Question

What is the average atomic mass of ³⁵₁₇Cl and ³⁷₁₇Cl ? And what is the formula to do so?

Answer 1

To calculate average atomic mass, we need the percentage of abondance of the isotps.

​​​​​​In your question , % of abondance is not given.

Generally , percentage of abondance of isotopes of clorin is,
17Cl35= 75% and 17Cl37 = 25%.​​

So,
average atomic mass = (75/101)*35 +( 25/100)*37
=26.25+9.25
=35.5 u


Formula,

Average atomic mass = f₁M₁ + f₂M₂ +… + f_nM_n


where ,
f is the fraction representing the natural abundance of the isotope

M is the mass number (weight) of the isotope.