Analytical Chem Experiment #8 Determining Equiv Weight of Unknown Acid

Okay, so I entered all this data & got a nice sigmoidal graph
I entered another series of change in pH on the y-axis & same volume on the x-axis. Got a wavering line with a nice peak
Here's where the fun begins
1. The peak is supposed to occur at the # of ml NaOH at equivalence point, right?
2. Just before the peak, the delta pH takes a big dip to almost zero. Anybody else get this?
3. From class, we can find concentration of unknown by:
M of NaOH to (expressed as moles/L) x volume of NaOH used = moles of NaOH used = moles of Acid in Sample
Moles of acid in Sample/Liters of sample = M concentration of Unknown acid
Is that right?

4. The lab was designed to find f.w. of the unknown acid. We should be able to do this:
grams in sample/moles of acid in sample = grams/mole = F. W.
I get almost 20% difference between the 2 samples. 190 & 160 (Heavy acid & I knew would get killed on SD &
RSD. That's the day I kept watching my beaker gain 0.0259 oz without adding anything.

Finally had my first anal chem nightmare. Guys, if you see me standing at the bench by the oven on Tuesday, sleeping, please wake me up.

Your Y axis should be change in ph/change in volume and x axis is total volume. I got a little dip before the actual equivalence point. I read in the lab book that this can happen, but the 2nd peak is the one we want to focus on.

We take equivalence point volume to find mols of base, then get mols of acid. We take grams of acid/mols acid to get formula weight.

I think he has already said this in class, but this experiment is only supposed to have 1 'real' equivalence point, yes?


Shauna J

There should only be 1 real e.p. I got a small peak before the giant peak and that is what I was referring to in the previous post.