Best Answer:
Summary: convert your concentration to a mole, mass, or volume fraction and multiply by 1 million.

Discussion:

Parts per million must be measured in consistent units:

mass per 1 million mass units

volume per 1 million volume units

moles per 1 million mole units

For example, if you add 1 milligram of substance to 1 kilogram of solution, the concentration is:

1 mg/1 kg = 0.001 g / 1000g = 1e-6

Multiply the resulting mass ratio by 1 million and you convert this to ppm

1e-6*1million = 1 ppm

I don't think it is correct to directly use molarity or molality times 1 million, since both of these are ammount/volume: the units are not consistent. You have to convert the "volume" in the concentration to a "mass" or "molar" amount consistent with the concentration measurement.

If you have a concentration in terms of a mole fraction, it is just the mole fraction times 1 million. A mole fraction of unity (one) is the same as 1 part per part (pure!).

If you have a concentration in terms of a volume fraction or mass fraction, you just multiply by a million. It is important to note that whether you do parts by mole, volume, or mass you will get different numbers for a given system. It is important to be clear which one you are talking about. You can read more on wikipedia, look up ppm.

Source(s):

Anonymous
· 1 decade ago