The of europium is 152 u.
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
##””_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.
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