The question asks you to determine what percent of the total mass of the compound sulfur dioxide (`~SO_2` ) is due to the mass of the S atom.

In order to determine the percent by mass of S in the compound `~SO_2` , we will need to divide the mass of the S atom by the mass of the entire compound, and then multiply our answer by 100 to put it into percent.

Step 1: Determine the atomic mass of each element from the periodic table.

• Mass of S is 32.065 g

• Mass of O is 15.9994 g

Step 2: Determine the mass of the entire compound. (`~SO_2` )

• Mass of `~SO_2` = (mass of S) + (2 x mass of O):

            (32.065 g) + (2 x 15.9994 g) = 64.064 g

    *Notice that we multiplied the mass of O by two, since there are two O atoms in the compound.

Step 3: Determine the percent by mass of S in `~SO_2` .

• % Mass of S in `~SO_2` = (mass of S/mass of `~SO_2` ) x 100

            (32.065 g/64.064 g) x 100 = 50.052 %

       So, 50.052 % of the mass of `~SO_2` comes from the S atom.

We can also determine the percent of O by mass in `~SO_2` .

• % Mass of O in `~SO_2`= (2 x mass of O/mass of `~SO_2`) x 100

              (2 x 15.9994 g/64.064 g) x 100 = 49.948 %

Notice that when you add together the percent by mass of the O atoms and the percent by mass of the S atom, your answer should equal 100%.

• 50.052 % + 49.948 % =100 %