What is the average atomic mass of element Y if Y consists of 57.25% of atoms with a mass of 123.90 amu and 42.75% of atoms with a mass of 122.90 amu?

The average atomic mass of Y is 123.4725 g/mol

Explanation:

The average mass of an element is calculated by taking the average of the atomic masses of its stable isotopes.

The enitre atomic mass = 100 % or 1

⇒ this consists of 57.25 % atoms with mass 123.90 g/mol

57.25 % = 0.5725

⇒ this consists of 42.75 % atoms with mass 123.90 g/mol

42.75 % = 0.4275

To calculate the mass of this isotope we use the following:

0.5725 * 123.90 + 0.4275 * 122.90 = 123.4725 g/mol