Question

3. There are 2 questions about isotopes.a) If an element has three isotopes with the following characteristics, what is theaverage atomic mass of the element?Isotope #1 is 10.0% abundant and has a mass of 23.8 amu. Isotope #2 is40.0% abundant and has a mass of 24.6 amu. Isotope # 3 has a mass of25.8 amu.b) An isotope has a half life of 5 minutes. How much of a 100 gram sample ofthis isotope is left after 20 minutes?

Answer 1

Question a

Step 1

The average atomic mass of the element is calculated as:

`Average\text{ atomic mass =}\frac{\sum_^(eachmass\text{ }x\text{ }\%abundance\rparen}{100}`

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Step 2

Information provided:

Isotope 1:

10.0 % abundant and 23.8 amu.

Isotope 2:

40.0 % abundant and 24.6 amu.

Isotope 3:

25.8 amu, the % abundant = 100 % - 10.0 % - 40.0 % = 50.0 %

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Step 3

Procedure:

Average atom mass = (10.0% x 23.8 amu + 40.0% x 24.6 amu + 50.0 % x 25.8 amu)/100 = 25.12 amu

Answer: 25.12 amu