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4.7.1: Mole Practice with Guidence (Exercises)

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    290940
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    These are homework exercises to accompany the Textmap created for Chemistry: A Molecular Approach by Nivaldo Tro. Complementary General Chemistry question banks can be found for other Textmaps and can be accessed here. In addition to these publicly available questions, access to private problems bank for use in exams and homework is available to faculty only on an individual basis; please contact Delmar Larsen for an account with access permission.

    Mole/Mass Conversions

    1. Find the amount of moles contained in each specimen.

    1. 7.87 kg H2O2
    2. 2.34 kg NaCl
    3. 12.5 g C2H6O
    4. 85.72 g NH3

    Strategy

    A. First, determine the units that are given for each sample. In order to convert these measurements to moles, each sample should first be written in grams. Two of the given samples are measured in kilograms. Use the following conversion factor to convert kilograms to grams, with \(x\) representing the given mass:

    \[x\, kg\cdot \dfrac{1000\, g}{1\, kg}\]

    B. Next, find the atomic masses of all the atoms in each compound by using the Periodic Table. Then, add these atomic masses together for each compound. The resulting value will be the number of grams of each sample that make up one mole.

    C. Convert the mass in grams of each sample to moles by multiplying it by the following conversion factor, called the molar mass, with x representing the mass of the given specimen and \(y\) representing the calculated atomic mass found in step B:

    \[x\, g\cdot \dfrac{1\, mole}{y\, g}\]

    A Numbers 1 and 2 first need to be converted into grams.

    1. \[7.87\, kg\, H_2O_2\cdot \dfrac{1000\, g}{1\, kg}=7,870\, g\, H_2O_2\]
    2. \[2.34\, kg\, NaCl\cdot \dfrac{1000\, g}{1\, kg}=2,340\, g\, NaCl\]

    B The following are the calculated molar masses of each of the given compounds. The atomic mass of each element has been rounded to 4 significant figures in these calculations.

    1. H2O2: \[2\, (1.008\, g\, H)+2\, (16.00\, g\, O)=\dfrac{34.02\, g}{1\, mole}\, H_2O_2\]
    2. NaCl: \[(22.99\, g\, Na)+(35.45\, g\, Cl)=\dfrac{58.44\, g}{1\, mole}\, NaCl\]
    3. C2H6O: \[2\, (12.01\, g\, C)+6\, (1.008\, g\, H)+\left ( 16.00\, g\, O \right )=\dfrac{46.07\, g}{1\, mole}\, C_2H_6O\]
    4. NH3: \[(14.01\, g\, N)+3\, (1.008\, g\, H)=\dfrac{17.03\, g}{1\, mole}\, NH_3\]

    C The number of moles in each specimen can now be calculated by multiplying the mass in grams of each sample by its molarity.

    1. \[7,870\, g\, H_2O_2\cdot \dfrac{1\, mole\, H_2O_2}{34.02\, g\, H_2O_2}=231\, moles\, H_2O_2\]
    2. \[2,340\, g\, NaCl\cdot \dfrac{1\, mole\, NaCl}{58.44\, g\, NaCl}=40.0\, moles\, NaCl\]
    3. \[12.5\, g\, C_2H_6O\cdot \dfrac{1\, mole\, C_2H_6O}{46.07\, g\, C_2H_6O}=0.271\, moles\, C_2H_6O\]
    4. \[85.72\, g\, NH_3\cdot \dfrac{1\, mole\, NH_3}{17.03\, g\, NH_3}=5.033\, moles\, NH_3\]

    Mole/Mass Conversions

    2. Calculate the number of moles in each of the following examples.

    1. 402.5 mg of NO2
    2. 2.7 kg of H2O
    3. 323 g of CBr4
    4. 2.9 kg of CaO

    Solution

    a)

    First, calculate the molar mass of NO2.

    14.007g/mol N + 2 (15.999 g/mol O) = 46.0055 g/mol

    Now, convert the sample from mg to g.

    402.5 mg NO2 1 g 0.4025 g NO2
      1000 mg  

    Finally, use the molar mass to determine how many moles your sample size contains.

    0.4025 g NO2 1 mol 0.0087 moles of NO2
      46.0055 g  

    This gives us the final answer, 0.0087 moles of NO2.

    From this, we can deduce that:

    \[\text{Moles of substance} = \text{Mass of substance (g)}{Molar mass of substance}\]

    b) 150 moles of H2O

    c) 0.974 moles of CBr4

    d) 52 moles of CaO

    Mass/Particle Conversions

    1. For each substance, convert the given molecules to mass in grams:

    1. 3.2 x 1024 Cl2 molecules
    2. 8.25 x 1018 CH2O molecules
    3. 1 carbon dioxide molecule

    Strategy:

    Step 1: Convert the given molecules to moles by dividing by Avogadro's number.

    Step 2: Multiply the number of moles by the Molar Mass of the substance to determine the grams.

    Step 3: Through correct Dimensional Analysis, the molecules and moles will cancel to leave grams.

    \[\dfrac{molecules}{Avogadro's}\rightarrow moles\rightarrow moles \cdot MM\rightarrow grams\]

    Helpful Hints:

    • Avogadro's number: 6.022 x 1023 mol of a substance
    • Molar mass (MM): the weight of a substance in the units g/mol, found by adding the atomic masses of each element.
      • Example: MM of H2O --> H(1.008 x 2) + O(15.999)= 18 g/mol

    Solution

    a) 3.8 x 1051 grams

    b) 4.11 x 1045 grams

    c) 7.31 x 1025 grams

     


    4.7.1: Mole Practice with Guidence (Exercises) is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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