We use the Raoult's Law to answer this question.
Partial Pressure = Mole Fraction*Total Pressure
When concentration is expressed in ppm, it means 'parts per million'. So, there can be 0.67 mol of ozone per 10⁶ mol of mixture.
Mole fraction = 0.67/10⁶ = 0.67×10⁻⁶
Partial pressure = 0.67×10⁻⁶(735 torr) = 4.92×10⁻⁴ torr
