Discussion
A reference solution is to be prepared with 5 PPM parabenzopyrene (molecular weight = 252). The quantity of parabenzopyrene required for preparing one litre 5 PPM solution is
Answer: C
A concentration of 1 ppm in dilute aqueous solution is 1 mg of solute per litre, since a litre of water weighs about one million milligrams. A 5 ppm solution therefore needs 5 mg of parabenzopyrene in one litre. The molecular weight of 252 is not needed, because the concentration is a mass ratio and not a molar one. A mass of 252 mg would be one millimole, 25.2 mg is a tenth of that, and 252 micrograms is far too small, so 5 mg is the only correct quantity.
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