It would be nice to have the density and to know the volume of water. Assuming 45.1 ug is so small and that the 250 mL of solution has a mass of 250 g, then % w/w = (mass solute/total mass)*100 = (45.1E-6g/250 g)*100.
Remember that percent w/w is parts per 100 (that's parts per 10^2). Multiply by 10,000(that's 10^4) to get ppm or by 10.000,000 (that's 10^7) to get ppb.
A quickie way I convert to ppm is 1 ppm = 1 mg/L which is a % w/v. That would be 45.1E-6 g or 0.0451 mg/250 mL = ? ppm
For dilute solutions ppm w/v and ppm w/w are essentially the same.
A 250.0 - ml aqueous solution contains 45.1 µg of a pesticide . Express the pesticide's concentration in weight percent , parts per million , and parts per billion .
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