[botanydolly]

Hi All,

I am new to this forum and realatively new to chemistry! I have undertaken a project that involves the chemistry of trees and so alot of chemistry experiments. So far, doing ok but I have hit a little wall with understanding concentrations!

My confusion comes from trying to follow my predecessors work (not a chemist!) in her thesis and HPLC profile for separating the monosaccharides we are interested in. The protocol we use for the HPLC calls for 2 solutions (A and B) where A = 400mM NaOH and B = 400 mM NaOAC. Great, have them made up and ready to go. I run 50% A and 50 % B, get my separations and ready to present my results. My understanding of concentrations is that I therefore have a 400mM NaOH solution in a 400mM NaOAc solution as I am not diluting with water, just mixing 2 solutions... Is this correct?

My predecessors thesis states that the monosaccharides were separated by a 200mM NaOH solution in a 200mM NaOAc solution and when I asked about this she said that by mixing 50% of A with 50% of B the concentration of the solutions is halved due to dilution (no water used) and therefore they are separated by a 200mM NaOH solution in a 200mM NaOAc solution.

I am sure it is a simple thing to confuse so maybe I need a greater understanding of solutions, concentrations, mixing, etc. If anyone could please respond with the correct problem I would greatly appreciate it!

[AlphaScent]

Do you know what mM stands for and what its definition is?

[Arkcon]

If you had 100 mL of 400 mM solution, and you added 100 mL of water, what would you have?  Does that really change if instead of water, it is a 400 mM solution of something else?  That is basically the question at hand, right?

[AlphaScent]

Botany,

Look up the definition of molarity.  With that and what arkon said you will have your answer.

[botanydolly]

Hi AlphaScent and Arkcon,

Thank you for your input!

mM is millimolar and it is the amount of something in a volume (eg amount of my NaOH in 1L water)

I think I have it right in saying that the molarity of my 400mM solutions do not change because I am mixing them i.e. remains a 400mM solution and not a 200mM solution just because they are being mixed?

Arkcon, if I add 100 ml water to 100 ml of a 400 mM solution then surely I am diluting it and thus reducing the concentration?

[Yggdrasil]

Solution B may have 400 mM NaOAc, but it has 0 mM NaOH.  Therefore, when you mix a solution that has 400 mM NaOH with a solution that has 0 mM NaOH, you get a solution with 200 mM NaOH.  The same goes with the NaOAc.  Solution A has 0 mM NaOAc, while solution B has 400 mM NaOAc.  NaOH and NaOAc are two different chemicals, so you have to consider their concentrations separately.

As an analogy, if you mix a dozen apples with one dozen oranges, you don't end up with two dozen apples.