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Worksheet 10: Atomic Mass . Topic 3

Set A: Calculating atomic mass Objective: To test your ability to calculate atomic mass of an element
from relative abundance and mass numbers of its isotopes

Calculate the atomic mass for each element given the relative percentages and the mass numbers of
the isotopes. Show ALL set up and work.

Need help? Study Book pg 38 – 40 Set 12 - 17
10
1) 19.78 % of B

80.22% of B
11

2) 93.12 % of K
39
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6.88 % of K
41

3) 78.70 % of Mg
24

10.13 % of Mg
25

11.17 % of Mg
26

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4) 80.0% of X
70

12.25% of X
69

7.75% of X
68

5) A sample of chlorine contains 75% of chlorine-35 and 25% of chlorine-37. What is the atomic mass of
chlorine? show work.

6) Element X has two naturally occurring isotopes. If 72% of the atoms have a mass of 85 amu
and 28% of the atoms have a mass of 87 amu, what is the atomic mass of element X. Show work.

7) The atomic mass of an element is dependent upon what two factors?

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