# [Neuroscience] Re: osmolarity adjustment sol with water

r norman via neur-sci%40net.bio.net (by r_s_norman from _comcast.net)
Thu Dec 6 19:44:08 EST 2007

```On Wed, 5 Dec 2007 15:14:56 -0800 (PST), Katya Melnik-Martinez
<verush from yahoo.com> wrote:

>Hi All,
>
>I would like to adjust the osm of a solution from 359
>to 340 mosm.  How do I calculate the amount of
>deionized water to add in order to lower the osm 10
>mosm?
>

I am sorry to say that if you were a student of mine, I would yell at
quickly if you intend to continue in experimental work!

Now that the lecture is over, here is how it works.

Let V be the volume of solution you want to modify in ml.  Then that
volume contains 359 * V mmole of solute (concentration = moles/volume
so moles = concentration * volume).  You want to make that into a new
volume V' so that the new concentration (359 * V)/V' = 340.

In other words 359 * V = 340 * V'

or   V' = (359/340) * V.

So if you start with V = 100 ml, then V' = (359/340)*100 = 105.59 ml.

That is, you add 5.59 ml of distilled water to 100 ml of the current
solution.

Of course, you have also diluted all the material currently in the
solution which may be a serious problem, but that is another story.

And if this really is a homework problem, which I very seriously
suspect, then you should really be ashamed of yourself and confess to
your instructor when you turn it in that you got help on it.

```