WEIGHTED AVERAGE MASS NUMBER:
1)The mass number on the periodic table is calculated
from the weighted average of the most abundant isotopes.
2) Abundance is presented as a percentage, however it
is expressed as a decimal in any calculations.
3) The most abundant isotope will have a mass number
closest to the weighted mass number average.
EXAMPLE FOR ELEMENT X, 30%(0.30) OF THE ATOMS ARE
X-40, 20%(0.20) ARE X-45 AND 50%(0.50) ARE X-47. NOTE
X-40 IS USUSALLY REPRESENTED AS 40X.
INTERPRETATION:
30 OUT OF 100 ATOMS ARE X-40
20 OUT OF 100 ATOMS ARE X-45
50 OUT OF 100 ATOMS ARE X-47
SKILL #2)--- WEIGHTED AVERAGE MASS NUMBER
CALCULATION.
1) PERCENTAGE ABUNDANCES TO DECIMALS CONVERT
ALL THE.
2) MULTIPLY THE DECIMAL ABUNDANCE BY THE MASS
NUMBER OF THAT ISOTOPE TO GET THE WEIGHTED
SUBTOTAL.
3) ADD THE WEIGHTED SUBTOTALS TO GET THE
WEIGHTED AVERAGE MASS NUMBER.
EXAMPLE)--- WEIGHTED AVERAGE MASS NUMBER CALCULATION.
CALCULATE THE MASS NUMBER OF ELEMENT X WHERE,: 30%(0.30) OF THE
ATOMS ARE X-40, 20%(0.20) ARE X-45 AND 50%(0.50) ARE X-47
1) CONVERT ALL THE PERCENTAGE ABUNDANCES TO DECIMALS.
20%/100 = 0.20 30%/100=0.30 50%/100=0.50
.
EXAMPLE)--- WEIGHTED AVERAGE MASS NUMBER CALCULATION.
CALCULATE THE MASS NUMBER OF ELEMENT X WHERE,: 30%(0.30) OF THE
ATOMS ARE X-40, 20%(0.20) ARE X-45 AND 50%(0.50) ARE X-47
1) CONVERT ALL THE PERCENTAGE ABUNDANCES TO DECIMALS.
20%/100 = 0.20
30%/100=0.30 50%/100=0.50
2)MULTIPLY THE DECIMAL ABUNDANCE BY THE MASS NUMBER OF THAT
ISOTOPE TO GET THE WEIGHTED SUBTOTAL
ISOTOPE
MASS # OF
ISOTOPE
(amu)
FRACTIONAL SUBTOTAL
ABUNDANCE
MASS(amu)
X-40
40
x
0.30 =
12.0
X-45
45
x
0.20 =
9.0
X-47
47
x
0.50 =
23.5
ISOTOPE
MASS # OF
ISOTOPE
(amu)
FRACTIONAL SUBTOTAL
ABUNDANCE
MASS(amu)
X-40 (40X)
40
x
0.30 =
12.0
X-45 (45X)
45
x
0.20 =
9.0
X-47 (47X)
47
x
0.50 =
23.5
+
ADD THESE TO GET
THE MASS AVERAGE
WEIGHTED AVERAGE
AS ON PERIDIC
TABLE
44.50 AMU