Question

Uranium is used as a fuel, primarily in the form of uranium(IV) oxide, in nuclear power plants. A small
sample of uranium metal (0.169 g) is heated to between 800 and 900 0C in air to give 0.199 g of a dark
green oxide, UxOy. (a) How many moles of uranium metal were used? (b) What is the empirical formula of
the oxide, UxOy?

Answer 1

mols U metal = grams/atomic mass = 0.169/238 = 0.00071
U + O2 ==> UxOy
mass O is UxOy = mass UxOy - mass U = 0.199 - 0.169 = 0.030 g
mols U metal from above = 0.000710
mol O = 0.030/16 = 0.00187
Now you want to find the mole ratio. The easy way to do this is to divide both numbers of the smaller number; i.e.,
mols U = 0.000710/0.000710 = 1.000
mols O = 0.00187/0.000710 = 2.63
Compounds don't have ratios that are not whole numbers so multiply by whole numbers until we find BOTH whole numbers. Multiply by 2 gives
U = 1.000 x 2 = 2.000
O = 2.634 x 2 = 5.27. Doesn't work so try 3.
U = 1.000 x 3 = 3.000
O = 2.634 x 3 = 7.902 which is close enough to round to 8 so the empirical formula is U3O8