Question

Chlorine has two isotopes \(\text{Cl}^{37}\) and \(\text{Cl}^{35}\) with percentage abundance of \(25\%\) and \(75\%\) respectively. Find average atomic mass of chlorine ?

• A. \(35\)
• B. \(37\)
• C. \(35.5\)
• D. \(36\)

Hint:

Average atomic mass is the abundance-weighted mean of the isotopic masses.

Answer 1

The Correct Option is C

Step 1: Understand the concept
The average atomic mass of an element is the weighted mean of the masses of its isotopes. Each isotope contributes in proportion to its percentage abundance in nature.

Step 2: Write the formula
\[ \text{Average mass} = \frac{(m_1 \times a_1) + (m_2 \times a_2)}{100} \]
Here \(m\) is the mass number of an isotope and \(a\) is its percentage abundance.

Step 3: Substitute the data
Cl-37 has abundance 25% and Cl-35 has abundance 75%.
\[ \text{Average mass} = \frac{37 \times 25 + 35 \times 75}{100} = \frac{925 + 2625}{100} = \frac{3550}{100} = 35.5 \]

Step 4: Check the other options
The value must lie between 35 and 37, and it must sit closer to 35 because Cl-35 is three times more abundant. So 35 and 37 are plain isotope masses, not averages, and 36 is the simple mean that ignores abundance. Only 35.5 fits the weighted result.

Final Answer:
The weighted average works out to 35.5 u. This is option (C). \[ \boxed{\text{(C) }35.5} \]