Answer
$ 3.1\ g\ U $
Work Step by Step
1.Use Avogadro's number and the molar mass of uranium as conversion factors in order to determine the number of grams of uranium.
2. $7.9\times10^{21}\ atoms\ U\times\frac{1\ mol\ U}{6.022\times 10^{23}\ atoms\ U}\times\frac{238.03\ g \ U}{1\ mol\ U} = 3.1\ g\ U $
