Calculate the “average” molar mass of

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Calculate the “average” molar mass of

(a) chlorobenzene (C6H5Cl) and

(b) bromobenzene (C6H5Br

The correct answer and explanation is:

To calculate the average molar mass of chlorobenzene (C₆H₅Cl) and bromobenzene (C₆H₅Br), we need to first determine the molar mass of each compound by summing the atomic masses of all the elements in their molecular formulas.

(a) Chlorobenzene (C₆H₅Cl)

The molecular formula for chlorobenzene is C₆H₅Cl, which consists of 6 carbon (C) atoms, 5 hydrogen (H) atoms, and 1 chlorine (Cl) atom.

  1. Carbon (C): The atomic mass of carbon is 12.01 g/mol.
    • For 6 carbon atoms: 6×12.01=72.06 g/mol6 \times 12.01 = 72.06 \, \text{g/mol}
  2. Hydrogen (H): The atomic mass of hydrogen is 1.008 g/mol.
    • For 5 hydrogen atoms: 5×1.008=5.04 g/mol5 \times 1.008 = 5.04 \, \text{g/mol}
  3. Chlorine (Cl): The atomic mass of chlorine is 35.45 g/mol.
    • For 1 chlorine atom: 1×35.45=35.45 g/mol1 \times 35.45 = 35.45 \, \text{g/mol}

Now, add up the individual contributions: Molar mass of chlorobenzene=72.06+5.04+35.45=112.55 g/mol\text{Molar mass of chlorobenzene} = 72.06 + 5.04 + 35.45 = 112.55 \, \text{g/mol}

(b) Bromobenzene (C₆H₅Br)

The molecular formula for bromobenzene is C₆H₅Br, consisting of 6 carbon (C) atoms, 5 hydrogen (H) atoms, and 1 bromine (Br) atom.

  1. Carbon (C): The atomic mass of carbon is 12.01 g/mol.
    • For 6 carbon atoms: 6×12.01=72.06 g/mol6 \times 12.01 = 72.06 \, \text{g/mol}
  2. Hydrogen (H): The atomic mass of hydrogen is 1.008 g/mol.
    • For 5 hydrogen atoms: 5×1.008=5.04 g/mol5 \times 1.008 = 5.04 \, \text{g/mol}
  3. Bromine (Br): The atomic mass of bromine is 79.90 g/mol.
    • For 1 bromine atom: 1×79.90=79.90 g/mol1 \times 79.90 = 79.90 \, \text{g/mol}

Now, add up the individual contributions: Molar mass of bromobenzene=72.06+5.04+79.90=157.00 g/mol\text{Molar mass of bromobenzene} = 72.06 + 5.04 + 79.90 = 157.00 \, \text{g/mol}

Conclusion:

  • Chlorobenzene (C₆H₅Cl) has a molar mass of 112.55 g/mol.
  • Bromobenzene (C₆H₅Br) has a molar mass of 157.00 g/mol.

This shows that bromobenzene has a significantly higher molar mass compared to chlorobenzene due to the larger atomic mass of bromine compared to chlorine.

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