What is a volumetric flask?

[Robert Zellmer]

I've seen one or more questions on every practice exam about a
volumetric flask.  While I haven't received any questions about what
this is I thought I would send out something about it since I didn't
cover it in class.

Volumetric flasks are used for diluting solutions when you want to
get a precise measure of the final volume.  They are much more
accurate than a flask or beaker.  Their volumes are usually accurate
to at least the first decimal place and sometimes the 2nd decimal
place.  There are pictures of them in Figures 4.15 and 4.16 in section
4.5 of the textbook.  These are meant to "contain" a given volume,
not deliver that volume.  A pipet or buret are meant to deliver a
particular volume.

When diluting something you need the final volume.  We generally
assume volumes are additive for water and aqueous solutions.
Strictly speaking this isn't necessarily true.  If you add 20 mL of
ethanol to 20 mL of water you do not get 40 total mL.  The final
volume is actually less than 40 mL.  It has to do with attractive
forces between the molecules (something we'll discuss in ch 11).

Thus, in real life if you have 20.0 mL of a NaCl solution and you
want to dilute it to 40.0 mL you really can't simply add 20.0 mL of
H2O (or another solution).  If you did, you might actually get more
or less than 40.0 mL.  Instead you put the 20.0 mL of NaCl in a
volumetric flask and you add enough water to the flask to bring it
up to the mark on the neck to give a total volume of 40.0 mL in
the flask.  Volumetric flasks come in a multitude of sizes (volumes)
and have a mark on the neck indicating where to fill it to get the
desired final volume.

If you're diluting a concentrated acid or base the process evolves
heat (heat is given off).  In fact, if the acid or base is concentrated
enough (like a 12 M HCl solution, about 36% HCl by mass) a
very large amount of heat is evolved.  So much so you have to
do the dilution in a special way.  If you know you need about
250.0 mL of the diluted solution you would have about 125 mL or
more of the water (about half full).  Then you would slowly pipet
the desired amount of acid or base to the flask.  Remember, water
has a high specific heat so a lot of energy can be added w/o the
temperature rising too much.

If you add the water to the concentrated acid or base, so much
heat is generated the water essentially instantaneously boils and
splatters at you so you wind up with conc. acid or base in your face.
One reason to make sure you're wearing your goggles.

Dr. Zellmer