# Question 62eaf

Feb 22, 2017

You cannot “convert” a percentage without a reference to the whole volume or mass.

#### Explanation:

A “percentage” of anything is just a ratio of something to the whole. That may be a mass (weight) or volume. A mole is a specific number of atoms or molecules. It can be derived from a complete description of elements and environment. Given a specific mass or a volume at a specific temperature and pressure, the moles of an element can be calculated. They cannot be calculated from only a “percentage” unless you also know WHAT it is a percentage OF.

For example, a one-liter (I’m defining a particular volume) container at STP (I’m defining the temperature and pressure) contains 19.8% oxygen. I don’t care what the other 81.2% is, as long as I know the conditions and volume of the container. This means that out of a total volume of 1.0L, I would have 0.198 * 1.0 = 0.198L of oxygen molecules. Using the ideal gas laws, we can calculate the number of moles from the conditions and this volume:
$n = \frac{P \cdot V}{R \cdot T}$

$n = \frac{1 \cdot 0.198}{0.082 \cdot 273} = 8.85 x {10}^{-} 3$`moles

If you are talking about a compound which contains oxygen and one other element (such as iron), here is what you would do.

#### Explanation:

Oxygen = 19.8%
Iron = 100 - 19.8 = 80.2%

Assume a 100g sample which would mean you have

19.8g oxygen

80.2g iron

Convert the mass of oxygen to moles by dividing by 15.999 (molar mass of O)

Convert the mass of iron to moles by dividing by 55.845 (molar mass of Fe)

This would allow you to determine the mole ratio of a compound.

Here is a similar problem...

Hope this helps!