**Question**

**The percentage of occurrence in a sample of _{5}Y^{10} and _{5}Y^{11} are 20% and 80% respectively. Find the atomic mass of the element.**

**Answer**

The atomic mass of the element can be calculated using the formula:

(average atomic mass) = (mass of isotope 1 × abundance of isotope 1) + (mass of isotope 2 × abundance of isotope 2)

Given that the sample contains 5 atoms each of Y10 and Y11, we can calculate the masses of the isotopes as:

Mass of Y^{10} = 5 atoms × 10 atomic mass units (amu)

= 50 amu

Mass of Y^{11} = 5 atoms × 11 atomic mass units (amu)

= 55 amu

Using the given percentages of occurrence, we can calculate the abundances of each isotope as:

Abundance of Y^{10} = 20% = 0.2 Abundance of Y^{11} = 80% = 0.8

Substituting these values in the formula, we get:

(average atomic mass) = (10 amu × 0.2) + (11 amu × 0.8)

= 2 amu + 8.8 amu

= 10.8 amu

Therefore, the atomic mass of the element is approximately 10.8 atomic mass units.