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    The nitrogen dioxide content of air on a particular day was 0.150 ppm by mass.
    The density of the air was 1.225kgm−3.
    What was the mass of nitrogen dioxide in 1m3
    of air on that day?

    A 1.83 × 10−7 kg
    B 1.83 × 10−10 kg
    C 1.84 × 10−7 kg
    D 1.84 × 10−10 kg

    answer is C

    A density of 1.225 kg m-3 means that a cubic metre has mass 1.225 kg.

    Given that 0.150 mass units in a million are NO2, you can set up a proportion and solve for the missing term:
    0.150 : 106 = x : 1.225 kg
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