Naturally occurring iodine has an atomic mass of 126.9045. A 12.3849-{\rm g} sample of iodine is accidentally contaminated with 1.00070 {\rm g} of ^{129}{\rm I}, a synthetic radioisotope of iodine used in the treatment of certain diseases of the thyroid gland. The mass of ^{129}{\rm I} is 128.9050 {\rm amu}.