Partial pressure & number of molecules of ozone per liter of air?

Question

an explaination of the answer is also needed please.

On a smoggy day in a certain city the ozone concentration was 0.42 ppm by volume. Calculate the partial pressure of ozone (in atm) and the number of ozone molecules per liter of air if the temperature and pressure were 20.0°C and 748 mmHg, respectively.

Answer 1

0.42 ppm  leads to the partial pressure , which will have that same ratio:

748Torr @ 0.42 / 1,000,000 =  3.14 e-4 mm of Hg

your first answer: O3's partial pressure = 3.1e-4mm of Hg

(I reduced the answer to 2 sigfigs , since that is all that 0.42ppm provides)

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find moles in 1 litre:

PV=nRT

(3.14e-4mm )(1 Litre) = n (62.36 Torr-Litre/mol-K)(293K)

1.72e-8 moles of ozone per litre

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1.72e-8 moles @ 6.022 e23 molecules / mol = 1.035 e16 molecules per litre

you answer(2 sig figs): 1.0e16 molecules of O3 per litre of air