Question

Lead (Pb) exists as four naturally occuring isotopes: 204Pb, 206Pb, 207Pb, and 208Pb. Using the data...

Lead (Pb) exists as four naturally occuring isotopes: 204Pb, 206Pb, 207Pb, and 208Pb. Using the data from the table below, calculate the atomic mass of 208Pb.

Isotope Atomic Mass (amu) Natural Abundance (%)
204Pb 203.973 1.4
206Pb 205.974 24.1
207Pb 206.976 22.1
208Pb ? ?
Average Atomic Mass 207.2 amu 100
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Answer #1

Given:

m1 = 203.973 amu

A1 = 1.4 %

m2 = 205.974 amu

A2 = 24.1 %

m3 = 206.976 amu

A3 = 22.1 %

m4 = 206.976 amu

A4 = 100 - A1 - A2 - A3

= 100 - 1.4 - 24.1 - 22.1

= 52.4 %

A = 207.2 amu

use:

atomic mass = sum of (mass of isotope * abundance) / 100

A = (m1*A1 + m2*A2 + m3*A3 + m4*A4)/100

207.2 = (203.973*1.4 + 205.974*24.1 + 206.976*22.1 + m4* 52.4)/100

207.2*100 = (203.973*1.4 + 205.974*24.1 + 206.976*22.1 + m4* 52.4)

20720 = (9823.70 + m4* 52.4)

m4* 52.4 = 10896.3

m4 = 10896.3 / 52.4

m4 = 207.945 amu

Answer: 207.945 amu

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