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4.4.1: Practice Problems- Formula Mass, Percent Composition, and the Mole

  • Page ID
    217267
  • PROBLEM \(\PageIndex{1}\)

    What is the total mass (amu) of carbon in each of the following molecules?

    (a) CH4
    (b) CHCl3
    (c) C12H10O6
    (d) CH3CH2CH2CH2CH3

    Answer a

    12.01 amu

    Answer b

    12.01 amu

    Answer c

    144.12 amu

    Answer d

    60.05 amu

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    PROBLEM \(\PageIndex{2}\)

    What is the total mass of hydrogen in each of the molecules?

    (a) CH4
    (b) CHCl3
    (c) C12H10O6
    (d) CH3CH2CH2CH2CH3

    Answer a

    4.032 amu

    Answer b

    1.008 amu

    Answer c

    10.08 amu

    Answer d

    12.096 amu

     
     

    PROBLEM \(\PageIndex{3}\)

    Calculate the molecular or formula mass of each of the following:

    (a) P4
    (b) H2O
    (c) Ca(NO3)2
    (d) CH3CO2H (acetic acid)
    (e) C12H22O11 (sucrose, cane sugar).

    Answer a

    123.896 amu

    Answer b

    18.015 amu

    Answer c

    164.086 amu

    Answer d

    60.052 amu

    Answer e

    342.297 amu

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    PROBLEM \(\PageIndex{4}\)

    Determine the molecular mass of the following compounds:

    (a)

    A structure is shown. A C atom is bonded to two C l atoms and forms a double bond with one O atom.

    (b)

    A structure is shown. Two C atoms form a triple bond with each other. Each C atom also forms a single bond with on H atom.

    (c)

    A structure is shown. Two C atoms form double bonds with each other. Each C atom also forms a single bond with an H atom and a B r atom.

    (d)

    A structure is shown. An S atom forms double bonds with two O atoms. The S atom also forms a single bond with an O atom which forms a single bond with an H atom. The S atom also forms a single bond with another O atom which forms a single bond with another H atom.

    Answer a

    98.906 amu

    Answer b

    26.018 amu

    Answer c

    185.818 amu

    Answer d

    98.072 amu

     

    PROBLEM \(\PageIndex{5}\)

    Determine the molecular mass of the following compounds:

    (a)

    A structure is shown. Two C atoms form double bonds with each other. The C atom on the left forms a single bond with two H atoms each. The C atom on the right forms a single bond with an H atom and with a C H subscript 2 C H subscript 3 group.

    (b)

    A structure is shown. There is a C atom which forms single bonds with three H atoms each. This C atom is bonded to another C atom. This second C atom forms a triple bond with another C atom which forms a single bond with a fourth C atom. The fourth C atom forms single bonds with three H atoms each.

    (c)

    A structure is shown. An S i atom forms a single bond with a C l atom, a single bond with a C l atom, a single bond with an H atom, and a single bond with another S i atom. The second S i atom froms a single bond with a C l atom, a single bond with a C l atom, and a single bond with an H atom.

    (d)

    A structure is shown. A P atom forms a double bond with an O atom. It also forms a single bond with an O atom which forms a single bond with an H atom. It also forms a single bond with another O atom which forms a single bond with an H atom. It also forms a single bond with another O atom which forms a single bond with an H atom.

    Answer a

    56.107 amu

    Answer b

    54.091 amu

    Answer c

    199.9976 amu

    Answer d

    97.9950 amu

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    PROBLEM \(\PageIndex{6}\)

    Compare 1 mole of H2, 1 mole of O2, and 1 mole of F2.

    (a) Which has the largest number of molecules? Explain why.

    (b) Which has the greatest mass? Explain why.

    Answer a

    1 mole is always 6.022 x 1023 molecules. They have the same number of molecules.

    Answer b

    F2; it has the highest molar mass

    PROBLEM \(\PageIndex{7}\)

    Which contains the greatest mass of oxygen: 0.75 mol of ethanol (C2H5OH), 0.60 mol of formic acid (HCO2H), or 1.0 mol of water (H2O)? Explain why.

