nimgoldman
Hazard to Others
Posts: 303
Registered: 11-6-2018
Member Is Offline
|
|
Preparing a solution with exact concentration
I have read several articles about making and diluting solutions, but one thing was never answered:
What if the volume of solution changes considerably by adding the solute?
This is not a big problem with, say, salt and water as the volume of water almost does not change by adding salt (it actually
decreases slightly).
The issue is more pronounced with solutions like ethanol and water where 100 ml water and 100 ml ethanol adds up to sth. like 195 ml, not 200 ml.
This is okay for most applications but may be a big problem in synthesis where exact molar amounts are needed.
The dilution equation also breaks in general, because the "packing factor" often changes with concentration.
Some solutions adds up to smaller volume than the sum of volumes until some concentration, and then adds up to bigger volume and even the factor
changes with concentraction in a non-linear fashion.
Is there any general dilution equation or tables/calculator for making solutions with precise concentrations?
I guess the "precision" cases call for w/w concentrations instead of w/v or molarities?
[Edited on 8-7-2018 by nimgoldman]
|
|
|
happyfooddance
National Hazard
Posts: 530
Registered: 9-11-2017
Location: Los Angeles, Ca.
Member Is Offline
Mood: No Mood
|
|
You take a volume of a substance of known purity, say, absolute ethanol, with a molar weight of 46 grams. If you want a 1M solution, you dissolve 46
grams of it in water, add it to a volumetric flask, and add water to the 1l mark.
There is a liter of liquid. It contains 1 mol of ethanol. Per liter. This is temperature dependent, the volumetric equipment should have the
temperature listed. You can also make this sort of equipment yourself, after a fashion. For example, fill a bottle with 1000 grams of room temperature
water. The water level marks approximately 1 liter.
Precision calls for precise measurement, this can be done with precise volumetric equipment or precise gravimetric equipment (and suitable technique).
Depending on what is available, convenient, or necessary for the task at hand will determine what methods are used.
[Edited on 7-8-2018 by happyfooddance]
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
Molarity can be used in precision cases; the solution is first prepared in slightly higher concentration than desired and then diluted to meet an
exact volume.
|
|
|
nimgoldman
Hazard to Others
Posts: 303
Registered: 11-6-2018
Member Is Offline
|
|
Ah ha. Thanks.
Yes I use volumetric glassware very often but most people have it rounded to some steps, volumetric flasks come in say 250 ml, 500 ml, 1 000 ml sizes.
I found some fancy ones with graduation (to get say 980 ml) but these seems to be needed only in advanced cases and I guess the amount of solvent
usually does not matter as much as the amount of solute and the concentration has some wiggle room in the reaction too (just not get it too
concentrated so the reaction goes smoothly enough but not too dilute either).
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
