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Eric, 

Say one was to want to make struvite ( NH4MgPO4·6H2O) from humonia. If we start with the baseline premise that one litre of human urine contains 1.4 grams of phosphate salts, which should equate to 0.11 grams of pure phosphorous...

Could you show us how you'd go about figuring out how much magnesium to add to the humonia in order get the maximum amount of MgPO4 out of the deal?

M:P molar ratio 1:1.1

I'd like to have a sort of baseline numbers to work with before I start doing this, and see what pans out. As it stands, the plan is to add very small amounts of Mg and note when the precipitate stops forming, but was wondering if you could help offer something a bit more scientific.

Thanks.

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I'm going a bit overboard here and I'm researching the chemical composition of urine. I found this "inked-out" study: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710023044_19...

UPDATE:

I got lost in the article--very lost. 

So, I can tell you that if you had say,  (NH4)3 (PO4)2 and you wanted to turn it into MgPO4  Then the balanced equation would be    (NH4)3 (PO4)2 + 3Mg ---------> Mg3(PO4)2 + 3NH4(which will immediately bond with something--I think). So the set up would be (if I had my periodic table with me I'd do the calculations myself):


(x)grams of Mg X 1mol of MgX 1mol of Mg3(PO4)2 262.9g Mg3(PO4)2 

            1            24.3g of Mg         3mol of Mg              1molMg3(PO4)2


I found the molar mass using this  website (http://www.convertunits.com/from/grams+Mg3(PO4)2/to/moles). Type in 1 mole for the mole part.

Sorry, I'm used to working with pure chemicals--If you could narrow down the questions I'd be more than happy to give you the equations. 'Till then I'll be reading through that study. 

Yeah, things in everyday life are usually complexes, or complex...and rarely pure..... I imagine if we suppose for a minute we're talking about 0.11 grams of phosphorous it would be easy for you to calculate the amount of magnesium required. But that is probably not a practical way of looking at things given the 'impurity' of the materials at hand...

Basically, I was trying to avoid a Hennig Brandt type situation...but no worries as both the precipitate (struvite) as well as the effluent will be put to good use...

Man, I've only made it about a third of the way through that first link, and my head is reeling...

I try not to pay attention to the stuff I'm not looking for--I needed to take a break for a bit. I just hope AP Chemistry won't be this hard.

Vlad Jovanovic said:

Yeah, things in everyday life are usually complexes, or complex...and rarely pure..... I imagine if we suppose for a minute we're talking about 0.11 grams of phosphorous it would be easy for you to calculate the amount of magnesium required. But that is probably not a practical way of looking at things given the 'impurity' of the materials at hand...

Basically, I was trying to avoid a Hennig Brandt type situation...but no worries as both the precipitate (struvite) as well as the effluent will be put to good use...

Man, I've only made it about a third of the way through that first link, and my head is reeling...

if you are just comparing moles to moles, why cant you just use their atomic numbers as ratios? 12/15 = (1 * x)/(1.1 * 0.11g) , x = 0.0968g of Mg to match a Mg:P of 1:1.1 That would be 4.84g of MgS04(epsom salt) needed or 2.34g of MgCl

Chris, just for clarities sake, do you mean 4.84 grams of Epsom salt (MgSO4-7H2O), or 4.86 grams of magnesium sulfate (MgSO4)..? Since the % of actual magnesium by weight would be different. About 10%Mg in Epsom salt as opposed to about 20% for magnesium sulfate...I think the weight of the water alone in Epsom salt accounts for about 51%

(Yes, I am equating mass with weight, for my purposes...and realize I may be scolded for doing so :)...

A mole is 6.02X10^23 or 6.02E23 atoms. This equates to 12.011 grams per mole of Carbon, or 15.9994 grams per mole of Oxygen (to name the ones I've memorized). That's what the molar mass is: grams/mole. Chris: the atomic number is the amount of PROTONS  in an atom. The molar mass is the atomic weight. (Some guy, maybe girl, did the dimensional analysis to figure this stuff out.) Vlad, with the constant of gravity on our planet, weight and mass are essentially interchangeable--just factor in gravity when doing density. I was going to explain how to do chemistry math in this thread, but I think a special thread is needed.

@Eric. I understand how my cowboy chemistry is inaccurate and unscientific which is like nails on a chalkboard to you. My point was as inaccurate and variable as pee and other involved chemical impurities can be, rounding off the math to something as simple as a relative mass ratio using the atomic number, which is within about a thousandth of a gram to the actual molar mass at this scale and the inaccuracy is negligible.

For your example Carbon (6) / Oxygen (8) = 0.75, which is close to 12.011g/mol / 15.9994g/mol = 0.75071

Assuming your not dealing with isotopes and you have an equal number of protons and neutrons, the g/mol of an element will be 2x the atomic number. 

The whole point of this group/discussion is to understand the real math like you have demonstrated, I was just suggesting an alternative shorthand if you only were looking to quickly figure out "how many grams of salt per liter do i need for this pickle bucket full of pee?"  We already knew how much P in grams their is, and what the desired molar ratio needed to be.

@Vlad. Totally missed that water in the epson salt. My bad.

I guess that ball parking estimate doesn't work as well with larger elements past the basic 20. I still love that old 2-8-8-2 rule from high school chem.

Sorry, Chris, I'm really precise, bordering on obsessive; after Calcium things tend to get weird and the atomic weight is all the isotopes averaged into one precise pot. Therefore I like to keep all the precise things precise, so I don't blow myself up (it's sort of happened before). 

Makes sense I guess. I should be more careful too. I lost the majority of my hearing for 2 weeks due to a high school chemistry class explosion, but not at my fault :)

What were you exploding?

Chris said:

Makes sense I guess. I should be more careful too. I lost the majority of my hearing for 2 weeks due to a high school chemistry class explosion, but not at my fault

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