Question #010b9

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: `CO_2`.

One "mole" of `CO_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 `CO_2`. The TOTAL moles of atoms in `CO_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 " for " C and 16 " for " O`.

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

Thus, the mass of a mole of `CO_2` is `44` and the mass percentages are:
`O = 32/44 = 0.727" mass"% O`
and
`C = 12/44 = 0.273" mass"% C`