Element A has two isotopes. The first isotope is present 24.01% of the time and has a mass of 131.96 amu. The second isotope has a mass of 113.66 amu. Calculate the atomic mass of element A.

• A. 118.74 amu
• B. 120.50 amu
• C. 117.35 amu
• D. 125.30 amu

Answer: (A)

Verified step by step guidance

1. Understand that the atomic mass of an element is the weighted average of the masses of its isotopes, based on their natural abundance.
2. Convert the percentage of the first isotope's abundance into a decimal by dividing by 100. For the first isotope, 24.01% becomes 0.2401.
3. Calculate the contribution of the first isotope to the atomic mass by multiplying its mass (131.96 amu) by its decimal abundance (0.2401). Use the formula: $\mathrm{mass}\times \mathrm{abundance}$.
4. Assume the second isotope's abundance is the remainder of 100% after accounting for the first isotope. Calculate this by subtracting the first isotope's abundance from 1: $1-0.2401$.
5. Calculate the contribution of the second isotope to the atomic mass by multiplying its mass (113.66 amu) by its decimal abundance. Add the contributions from both isotopes to find the atomic mass of element A.