What is a volumetric flask?

[Zellmer, Robert]

I got a question about this and I've seen one or more questions on every MT 2 practice

exam about a volumetric flask.  I decided to send 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 start with

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
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