18-BULTO ty College-CHEM 155-Spring18-BULLOCK > Activities and Due Dates > Sapli

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18-BULTO ty College-CHEM 155-Spring18-BULLOCK > Activities and Due Dates > Sapling Assignment #2 ) 2/14/2018 11:55 PM 7.8/102/13/2018 04:38 PM Gradebool Print ill Calculator Periodic Table uestion 5 of 10 General Chemistry 4th Edition University Science Books presented by Sapling Learning Gallium is a metal which forms compounds with a wide variety of uses. Some of the applications of gallium compounds include computer memory chips, light emitting diodes and lasers. Radioactive isotopes of gallium are used to image the human body and locate tumors. Naturally oocurring gallium consists of two isotopes. One of those isotopes is Ga with an isotopic mass of amu and an abundance of 39.892%, what is the mass number of the other isotope? Number Preveus check Answer Next tl Ext about us | careersprivacy policy terms of use contact us

Explanation / Answer

The idea is

Relative atomic mass = (relative abundance x isotopic mass) over all the naturally occuring isotopes.

So..
(1) Look on a periodic table to get relative atomic mass.
(2) Setup the equation and solve for isotopic mass of the other isotope.
(3) Estimate mass number from that isotopic mass. For example, 70.9247050 amu probably has mass number = 71.

69.72 = (0.39892 x 70.9247050) + [ ( 1 - 0.39892 ) x Z ]
or, 69.72 = 28.2932833 + [ ( 0.60108 ) x Z ]
or, ( 0.60108 ) x Z = 41.4267167
or, Z = 41.4267167 / 0.60108
or, Z = 68.920471

So we'll guess the other isotope is 69Ga.

Answer is 69.

Now, "Ga-69 and Ga-71" make up the isotope in the periodic table as Ga-69.723 amu." is not correct. The naturally occuring isotopes of Ga are Ga-69 and Ga-71. But they don't combine to make "1 isotope as shown on a periodic table". They make a mixture.

If someone were to take a take a sample of 100,000 atoms of naturally occuring Ga and count how many were Ga-69, they would find count 60108. If they counted the number of Ga-71 atoms, they would count 39892. They would never find any Ga-69.723 atoms. The mass of those 100,000 atoms would be 69.723x10^5 amu.

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