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Homework 21

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    Isotopes are atoms that have the same number of protons but differ in the number of neutrons. Write the chemical symbols for each of the following isotopes:

    a) the chlorine isotope with 18 neutrons

    b) the chlorine isotope with 20 neutrons

    c) the calcium isotope with 20 neutrons

    d) the neon isotope with 11 neutrons


    Step 1: Determine the given element's atomic number from the periodic table.

    Step 2: Add the number of neutrons given in the question to the atomic number to calculate the mass number of the isotope.

    Step 3: The mass number will be represented in the upper left corner of the chemical symbol, while the atomic number will be represented in the lower left corner.

    Helpful Hints:

    • A chemical symbol is written as \[_{Z}^{A}\textrm{X}\] . The A stands for the number of protons and neutrons, also known as the mass number. The Z stands for the number of protons, also known as the atomic number.
    • Subtracting Z from A will give you the number of neutrons.
      • Example: \[_{19}^{39}\textrm{K}\] --> 39-19= 20 neutrons
    • Therefore, adding the given number of neutrons to the atomic number, Z, gives you the mass number.


    a) \[_{17}^{35}\textrm{Cl}\]

    b) \[_{17}^{37}\textrm{Cl}\]

    c) \[_{20}^{40}\textrm{Ca}\]

    d) \[_{10}^{21}\textrm{Ne}\]



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

    a) 3.2 x 1024 Cl2 molecules

    b) 8.25 x 1018 CH2O molecules

    c) 1 carbon dioxide molecule


    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.

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

    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


    a) 3.8 x 1051 grams

    b) 4.11 x 1045 grams

    c) 7.31 x 1025 grams

    Homework 21 is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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