[light]

when we make a 70% ethanol from a 95% ethanol, why can't we use this formula like for all the other dilutions;

95% X V= 70% X 1L
V= 736.8mL ethanol.
and 263,2 mL dH20


BUT we have to use 700mL 95% ethanol and add dH20 to a final volum of 950mL? see link; http://www.rockystar.com/home/chemistry/chem_prep.htm

hope for replies!
thanks!

[Donaldson Tan]

well.. when u directly add pure ethanol with water, i think the hydration energy from ethanol-water interaction will cause some water and ethanol loss, hence u wont be able to arrive at 95%

[Demotivator]

You can use the dilution formula or the website recipe. The website recipe is simply a shortcut to mix the 95% without having to do a calculation.
 
from your formula: 736.8ml/1000ml = .7368
from website: 700ml/950ml = .7368
They're both the same ratio.

[jdurg]

A neat lab I did in high school was taking a tube and filling it halfway with distilled water, and topping it off with absolute ethanol.  (Carefully adding so they don't mix).  The tube was then sealed and we mixed the liquids by inverting the tube multiple times.  It was pretty neat seeing a good deal of empty space form as the two liquids mixed.  I think it would be much easier to make a 70% ethanol solution based on weight than volume.

[AWK]

i think the hydration energy from ethanol-water interaction will cause some water and ethanol loss
There is no water or ethanol loss, there is loss of volume called contraction of volume