10. Solute—The component of a solution that is dissolved in the solvent.

Short answers (3 points each; 30 points total)
      1. Show how to convert 21.3 km to inches. Work out the answer, and express it to the proper number of significant figures. Assume that the 21.3 is a measured value. (1 in = 2.54 cm exactly)

21.3 km x (1000 m/km) x (100 cm/m) x (1 in/2.54 cm) = 839,000 in = 8.39 x 10⁵ in
(rounded to 3 sig figs)
(Note that all three conversion factors are exact.)

      2. Give the chemical symbols for these elements: sodium (Na) beryllium (Be) cobalt (Co) phosphorus (P) bromine (Br) vanadium (V)
      3. How many significant figures are there in each of these numbers? (a) 4051 (4) (b) 0.0169 (3) (c) 5.00 x 10⁹ (3)
      4. Name these compounds: (a) CH₃CH₂CH₂CH₃  butane (b) Ca(OH)₂ calcium hydroxide (c) B₂Cl₄ diborontetrachloride
      5. For each of these compounds, state whether it should be ionic or molecular and give the reason: (a) PCl₅ molecular—two nonmetals (b) KBr ionic—metal plus nonmetal (c) MnO₂ ionic—metal plus nonmetal
      6. Name these ions: (a) NH₄⁺ ammonium (b) Co²⁺ cobalt (II) (c) SO₃^-2-sulfite
      7. Can a series of measurements be accurate without being precise? Yes. Explain why or why not. The average can be close to the correct value, but the individual measurements widely scattered around it.
      8. Explain the Arrhenius theory of acids and bases (the original definition of acids and bases). Acids dissolve in water to give hydrogen ions, bases to give hydroxide ions.
      9. Metal or nonmetal? (a) F nonmetal (b) C nonmetal (c) Pd metal
      10. In the manner of ₂₄⁵²Cr, represent these nuclides: (a) strontium-86 ₃₈⁸⁶Sr (b) europium-153 ₆₃¹⁵³Eu (c) gallium-69 ₃₁⁶⁹Ga

Problems (10 points each; 50 points total)
      1. Find the answer and express it to the proper number of significant figures, assuming that all the numbers have come from experimental measurements:

[418.7 x 31.8]/[19.27 – 18.98] = 13,314.66 (3)/0.29 (2) = 45,913 (2) = 46,000
(Number of significant figures shown in italics)

      2. Calculate the percentage composition of Ba(ClO₄)₂ to three significant figures.
         (1) Total mass = 1(137.327) + 2(35.4527) + 8(15.9994) = 336.2276 amu.
          (2) Fraction Ba = 137.327/336.2276 = 0.4084; fraction Cl = 70.9054/336.2276 = 0.2109; fraction O = 127.9952/336.2276 = 0.3807.
          (3) Percents: 40.8% for Ba, 21.1% for Cl, and 38.1% for O.

      3. Balance this equation: WO₃ + H₂® W + H₂O. Show the steps in your work.

WO₃ + H₂® W + 3H₂O
WO₃ + 3H₂® W + 3H₂O
Check W

      4. How many milliliters of 3.84 M HCl are required to consume 4.12 grams of zinc in the reaction (unbalanced):

Zn(s) + HCl(aq) ® ZnCl₂(aq) + H₂(g)

      This problem contains two steps: (1) determine the mass of HCl needed to react with the Zn, and (2) determine the volume of 3.84 M HCl that contains it.
      Step 1: Balance the equation and calculate the required mass of HCl in the usual way. The balanced equation is:

Zn(s) + 2HCl(aq) ® ZnCl₂(aq) + H₂(g)

      The proportion is 65.39 amu/4.12 g Zn = 72.92 amu/X g HCl. X = 4.5944 g HCl.
      Step 2: C = M/V ® V = M/C = [4.5944 g HCl]/[(3.84 mol HCl/L)(36.46 g HCl/mol HCl)] = 0.0328 L HCl = 32.8 mL HCl (to 3 significant figures)

      5. Pb(II) oxide reacts with ammonia according to the following reaction (which is not balanced):

PbO(s) + NH₃(g) ® Pb(s) + N₂(g) + H₂O(l)

            (a) When 75.0 g PbO reacts with 15.5 g NH₃, which reactant is limiting?

      Procedure: balance the equation and see how much NH₃ is required.

3PbO(s) + 2NH₃(g) ® 3Pb(s) + N₂(g) + 3H₂O(l)

      The mass proportion from the left side is 669 amu/75.0 g PbO = 34 amu/X g NH₃. X = 3.8 g NH₃. Since 15.5 g of NH₃ is available, it is in excess and the PbO is limiting.

            (b) Under the conditions in (a), what masses of Pb and N₂ will be formed?

      Calculating from the PbO, the mass proportion for Pb is 669 amu/75.0 g PbO = 621 amu/X g Pb. X = 69.6 g Pb.
      Doing the same for N₂, the mass proportion is 669 amu/75.0 g PbO = 28 amu/X g N₂. X = 3.14 g N₂.

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