Answer to Question #135978 in Chemistry for junaisah

Question

An unknown element (X) has two stable isotopes. 18X with a mass of 17.55 amu: has a % abundance of 83.55% while the other isotope is (19X) whose mass is 18.67 amu has a % abundance of 16.45%. What is the average and relative atomic mass of the unknown element?

Answer 1

Solution:

1) The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance.

Average atomic mass = f₁M₁ + f₂M₂ +… + f_nM_n, where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.

Thus:

Average atomic mass of X = (0.8355) × (17.55 amu) + (0.1645) × (18.67 amu) = 17.73 amu

2) The relative atomic mass is the average of the mass numbers of the different isotopes.

The relative atomic mass of X = (0.8355) × (18) + (0.1645) × (19) = 18.1645