Converting between units

[gogogadget]

Hello so I am having issues with trying to successfully convert between units between these two problems. My prof has provided the answers but didn't provide the calculations needed to get it.


1. You have a 1M Tris (pH 8.0) stock solution and you want to make 250 ml of 100mM Tris (pH 8.0). How will you make your solution?

I started with 250 mL and divided by 1000 mL to get to one L. From there, converted the L to M as M = mol per liter. She provided info in that the density of Tris base is 121.2 g which I know is equal to one mole. From there though, I cannot figure out what to do with 100 mM or how to convert to it. Is it milli moles or milli M (mol/liter). I kept getting an answer at around 30 but my professor's answer was to add 25 mL of 1 M Tris (pH 8.0) to 225 mL of water.

2. 1x TE is 10mM Tris (pH 8.0) and 1mM EDTA. How would you make 500 ml of 1x TE from stock solutions of 1M Tris (pH 8.0) and 0.5M EDTA?

From this problem, it appears that two seperate calculations will be made. However, again I am stumped on what to do with mM....Any help would be really appreciated! My professor's answer for this one was to add 5 mL of 1 M Tris (pH 8.0) and 1 mL of 0.5 M EDTA to 494 mL of water.

 

[gogogadget]

Can you change your problems to something readable...like X% salt solution
changed to Y% salt solution (Y < X) by adding water...

I apologize but I'm not quite sure what you mean...I'm trying to figure these out for my lab and these were a couple of practice problems my professor had posted with answers. I just was not sure how to convert between each of the units to get the answer she got. :?

 

[gogogadget]

What Denis is saying is that either the question as posed by your professor requires a bunch of specific information that none of us may have because we are not taking your course or that you have not quoted the question correctly.

I read your question, but could not understand it. I had to look up tris to find out it is the abbreviation for an organic compound. I suggest you ask a random sample of 100 what tris is to find out what they say. It is not a generally know abbreviation. In any case, you could have said solute and saved me some time. You say you have a solution of tris equal to 1 mole. Do you have 1 mole, a weight, of the solute or a solution containing the solute with a concentration of 1 mole per something. You imply that you are diluting this solution but end up with the same pH. Unless I have forgotten all my chemistry (which I admit is possible), dilution causes pH to move toward 7.0. You give us what purports to be a density measured in weight, but density is given in weight over volume. All of this may be absolutely clear in the context of your course, but we are not diviners.[/QUOTE

Oh I see, thank you for clearing that up! I do see now that it does come off as confusing...Especially since it seems like either I myself am missing some information or did not see something she may have up put up. In any case, I'll present it to her to see if she can clarify on some things. Thank you both for looking at them though!