Cliffs AP Chemistry, 3rd Edition

First change all the grams to moles:

32.0 grams of O 2 = 1.00 mole

32.0 grams of CH 4 = 2.00 moles

32.0 grams of SO 2 = 0.500 mole

Mole fraction of oxygen gas:

Mole fraction of oxygen gas: 350.^1 mole Ototal moles^2 =0 286. mole fraction

7. Element X is found in two forms: 90.0% is an isotope that has a mass of 20.0, and 10.0%
   is an isotope that has a mass of 22.0. What is the atomic mass of element X?

A. 20.1

B. 20.2

C. 20.8

D. 21.2

E. 21.8

Answer: B

To solve this problem, multiply the percentage of each isotope by its atomic mass and add
those products.

(0.900 ×20.0) + (0.100 ×22.0) = 20.2 atomic mass of element X

8. What is the formula of a compound formed by combining 50. grams of element X
   (atomic weight = 100.) and 32 grams of oxygen gas?

A. XO 2

B. XO 4

C. X 4 O

D. X 2 O

E. XO

Answer: B

According to the information given, you have 0.50 mole of element X (50. g/100. g ⋅mole–1=

0.50 mole). For the oxygen, remember that you will use 16 g/mole for the atomic weight, giv-
ing you 2.0 moles of oxygen atoms. A 0.50:2.0 molar ratio is the same as a 1:4 molar ratio, so
the answer is XO 4.

Part II: Specific Topics