Average Atomic Mass s1

Average Atomic Mass

. can be calculated given data from a mass spectrophotometer.

. % or relative abundance and isotope masss are needed to find a weighted average

. The final result will have units of amu (atomic mass units).

Sample calculations

1. A sample of neon gas was injected into a spectrophotometer and the print out included three peaks at 20,21,and 22 amu. The relative areas of the peaks were: 0.9092, 0.00257, and 0.0882 repectively. Calculate the atomic mass of neon

2. Chlorine has two stable isotopes. The mass of one is 34.97 amu and its relative abundance is 75.53% What is the mass of the other stable isotope?

3. The element indium exists as 2 isotopes. 113 In has a mass of 112.9043 amu and 115In has a mass of 114.9041 amu. The average atomic mass of indium is 11In has a mass of 114.9041 amu. The average atomic mass of indium is 114.82 Calculate the relative abundance of the two isotopes of indium. Complete the following:

4. The element magnesium has three stable isotopes with the following masses and abundances:

Isotope Mass (amu) Abundance Magnesium-25 23.9850 78.99% Magnesium-25 24.9858 10.00% Magnesium-26 25.9826 11.01%

Calculate the average atomic mass of magnesium.

5. An unknown sample of mystery element “T” is injected into a mass spectrophotometer. 7.42% of the element is present as 6T and 92.58% as 7T. the mass value for 6T is 6.02 amu 7.02 amu for 7T. What is the average atomic mass and what is the identity of the element?

6. Silicon has three stable Isotopes. Fill in the missing information. Hint: find Si average atomic mass on the periodic table.

Isotope Mass (amu) %abundance Silicon-28 27.89 ? Silicon-29 ? 4.70% Silicon-30 29.97 3.09%

7. Gallium has two stable isotopes of masses 68.38 (69Ga ) and 70.92 amu (71Ga). What are the relative abundances of the two isotopes?