1 Rubidium has two common isotopes 85 Rb and 87 Rb If the abundance of 85 Rb is 722 and the abundance of 87 Rb is 278 what is the average atomic mass of rubidium PROCEDURE ID: 751570
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Slide1
Average atomic mass worksheet
#1: Rubidium has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?
PROCEDURE: 1. Convert the 1st isotope’s percentage to a decimal (move decimal 2 places to the left)
72.2% → .722;
2. Multiply the decimal by the mass # for the isotope
.722 x 85 = 61.37
3. Write the product down; this is a SUB-TOTAL
4. Repeat steps 1-3 for each of the remaining isotopes listed, recording all SUB-TOTALS
27.8% → .278;
.278 x 87 = 24.186
5. Add your sub-totals together, and round to the lowest decimal place of the 2 subtotals - THIS IS THE ATOMIC MASS!
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85.56Slide2
Average atomic mass worksheet
#2: Uranium has three common isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is 0.71% and the abundance of 238U is 99.28%, what is the average atomic mass of Uranium?
1. Convert the 1st isotope’s percentage to a decimal (move decimal 2 places to the left)
0.01% → .0001;2. Multiply the decimal by the mass # for the isotope
.0001
x 234 = .02343. Write the product down; this is a
SUB-TOTAL4. Repeat steps 1-3 for each of the remaining isotopes listed, recording all
SUB-TOTALS
.71% → .0071;
.0071 x 235 = 1.6685 5. Add your sub-totals together, and round to the lowest decimal place of the 2 subtotals -
THIS IS THE ATOMIC MASS!
+
=237.9783
99.28% → .9928;
.9928
x 238 =
236.2864
Slide3
USE YOUR “READING THE PERIODIC TABLE: ….” NOTES – BOTTOM OF FRONT SIDE OF NOTEPAGE-IF YOU NEED GUIDANCE WITH THE MATH: