# Question 010b9

Dec 24, 2017

Convert the moles to the mass of the element.

#### Explanation:

Example:

What is the molar percentage and mass percentage of each element in carbon dioxide?

Carbon dioxide has two oxygen atoms combined with one carbon atom: $C {O}_{2}$.

One "mole" of $C {O}_{2}$ thus contains one "mole" of carbon, $C$, and one "mole" of oxygen, ${O}_{2}$, or TWO "moles" of $O$ (it's important to know how you are defining your elements).

Let's use the ATOMIC definition for this example, so we have two "moles" of $O$ in each mole of $C {O}_{2}$. The TOTAL moles of atoms in $C {O}_{2}$ is then three (3).

So then, our "molar percentages" of $O$ and $C$ are:
O = (2 " mole " O)/(3 " moles") = 0.667 " mole"% O
and
C = (1 " mole " C)/(3 " moles") = 0.333" mole"% C

To find the mass percentage we need to multiply a mole unit by the molecular weight. This is $12 \text{ for " C and 16 " for } O$.

Again, we have 2 moles of "O", so it will have a mass of $2 \times 16 = 32$
The carbon will have a mass of $12$ because there is only one mole.

Thus, the mass of a mole of $C {O}_{2}$ is $44$ and the mass percentages are:
O = 32/44 = 0.727" mass"% O
and
C = 12/44 = 0.273" mass"% C#