ttystikk, I think you only need to know the ppm conversion factor when you're talking about a commonly used ppm meter reading. We're calculating the hard ppm of elements in solution, there isn't really a correlation between the numbers we figure out and a ppm meter reading. For example, you can't come up with a profile of 150N, 50P, 200K etc., calculate the ppms of all elements, mix up the salts in a solution and say "well then my hanna meter should read whatever all those ppms add up to.". As you might already know the common ppm meters are only EC meters that use one of a couple conversion factors(sodium or NaCl and others) to display ppm and since different elements and molecules contribute differently to the overall EC, theres no real way to accurately convert between the two that I know of. There may be some crazy math you could do, but I don't know anything about it.
The molar mass of MgSO4 is about 120u (all and the elemental percentage breakdown is Mg=20.1922%, S=26.6395%, O=53.1683%. You really need to know the mass (weight) of MgSO4 and the exact volume of water you're going to dissolve it in to figure out ppm. If you put one gram of MgSO4 in one liter of water you will have 201.922ppm of Mg, 266.395ppm of S and 531.683ppm of O. Remember ppm in water is the same as mg/L. Also to tie in the above, if you did put one gram of MgSO4 in a liter of water your Hanna meter would not read 1000ppm but you would still have the listed elemental ppms in solution. And not to jumble things up too bad, maybe someone else can help clarify, but I think when you mix MgSO4 in water it picks up 7 H2O molecules to become the hydrate form of the salt. What I'm not sure about is whether or not to include those extra H and O atoms into the calculations or not and also if all salts have something like that when dissolved. I just assume that most other salts simply break into their respective elements upon solution. If someone could help clear that up or offer an other input I'd appreciate it.
Also if there's any chemists out there, I'm have a little trouble with some defintions. It seems like the term atomic mass takes into consideration different isotopes of elements and is a very exact, sample by sample measurement of something. Atomic weight on the other hand seems to be more of a "most commonly found" number and includes an average of most common isotopes. Do I have that right and is it important to keep them straight or are they used interchangably (even though they're not interchangable)? Also are g/mol and daltons(atomic mass units) somehow interchangable? When I was figuring out what I thought was the molecular mass of MgSO4, I found the atomic weights of the individual elements and did the math and it came out to about 120u. Then I checked on an online calculator and it gave me 120 g/mol, how are these two units related or what am I not understanding about their relationship?