I don't know if this is even the right forum for this question, but perhaps someone can steer me in a better direction if it isn't.

I did some experiments on duckweed, culturing them in a standard DynaGro bag culture solution of 1 tsp/gallon (I actually used 1.25 mL/L, which is virtually the same). Turns out I need to convert this value to its molar equivalent (e.g., millimoles per liter), and although I've gone round and round about this, I can't seem to crack it. N has a molecular weight of 14 g, so a 1 mM solution should contain 0.014 g N. I get that. DynaGro is listed at 7% N, so the 1.25 mL in that L should contain, um, 0.0875 mL N? How do I make the jump from volume to mass?

Thanks in advance for any insight on this!

Cheers,

Alan

Thanks, but let me just doublecheck this. The 7% is by mass, not volume, but the dilution (1.25 mL/L) is by volume, not mass. If the DynaGro was plain water, it wouldn't matter (1 mL water having a mass of 1 g and all), but I presume that a concentrated fertilizer solution would be denser than water. Is that right, or is the difference too slight to matter?

Cheers!

In a solution, percent could mean either mass per unit volume or mass per unit mass (or even volume per unit volume in the case of two liquids). Often, there is no indication as to which one is meant.

If you assume the percentage was calculated on a mass/volume basis, you don't have to worry about the density of the solution. As George said, it should contain 7 g N / 100 mL. So your 1.25 mL will contain 0.0875 g N, which translates to 6.25 mmol of N per liter of final solution.

If the percentage was calculated on a mass/mass basis, you will need to know the density of the solution so you can get the mass of your 1.25 mL - at least to be accurate. You could always measure the Dyna Gro with a hydrometer if you really needed its density.

If it was me, I wouldn't worry about the second possibility, because they probably did use the first method, and you wouldn't be off by a whole lot even if they didn't.

ThinMan