Solutions: Molecules and Compounds Homework Set

This problem set was developed by S.E. Van Bramer for Chemistry 145 at Widener University.

  1. How many protons, neutrons and electrons are present in one atom of the following:
    2. IsotopeProtonsNeutronsElectrons
    1H101
    2H111
    3H121
    13N767
    14N777
    15N787
    79Br354435
    81Br1-354636
    262Bh 107155107

  2. What is the Atomic Weight of chlorine given that:
    Isotope Mass Abundance Product
    35Cl 34.9689 75.53% 26.412
    37Cl 36.9659 24.47% 9.046
    Sum 35.458
    Is this the answer that you expected?

    Comparing this result with the value given in the periodic table of 35.4527 agrees to the expected number of significant figures (4).

  3. What is the Atomic Weight of ruthenium given the following information from WebElements:

    Isotope Abundance Exact MassProduct
    96Ru 5.54% 95.9075995.313
    98Ru 1.87% 97.9052871.831
    99Ru 12.76% 98.905938912.620
    100Ru 12.60% 99.904219212.588
    101Ru 17.06% 100.905581917.214
    102Ru 31.55% 101.904348532.151
    104Ru 18.62% 103.90542419.347
    Sum101.065
    Is this the answer that you expected?

    Comparing this result (101.07) with the value given in the periodic table of 101.07 agrees to the expected number of significant figures (2). Notice that the atomic weights (average atomic mass) is not known to the same precision for all elements. This is largely because the natural abundance of each isotope varies more for some elements. It is possible to measure the exact mass and the abundance with VERY great precision.

  4. Using a blank periodic table, fill in the atomic number, name, atomic symbol, and atomic weight of the first 36 elements. Also label the noble gases, the halogens, the alkali metals, the metals, and the non-metals.

    This may be a bit tedious, but you will use these elements frequently throughout the course, and in any future chemistry courses. You need to know the symbols for AT LEAST the first 36 elements from memory and will be responsible for them on future exams, quizzes, and homework.

  5. Use the mass spectrum for chromium given below to determine the atomic weight.

    For previous problems like this you were given the exact mass and the abundance for each isotope of an element. This information is obtained from the mass spectrum of the element. In this problem, you are given the mass spectrum and need to determine the exact mass and abundance of each isotope. In the mass spectrum the total abundance of each peak is scaled so that the larges has a signal of 100. From the spectrum I obtain the following values (yours may differ slightly, that is OK)

    Isotope Mass Intensity Abundance (%) Product
    50Cr 49.95 5 4.348 2.172
    52Cr 51.95 100 86.957 45.174
    53Cr 52.95 10 8.696 4.604
    Sum 51.950
    This is very close to the value given in the periodic table (51.9961). How many significant figures does this solution have?

  6. For the following molecules; write the chemical formula, determine how many atoms of each element are present in one molecule, determine the molecular weight, determine the number of moles in exactly 1 gram, and the number of moles in exactly 10-6 gram.
    1. carbon dioxide
    2. iron (II) chloride
    3. zinc (I) oxide
    4. dinitrogen pentoxide
    5. potassium fluoride
    6. dihydrogen monoxide
    7. iron (III) sulfate
    8. lithium acetate

    Name Formula Atoms MW mole (1 g) mole (10-6 g)
    carbon dioxide CO2 3 44.010 0.022722 2.2722x10-8
    iron (II) chloride FeCl2 3 126.752 7.88940x10-3 7.88940x10-9
    zinc (I) oxide Zn2O 3 146.78 6.8129x10-3 6.8129x10-9
    dinitrogen pentoxide N2O5 7 108.0105 9.258360x10-3 9.25836x10-9
    potassium fluoride KF 2 58.0967 1.72127x10-2 1.72127x10-8
    dihydrogen monoxide H2O 3 18.0153 5.55084x10-2 5.55084x10-8
    iron (III) sulfate Fe2(SO4)3 17 399.885 2.50072x10-3 2.50072x10-9
    lithium acetate LiCH3COO 8 65.986 1.5155x10-2 1.5155x10-8

  7. Name the following compounds, determine the molecular weight, determine the percent composition, and determine how many moles in 8.35 grams of the compound.
    1. KI
    2. CaF2
    3. Cu2SO4
    4. Cr(NO3)6
    5. NO
    6. NO2
    7. N2O
    8. N2O4
    9. LiOH
    10. SO2

    Formula Name MW % Comp mole in 8.35 g
    KI potassium iodide 166.0028 K = 23.5528
    I = 76.44719    		 5.03x10-2
    CaF2 calcium fluoride 78.075 Ca = 51.333
    F = 48.667    		 0.107
    Cu2SO4 copper (I) sulfate 223.156 Cu = 28.476
    S = 14.369
    O = 28.6785    		 3.74x10-2
    Cr(NO3)6 chromium (VI) nitrate 424.0206 Cr = 12.2626
    N = 19.81989
    O = 67.9188    		 1.97x10-2
    NO nitrogen monoxide 30.0061 N = 46.8818
    O = 53.3205    		 0.278
    NO2 nitrogen dioxide 46.0055 N = 30.5776
    O = 69.5543    		 0.181
    N2O dinitrogen monoxide 44.1342 N = 63.7483
    O = 36.2517    		 0.189
    N2O4 dinitrogen tetroxide 92.1324 N = 30.5374
    O = 69.4626    		 9.06x10-2
    LiOH lithium hydroxide 23.948 Li = 28.98
    O = 66.786
    H = 4.2089    		 0.349
    SO2 sulfur dioxide 64.065 S = 50.052
    O = 49.995    		 0.130

  8. Give the chemical formula (or atomic symbol), molecular (or atomic) weight, and charge for the following ions:
    1. sulfate
    2. sulfite
    3. nitrate
    4. hydroxide
    5. nitrite
    6. chloride
    7. nitride
    8. bromide
    9. acetate
    10. carbonate

    Ion formula MW charge
    sulfite SO32- 80.0642 2-
    nitrate NO31- 62.0656 1-
    hydroxide OH1- 17.0073 1-
    nitrite NO21- 46.0662 1-
    chloride Cl1- 35.4527 1-
    nitride N3- 14.00674 3-
    bromide Br1- 79.904 1-
    acetate CH3COO1- 59.045 1-
    carbonate CO32- 60.009 2-

Please send comments or suggestions to svanbram@science.widener.edu

Scott Van Bramer
Department of Chemistry
Widener University
Chester, PA 19013

© copyright 1996, S.E. Van Bramer
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Last Updated: Saturday, May 18, 1996