Re: How do you convert between uM & g/lit?

While Molarity (M) is a measure of concentration like grams per liter (g/l), they are not equivalent, and converting from one to the other requires knowing the formula weight (F.W., or more correctly the molecular mass) of each compound. Here's how the units work: a gram, as you know, is an amount of mass; while a mole is an amount of molecules (6.022 x 10²³ molecules to be exact) of a compound. Since molecules have mass, the mass of a mole is the sum of the masses of the molecules. Thus, the mass of a mole of a small molecule, like water (18 grams per mole), is much smaller than the mass of a mole of large molecules, like proteins (up to 500,000 grams per mole). This conversion (grams per mole) is the Formula Weight. Since Molarity is moles per liter, the conversion is simply:

M (moles/l) x F.W. (g/mole) = grams per liter (g/l)

So how, you may ask, does one find the Formula Weight of a compound? Well, there are two ways to do this. The hard way, which is taught in Chemistry, is to break the compound into its components and add the atomic masses together:

PO₄⁺³ (phosphate) = 1(P) + 4(O) = 1(31) + 4(16) = 95 g/mole

(Atomic masses can be found on the periodic table.)
The easy way is to look up the MSDS (Material Safety Data Sheet) for the compound, which should give the Formula Weight directly. Unfortunately, ions, like phosphate and nitrate, are not commercially available as such (though their salts and free acids are), thus they don't have MSDS's so you may be stuck with getting the Formula Weights the hard way.

So getting back to the equation: 1 µM PO₄⁺³ (µ, "micro" stands for 1 millionth, or 10^-6) x 95 g/mole (F.W.) = 95 µg/liter.

So now working back the other way, my question to you is, "What is the Molarity of Water?"