[psychoNOT]

I am sure this experiment has been talked about, but I have a specific question.  In our experiment, 20mL of approximately 0.1M HCl was titrated with 0.1M (standardized) NaOH in order to determine the true concentration of the HCl.  From the equation HCl + NaOH :rarrow:NaCl + H₂O, it is known that 1 mol of HCl reacts with 1 mol of NaOH, therefore we would assume that about 20mL of NaOH would be needed to neutralize the 20mL of HCl.  However in our experiment, to reach the end point it only took 15.5 and 15.6 mL of NaOH to neutralize 20mL of HCl.  Granted, it could have been a little bit less HCl as a pipet was used which could not be completely emptied, but my question is why is this so significantly off?  From this data we calculated a molarity of 0.07775M HCl solution, which is a far cry from the 0.1M which was listed on the bottle.  As the goal of the experiment was to determine the actual concentration, obviously we wouldn't expect it to be exactly 0.1M, but I feel (and my TA felt) that this was significantly off.  We know that the titration was performed correctly, and the two trials were very precise, which is why it is confusing.  Do you think the difference in concentration has to do with the HCl solution (perhaps it is hygroscopic or something), or perhaps our NaOH solution drew a lot of CO2 from the air (although this would have made it require more NaOH solution to neutralize the HCl), or perhaps a problem with the titration?
Any info is appreciated.

[Borek]

No simple explanation, other than lousy preparation of solutions for students

For general discussion of acid-base titration see http://www.titrations.info/acid-base-titration

[gluedudeguru]

Some other undergrad made that solution (which i did before) and could have been sloppy on both solutions.  I was in that persons shoes not too long ago and when someone asked me about how badly my solution was off i would say i was just keeping them on their toes... It was enjoyable... Trust what u get in the lab, especially after duplicates

[JGK]

" In our experiment, 20mL of approximately 0.1M HCl was titrated with 0.1M (standardized) NaOH in order to determine the true concentration of the HCl"

How approximate? from your data it would seem that it was somewhere between 0.075 and 0.08M assuming the standadized 0.1M NaOH is accurate. Obviously, the HCl was a "Friday afternoon" preparation, where the tech had other things on their mind.

[Borek]

Obviously, the HCl was a "Friday afternoon" preparation, where the tech had other things on their mind.

Could be also Monday morning, where the tech mind was still at home, curing hangover

[psychoNOT]

Thanks a lot guys.  The only issue for me was that the TA said that the amount of NaOH needed was odd, and I assumed this meant different than the other groups, but as we were the first to finish perhaps he was just surprised.