[threedaysjane]

Hi everyone,
It's been a long time since I've done these calculations so I'm feeling rusty and have now confused myself.  I prepare formulations in molar concentrations and typically just use a molarity calculator to determine appropriate weight/volume based on our SOPs.  I noticed that previous grad students have multiple salt forms or the free acid/base for some of the compounds.  If I want a 100 nM solution, for example, and have multiple salt forms, it's my inclination that I should adjust for the salt form (and probably water of hydration) and express as free acid/base. 

Would anyone be willing to share their thoughts on this and give a quick how-to? My first inclination was a mole fraction calculation but that doesn't seem quite right. 

Many thanks!

[Arkcon]

It seems like you might have the right idea, but its hard to tell.  Maybe you can give us an example of "multiple salt forms" for your particular application, and show us how you'd do it -- or perhaps show us possible competing methods, so we can see the problem.

[threedaysjane]

For example:  kynurenic acid (C10H7NO3) is available both as a free acid (m.w. = 189.17) and as a sodium salt (C10H6NNaO3 * 1 1/2 H20;  m.w. = 238.17). 
For the free acid, 1.8917  milligrams to 10 mL would be 1 millimolar.   I'm getting stuck at the next point, e.g., if I want to use the sodium salt form instead.  If I want to ensure that that 1 millimolar solutions prepared using either the freeacid or salt form are equivalent, what is the best way to do that?  A ratio of KYNA to KYNA*NaCl is 0.794.   Therefore, can I apply that as a correction factor? 

1.8917 * 0.794 = 1.502.    Is 1.502 mg kynurenic acid sodium salt equivalent to 1.8917 mg kynurenic acid?

The math doesn't seem quite right:
Using the same example as above:  1.8917 mg kynurenic acid = 0.35785 moles
versus: 1.502 * 0.23817 = 0.3577 moles, so I'm not sure about my reasoning.   

[Borek]

A ratio of KYNA to KYNA*NaCl is 0.794

No idea what KYNA*NaCl is intended to be. But the number makes sense.

1.8917 * 0.794 = 1.502.    Is 1.502 mg kynurenic acid sodium salt equivalent to 1.8917 mg kynurenic acid?

No, you got it reversed. Sodium salt has a higher m.w. so for equivalent amount you need more, not less.

Using the same example as above:  1.8917 mg kynurenic acid = 0.35785 moles

No. No idea where you got these numbers from. Molar mass is 189.17 g/mol, so 189.17 g is 1 mole, while 1.8917 mg is exactly 10000 times less than that.

versus: 1.502 * 0.23817 = 0.3577 moles, so I'm not sure about my reasoning.   

No. Have you checked your units? I guess you are multiplying mass by molar mass, so you end with mass²/mole, not moles.