Question

Two isotopes of lithium are found in nature Li6 has a mass of 6. 02u and Li7 has a mass of 7.02u . Use the atomic weight of lithium found on the periodic table to determine which isotope is more abundance

Answer 1

The isotope that is more abundant, given the data is isotope Li7

Assumption

• Let Li6 be isotope A
• Let Li7 be isotope B

How to determine whiche isotope is more abundant

• Molar mass of isotope A (Li6) = 6.02 u
• Molar mass of isotope B (Li7) = 7.02 u
• Atomic mass of lithium = 6.94 u
• Abundance of A = A%
• Abundance of B = (100 - A)%

Atomic mass = [(mass of A × A%) / 100] + [(mass of B × B%) / 100]

6.94 = [(6.02 × A%) / 100] + [(7.02 × (100 - A)) / 100]

6.94 = [6.02A% / 100] + [702 - 7.02A% / 100]

6.94 = [6.02A% + 702 - 7.02A%] / 100

Cross multiply

6.02A% + 702 - 7.02A% = 6.94 × 100

6.02A% + 702 - 7.02A% = 694

Collect like terms

6.02A% - 7.02A% = 694 - 702

-A% = -8

A% = 8%

Thus,

Abundance of B = (100 - A)%

Abundance of B = (100 - 8)%

Abundance of B = 92%

SUMMARY

• Abundance of A (Li6) = 8%
• Abundance of B (Li7) = 92%

From the above, isotope Li7 is more abundant.

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