Question

Review Constants Periodic Table Part A Atomic mass is a weighted average of the masses of the naturally occurring isotopes of that element. For example, consider isotopes of lithium: Abundance Mass Isotope (%) (amu) Li 7.5 6.015 On a mission to a newly discovered planet, an astronaut finds copper abundances of 69.15 % for Cu and 30.85 % for 65Cu. What is the atomic mass of copper for this location? The mass of Cu is 62.9300 amu . The mass of 65Cu is 64.9200 amu. Express your answer to two decimal places, and include the appropriate units. View Available Hint(s) 7 Li 92.5 7.016 The atomic mass is therefore (0.075 x 6.015) + (0.925 x 7.016) = 6.94 amu Notice that the atomic mass is closer to 7 than it is to 6. That is because the heavier isotope is more abundant. HA ? Value Units Submit Part B The astronaut then measures the abundance of silicon on the new planet, obtaining the following results: Isotope Abundance Mass (%) (amu) 28 Si 73.71 27.98 29 Si 14.93 28.98 30 Si 11.36 29.97 What is the atomic mass of silicon for this planet? Express your answer to two decimal places, and include the appropriate units. View Available Hint(s) HA ?

Answer 1

Part A:
Isotopes are atoms of the same element, having different atomic masses.
Average atomic mass of the iostopes = weighted average of the mass of all the isotopes
Average mass of the copper discovered = 69.15% of 63 amu + 30.85% of 65 amu
Average mass of the copper discovered = {(69.15/100) x 63 amu} + {(30.85/100) x 65 amu}
= 43.5645 + 20.0525 = 63.62 amu
NOTE: The average atomic mass value is closer to the atomic mass of the more abundant isotope.

Part B:
Average mass of the silicon discovered = 73.71% of 27.98 amu + 14.93% of 28.98 amu + 11.36% of 29.97 amu
Average mass of the silicon discovered = {(73.71/100) x 27.98 amu} + {(14.93/100) x 28.98 amu} + {(11.36/100) x 29.79 amu}
Average mass of the silicon discovered = 20.624058 + 4.326714 + 3.384144 = 28.33 amu