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 %