Question

Cl , Br, I follow Dobereiner's triads, the atomic masses of $Cl$ and $I$ are $35.5$ and $127$ respectively. The atomic mass of $Br$ is :
A. $162.5$
B. $91.5$
C. $81.25$
D. $45.625$

Answer 1

As per the Dobereneir’s triads, the atomic mass of is equal to the average of the atomic masses of two compounds. And also the properties of is intermediate to those two compounds. So just by knowing the atomic masses of those two compounds, questions can be easily answered.

Complete answer:
As per the Dobereneir’s triads, the atomic mass of $Br$ is equal to the average of the atomic masses of two compounds which are $Cl$ and $I$ .
The atomic mass of $Cl$ is given and also as we know is equal to $35.5$
And similarly the atomic mass of $I$ is equal to $127$.
Now, the atomic mass of $Br$ $= \dfrac{{35.5 + 127}}{2}$
After solving,
$\Rightarrow 81.25$
Hence, the atomic mass of $Br$ is $81.25$

Therefore, Option C is correct.

Additional Information:
Chlorine is actually a chemical element having the symbol $Cl$ and its atomic number is 17. It is the second lightest of the halogens, and it appears between fluorine and bromine in the chemical periodic table and its properties are also mostly intermediate b/w them. Chlorine is a yellow-green colour gas at room temperature.
Chlorine is used to kill bacteria as it is a disinfectant. It is used in treating water for drinking and swimming. It is used to make many consumer products from paper to paints, and also from textiles to insecticides. Around 20 percent of chlorine produced is actually used to make PVC.
Iodine is a very essential mineral mostly found in seafood. Our thyroid gland uses it to make thyroid hormones, which further help control growth, also repair damaged cells and support in a healthy metabolism. Although, up to a third of people worldwide are actually at risk of an iodine deficiency unfortunately.
Bromine is a chemical element having the symbol $Br$ with the atomic number 35. It is actually the third lightest halogen, and is generally a fuming red-brown liquid at room temperature which evaporates heavily to produce a similarly coloured gas. Its properties are actually intermediate b/w those of chlorine and iodine.

Note: According to the Dobereneir’s triads, the atomic mass of $Br$ is equal to the average of the atomic masses of two compounds and those two compounds are chlorine and iodine whose atomic masses are already given.