    Answer

    Formic acid. Its formula has twice as many oxygen atoms as the other two compounds (one each). Therefore, 0.60 mol of formic acid would be equivalent to 1.20 mol of a compound containing a single oxygen atom.

    PROBLEM \(\PageIndex{8}\)

    Calculate the molar mass of each of the following:

    (a) S8
    (b) C5H12
    (c) Sc2(SO4)3
    (d) CH3COCH3 (acetone)
    (e) C6H12O6 (glucose)

    Answer a

    256.528 g/mol

    Answer b

    72.150 g/mol

    Answer c

    378.103 g/mol

    Answer d

    58.080 g/mol

    Answer e

    180.158 g/mol

    PROBLEM \(\PageIndex{9}\)

    Calculate the molar mass of each of the following:

    (a) the anesthetic halothane, C2HBrClF3
    (b) the herbicide paraquat, C12H14N2Cl2
    (c) caffeine, C8H10N4O2
    (d) urea, CO(NH2)2
    (e) a typical soap, C17H35CO2Na

    Answer a

    197.382 g/mol

    Answer b

    257.163 g/mol

    Answer c

    194.193 g/mol

    Answer d

    60.056 g/mol

    Answer e

    306.464 g/mol

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    PROBLEM \(\PageIndex{10}\)

    Determine the number of moles of compound and the number of moles of each type of atom in each of the following:

    (a) 25.0 g of propylene, C3H6
    (b) 3.06 × 10−3 g of the amino acid glycine, C2H5NO2
    (c) 25 lb of the herbicide Treflan, C13H16N2O4F (1 lb = 454 g)
    (d) 0.125 kg of the insecticide Paris Green, Cu4(AsO3)2(CH3CO2)2
    (e) 325 mg of aspirin, C6H4(CO2H)(CO2CH3)

    Answer a

    0.595 mol C3H6

    1.78 mol C

    3.57 mol H

    Answer b

    4.08 × 10-5 mol C2H5NO2

    8.16 × 10-5 mol C

    2.04 × 10-4 mol H

    4.08 × 10-5 mol N

    8.16 × 10-5 mol O

    Answer c

    40.106 mol C13H16N2O4F

    521.37 mol C

    641.70 mol H

    80.212 mol N

    160.42 mol O

    40.106 mol F

    Answer d

    0.202 mol Cu4(AsO3)2(CH3CO2)2

    0.808 mol Cu

    0.404 mol As

    2.02 mol O

    0.808 mol C

    1.212 mol H

    Answer e

    0.00181 mol C6H4(CO2H)(CO2CH3)

    0.01625 mol C

    0.0144 mol H

    0.0072 mol O

    PROBLEM \(\PageIndex{11}\)

    Determine the mass of each of the following:

    (a) 0.0146 mol KOH
    (b) 10.2 mol ethane, C2H6
    (c) 1.6 × 10−3 mol Na2 SO4
    (d) 6.854 × 103 mol glucose, C6 H12 O6
    (e) 2.86 mol Co(NH3)6Cl3

    Answer a

    0.819 g

    Answer b

    307 g

    Answer c

    0.23 g

    Answer d

    1.235 × 106 g (1235 kg)

    Answer e

    765 g

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    PROBLEM \(\PageIndex{12}\)

    Determine the number of moles of the compound and determine the number of moles of each type of atom in each of the following:

    (a) 2.12 g of potassium bromide, KBr
    (b) 0.1488 g of phosphoric acid, H3PO4
    (c) 23 kg of calcium carbonate, CaCO3
    (d) 78.452 g of aluminum sulfate, Al2(SO4)3
    (e) 0.1250 mg of caffeine, C8H10N4O2

