# Question #f833d

##### 1 Answer

#### Explanation:

All you have to do to solve this problem is use the fact that the atomic mass of *uranium* depends on the isotopic masses of its isotopes and on their respective *abundances*.

#"atomic mass" = sum_(i) "isotope mass"_i * "abundance"_i#

You know that you're dealing with three isotopes, *uranium-234*, *uranium-235*, and *uranium-238*, for which you know

#""^234"U" -> "234.041 u", 0.0057%#

#""^235"U" -> x, 0.72%#

#""^238"U" -> "238.051 u", 99.27%#

This means that you an set up your equation like this - I'll use *decimal abundances*, which are simply percent abundances divided by

#0.000057 * 234.041 + 0.0072 * x + 0.9927 * 238.051 = 238.019#

All you have to do now is solve for

#x = (238.019 - 0.0133403 - 236.313)/0.0072#

#x = 1.692658/0.0072 = color(green)("235.091 u")#