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

127.0665 amu

Explanation:

Firstly, to answer the question correctly, we need to access the percentage compositions of the iodine and the contaminant iodine. We can do this by placing their individual masses over the total and multiplying by 100%.

We do this as follows. Since the mass of the contaminant iodine is 1.00070g, the mass of the 129I in that particular sample will be 12.3849 - 1.00070 = 11.3842g

The percentage abundances is as follows:

Synthetic radioisotope % = 1.0007/12.3849 * 100% = 8.1%

Since there are only two constituents, the percentage abundance of the 129I would be 100 - 8.1 = 91.9%

Now, we can use these percentages to get the apparent atomic mass. We get this by multiplying the percentage abundance's by the atomic masses of both and adding together.

That is:

[8.1/100 * 128.9050] + [91.9/100 * 126.9045] = 10.441305 + 116.6252355 = 127.0665 amu