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 the gaseous mixture of helium and oxygen, we will follow these steps:

### Step 1: Convert the given pressure to atm
The pressure is given as 720 torr. We need to convert this to atm using the conversion factor \(1 \text{ atm} = 760 \text{ torr}\).

\[ P = \frac{720 \text{ torr}}{760 \text{ torr/atm}} = 0.9474 \text{ atm} \]
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