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Naturally occurring europium (Eu) consists of two isotopes with a mass of 151 and 153. 151/63 Eu has an abundance of 48.03% and 153/63 Eu has an abundance of 51.97%. What is the atomic mass of europium?
The of europium is 152 u.
Method 1
Assume that you have 10 000 atoms of Eu.
Then you have 4803 atoms of ##""_63^151"Eu"## and 5197 atoms of ##""_63^153"Eu"##.
Mass of ##""_63^151"Eu"## = 4803 × 151 u = 725 253 u
Mass of ##""_63^153"Eu"## = 5197 × 153 u = 795 141 u
Mass of 10 000 atoms = 1 520 394 u
Average mass = ##(1520394"u")/10000## = 152 u
Method 2
##""_63^151"Eu"##: 48.03 % × 151 u = 72.5253 u
##""_63^153"Eu"##: 51.97 % × 153 u = 79.5141 u
Total = "152.0394 u" = 152 u
Note: In both methods, the answer can have only three , because that is all you gave for masses of the two .
Hope this helps.