Question

Element 'x' has two stable isotopes:

215X present at 25%, and 217X present at 75%.
what would be the theoretical atomic mass for it? show/explain your work

Answer 1

216.5 amu

Step-by-step explanation:

Step 1: Given data

• Mass of isotope ²¹⁵X (m²¹⁵X): 215 amu

• Abundance of isotope ²¹⁵X (ab²¹⁵X): 25%

• Mass of isotope ²¹⁷X (m²¹⁷X): 217 amu

• Abundance of isotope ²¹⁷X (ab²¹⁷X): 75%

Step 2: Calculate the average atomic mass for the element

When an element has 2 or more isotopes, we calculate the average atomic mass (am) using the following expression.

am = ∑ mi × abi / 100%

where,

mi: mass of each isotope

abi: abundance of each isotope

am = (m²¹⁵X × ab²¹⁵X + m²¹⁷X × ab²¹⁷X) / 100%

am = (215 amu × 25% + 217 amu × 75%) / 100% = 216.5 amu