- An oxide is known to have the formula X 2 O 7 and to contain 76.8% X by mass. Which of
the following would you use to determine the atomic mass of X?
A.
...16 023 2
2776 8
ccmm#B.
...23 216 0
7276 8
ccmm#C.
...16 023 2
7276 8
ccmm#D.
...23 216 0
2776 8
ccmm#E....76 823 216 0
2
ccmm#^7Answer: C
From the information provided, you know that the oxide X 2 O 7 contains 76.8% X and 23.2%
oxygen by weight. If you had 100.0 g of the oxide, you would have 76.8 g of X and 23.2 g of O
(or 23.2/16.0 moles of O atoms). Because for each mole of O in the oxide you have 2/7 mole of
X, you have in effect (23.2/16.0) ×2/7 mole of X. Since the units of atomic mass are g/mole,
the setup is:
...16 023 2
7276 8
ccmm#Finding the solution is unnecessary for selecting an answer; however, here is the solution for
your information.
./..
/
g O mole OgO
mole Omole XgX
g mol
16 023 2
7276 8
186
#=
e o e o(The element is rhenium.)
Gravimetrics