A gasesous mixture of helium and oxygen is found to have a density of `0.518 g dm^(-3)` at `25 ^(@)C` and `720` torr. What is the precent by mass of helium in this mixture?

Explanation

To find the percent by mass of helium in a gaseous mixture of helium and oxygen, we can follow these steps:

### Step 1: Use the Density-Pressure-Temperature Relation
We start with the formula that relates density (d), pressure (P), temperature (T), and molar mass (M):
\[ PM = dRT \]
Where:
...