chemistry
posted by sara .
A 375 g sample of well water is found to contain 17.0 mg of lead. How much lead (in parts per million) is in the well water? Round to the nearest tenth. Don't forget the unit.

1 ppm = 1 part/1,00,000 parts.
So you convert 17 mg/375g to g/10^6 g.
17 mg = 0.017 g
The easiest way to do this I think is to multiply numerator and denominator by some numbers such that 375 becomes 1,000,00. That's asking 375g x ? = 1,000,000g and calculate ? as 1,000,000/375 = 2,666.7
Now 0.017g x 2,666.7 = approximately 45.
Another way of doing it, and it may be a little clearer this way, is
17 mg/375g = 0.017g/375g = 4.53E5gPb/1 gH2O. Multiply both numerator and denominator by 1E6 to convert to g/1,000g or ppm. Then 4.53E5*1E6/1E6 = 45.3 ppm.