By Dr. Donald G. McGahan
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The molecular weight of a substance is the weight of 6.022 x 1023 molecules (1 mole). When the molecular weight is expressed in grams it is sometimes more explicitly called the gram molecular weight (1 gmol).
Molality is defined as the number of moles of an element or a compound, the solute, that is added per kg of solvent (not solution): molarity cannot be computed from molality without the density of the solvent.
Molarity is defined as the number of moles of an element, ion or compound per litter of solution.
Normality is defined as the number of equivalent weights of a substance per litter of solution. (Equivalent weight is the amount of a substance that will produce, neutralize, replace, or exchange for one mole (l g) of hydrogen ions).
Be aware that normality can be applied in a more broad way such as in medical applications where normality is the reactant that is not necessarily an equivalent as defined in the paragraph above. In science where soil is involved the formal definition of: Equivalent weight is the amount of a substance that will produce, neutralize, replace, or exchange for one mole (l g) of hydrogen ions is retained.
Parts per million (ppm)
ppm is a fraction expression on a million basis; 1 ppm represents 1 g of an element per million grams of solution or 1 lb of an element per million pounds of solution or soil. Since it can be solution, or soil, (a solution or solid) it is a depreciated term and scientists now work to avoid the term.
The most commonly used concentrations in soils work are molarity (M), normality (N), and ppm.