[Histonet] a silly question

[Nicola J Broadbent]

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