Four isotopes of Lead occur naturally:

204Pb (mass = 203.973037 amu)
206Pb (mass = 205.974455 amu)
207Pb (mass = 206.975885 amu)
208Pb (mass = 207.976641 amu)

If the percent abundance of 204Pb and 208Pb are 1.400 and 52.40, respectively, what is the percent abundance of 206Pb?

Pb average atomic mass = 207.2

1 answer

abundance 204Pb = 1.400%
abundance 208Pb = 52.40%
abundance 206Pb = Y (as a fraction)
abundance 207Pb = Y-1.400-52.40

0.14(203.973037) + Y(205.974455) + (Y-1.400-52.50)(206.975885) + 0.5240(207.976641) = 207.2
Solve for Y