    Answer a

    0.0178 mol KBr

    0.0178 mol K

    0.0178 mol Br

    Answer b

    0.00152 mol H3PO4

    0.00455 mol H

    0.00152 mol P

    0.00607 mol O

    Answer c

    230 mol CaCO3

    230 mol Ca

    230 mol C

    690 mol O

    Answer d

    0.229 mol Al2(SO4)3

    0.459 mol Al

    0.688 mol S

    2.75 mol O

    Answer e

    6.44 × 10-7 mol C8H10N4O2

    5.15 × 10-6 mol C

    6.44 × 10-6 mol H

    2.58 × 10-6 mol N

    1.29 × 10-6 mol O

    PROBLEM \(\PageIndex{13}\)

    The approximate minimum daily dietary requirement of the amino acid leucine, C6H13NO2, is 1.1 g. What is this requirement in moles?

    Answer

    0.0084 mol C6H13NO2

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    PROBLEM \(\PageIndex{14}\)

    Determine the mass in grams of each of the following:

    (a) 0.600 mol of oxygen atoms
    (b) 0.600 mol of oxygen molecules, O2
    (c) 0.600 mol of ozone molecules, O3

    Answer a

    9.60 g

    Answer b

    19.2 g

    Answer c

    28.2 g

    PROBLEM \(\PageIndex{15}\)

    A 55-kg woman has 7.5 × 10−3 mol of hemoglobin (molar mass = 64,456 g/mol) in her blood. How many hemoglobin molecules is this? What is this quantity in grams?

    Answer

    4.52 × 1021 molecules of hemoglobin

    483.42 g of hemoglobin

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    PROBLEM \(\PageIndex{16}\)

    One 55-gram serving of a particular cereal supplies 270 mg of sodium, 11% of the recommended daily allowance. How many moles and atoms of sodium are in the recommended daily allowance?

    Answer

    0.107 mol Na; 6.45 × 1022 atoms Na

    PROBLEM \(\PageIndex{17}\)

    A tube of toothpaste contains 0.76 g of sodium monofluorophosphate (Na2PO3F) in 100 mL.

    1. What mass of fluorine atoms in mg was present?
    2. How many fluorine atoms were present?
    Answer a

    100.27 mg F

    Answer b

    3.18 × 1021 atoms F

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    PROBLEM \(\PageIndex{18}\)

    Which of the following represents the least number of molecules?

    1. 20.0 g of H2O (18.02 g/mol)
    2. 77.0 g of CH4 (16.06 g/mol)
    3. 68.0 g of CaH2 (42.09 g/mol)
    4. 100.0 g of N2O (44.02 g/mol)
    5. 84.0 g of HF (20.01 g/mol)
    Answer

    20.0 g of H2O represents the smallest number of moles, meaning the least number of molecules present. Since 1 mole = 6.022 × 1023 molecules (or atoms) regardless of identity, the least number of moles will equal the least number of molecules

    PROBLEM \(\PageIndex{19}\)

    Calculate the following:

    (a) the percent composition of ammonia, NH3
    (b) the percent composition of photographic “hypo,” Na2S2O3
    (c) the percent of calcium ion in Ca3(PO4)2

    Answer a

    % N = 82.24%

    % H = 17.76%

    Answer b

    % Na = 29.08%

    % S = 40.56%

    % O = 30.36%

    Answer c

    % Ca2+ = 38.76%

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    PROBLEM \(\PageIndex{20}\)

    A compound of carbon and hydrogen contains 92.3% C and has a molar mass of 78.1 g/mol. What is its molecular formula?

    Answer

    C6H6

    PROBLEM \(\PageIndex{21}\)

    Dichloroethane, a compound that is often used for dry cleaning, contains carbon, hydrogen, and chlorine. It has a molar mass of 99 g/mol. Analysis of a sample shows that it contains 24.3% carbon and 4.1% hydrogen. What is its molecular formula?

    Answer

    C2H4Cl2

    Click here to see a video of the solution

    In the second edition, this problem was moved. The number in the title of the video may be incorrect, but the solution is correct.

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