Question

Lithium has two isotopes.

Li-6 and Li-7.
Li-6 mass = 6.015121amu
Li-7 mass = 7.016003amu
Lithium Mass = 6.94049amu

What is percent abundance of Li-6?

I know that there is an equation:
6.9409 = (% Abundance Li-6 * Weight of Li-6) + (% Abundance Li-7 * Weight of Li-7)

I don't know how to use it.

Answer 1

Answer: This is in the assumption that these are the only 2 isotopes. This means that:

abundance of Li-6 = X

abundance of Li-7 = 1-X

Hence from your equations

6.9409 = (X * 6.015121amu) + ((1-X) * 7.016003amu)

Solve for X = 0.075 or 7.50%

Step-by-step explanation:

Answer 2

This is in the assumption that these are the only 2 isotopes. This means that:
abundance of Li-6 = X
abundance of Li-7 = 1-X

Hence from your equations
6.9409 = (X * 6.015121amu) + ((1-X) * 7.016003amu)
Solve for X = 0.075 or 7.50%