Gallium has two naturally occurring isotopes: Ga-69 with a mass of 68.9256 amu and a natural abundance of 60.11%, and Ga-71. If the atomic mass of gallium is 69.723 amu, what is the mass of Ga-71?

• A. 70.9247 amu
• B. 69.723 amu
• C. 68.9256 amu
• D. 71.926 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 abundances.
2. Set up the equation for the average atomic mass of gallium using the formula: \( \text{Atomic Mass} = (\text{Mass of Ga-69} \times \text{Abundance of Ga-69}) + (\text{Mass of Ga-71} \times \text{Abundance of Ga-71}) \).
3. Substitute the known values into the equation: \( 69.723 = (68.9256 \times 0.6011) + (\text{Mass of Ga-71} \times (1 - 0.6011)) \).
4. Solve for the mass of Ga-71 by isolating it on one side of the equation. First, calculate the contribution of Ga-69 to the atomic mass, then rearrange the equation to solve for the mass of Ga-71.
5. Perform the necessary arithmetic operations to find the mass of Ga-71, ensuring that all calculations are done with appropriate significant figures.