Question

Naturally occurring copper is 69.15% copper-63 (atomic mass = 62.93 amu) and 30.85% of some other isotopic form of copper. What must the atomic mass of this second isotope be in order to account for the 63.55 amu average atomic mass of copper? (Write the answer to two decimal places.)

Answer 1

64.94 First, write out an expression about what we know 0.6915 * 62.93 + 0.3085 * x = 63.55 Now solve for x 0.6915 * 62.93 + 0.3085 * x = 63.55 43.5161 + 0.3085 * x = 63.55 0.3085 * x = 20.03391 x = 64.93972 Rounding the answer to 2 decimal places gives x = 64.94.