Hi Nicola. This is actually not a stupid question at all. When I first
started my MSc, I had no friggin' clue how to make a solution and over
the years I've had to teach myself the basics and the more advanced
stuff. In addition, I've come across numerous PhDs who *still* don't
know how to make solutions!
The best way to consider a percent solution is to think of things in
terms of 100 mL. A 10% solution (w/v) is 10 grams of your solute in a
total volume of 100 mL (this final volume being the volume of your
solute + solvent).
Anyway, it's a good thing you checked b/c adding 400 mL of saline would
be too much. The way to make such a solution is to add your powder (10
mg) to a microfuge tube and *raise* the level of the solution to 400 uL
(microlitres) using saline, to get a 2.5% (w/v) solution; however, this
is rather difficult using such small volumes (and microfuge tube
gradations are not that accurate), so you can totally get away with
simply adding 400 uL of saline to your 10 mg of drug in the tube.

I got this volume by crossmultiplication:
0.01 g (drug)/X (volume in mL) = 2.5 g/100 mL (to give you 2.5%
solution)
Solve for x:
X = (0.01 g x 100 mL)/2.5 g (cross out units and you're left with mL)
= 0.4 mL = 400 uL

Note that b/c your concentration is relatively small (2.5%) you can get
away with measuring your powder out and adding the final volume;
however, you cannot do this with highconcentration (eg. 2580% w/v)
solutions. What you need to do in this case is measure your powder, add
it to the flask and raise the solution to the final volume you want. If
you don't do this and you simply add the volume to your powder, your
final volume will be *much* higher than you want. This translates into
a more dilute solution.
I hope I'm making myself clear? It's rather difficult explaining this
via email instead of drawing pictures and doing the calculations in
front of the students, like I normally do. Anyway, if you need more
info, please feel free to let me know.
Jacqui
Jacqui Detmar, Postdoctoral Fellow
Samuel Lunenfeld Research Institute,
Mount Sinai Hospital
25 Orde Street, room 61001 AJ,
Toronto, ON, Canada
M5T 3H7
phone: 4165864800 x5607
fax: 4165868588
email: detmar@mshri.on.ca
Original Message
From: histonetbounces@lists.utsouthwestern.edu
[mailto:histonetbounces@lists.utsouthwestern.edu] On Behalf Of
Truscott, Tom
Sent: Thursday, July 24, 2008 1:02 PM
To: nbroadbent@ucsd.edu; histonet@lists.utsouthwestern.edu
Subject: RE: [Histonet] a silly question
Hi Nicola, Ask yourself, Would adding 10mg to .4ml be the same as adding
2.5 gm to 100 ml? or would adding 10mg. to 400ml be the same as adding
2.5 gm to 100ml? Good luck< Tom T.
Original Message
From: histonetbounces@lists.utsouthwestern.edu
[mailto:histonetbounces@lists.utsouthwestern.edu] On Behalf Of Nicola J
Broadbent
Sent: Thursday, July 24, 2008 8:37 AM
To: histonet@lists.utsouthwestern.edu
Subject: [Histonet] a silly question
Hi,
I'm embarrassed to ask this, but I have a basic question about making
percent solutions (% w/v). I wish to make a 2.5% solution of drug A in
saline. I have 10mg of drug A and want to know how much saline to add
to
get a 2.5% solution. The formula I found for making % solutions is:
%solution = (dry mass in grams/volume in mls) *100
According to this formula, I would need to add 0.4ml to 10mg to get a
2.5%
solution. I am not sure whether this is correct however, as my
intuition is
to keep the units the same (mg/mls) and here the amount of saline to be
added would be 400ml. I know this is a bit stupid and I can only claim
that
it is close to Friday and my brain is shutting down...Can someone
resolve
this for me? Help!
Nicola J. Broadbent
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