Question

The isotopes of chlorine with mass numbers $35$ and $37$ whose average mass is 35.5 exist in the ratio of:

• A. $1:1$
• B. $3:1$
• C. $3:4$
• D. $3:2$

Answer 1

The correct option is B $3:1$

Let the fraction of $C{l}^{35}$ isomer be $p$. The fraction of $C{l}^{37}$ isomer will be $1-p$.

Average atomic mass $=p\times m_1+(1-p)m_2$
$\therefore 35.5=p\times 35+(1-p)\times 37$

So, $p=0.75$

Hence, the ratio of two isotopes will be $p:(1-p)=0.75:(1-0.75)=0.75:0.25=3:1$

Option B is correct.