University of South Carolina Aiken PH of Weak Acids & Lactic Acid Worksheet Answer the following assignment using the information included in the file. Please include your work. pH of Weak Acids & Weak Base Aqueous Solution
Demo Lab Data and Information
This lab begins the introduction to weak acid and base chemistry and introduces the use of a pH
meter and probe to general lab work. The pH is a shorthand method of describing very small
concentrations. Scientific notation is in base-10 which is also the number-base for our number
system. If we have a concentration of 1×10?5 M, this scientific notation number serves a useful
purpose by saving us the need to use a bunch of zeroes to locate the decimal point. If these
small concentrations are used all of the time in a certain chemistry, might there be a way to
make the numbers even shorter, easier to use, and more relevant. The pH system and the pH
Cycle is such a use. Please study the pH Cycle so that quickly moving from pH to pOH to
concentrations is easily done.
The pH probe was an invention that several chemists and engineers had worked on because the
acidity of a solution was important to know quickly in a variety of practical situations. Besides,
the hydrogen ion, H+, is reactive as is the hydroxide ion, OH?, so knowledge of their
concentrations is fundamental to chemistry. The pH probe is an electrochemical arrangement
that produces a small voltage much like a battery, but the chemical components inside are
exposed to the outside via a special sintered glass or synthetic glass material. The chemistry
inside is disturbed slightly as a result of the presence of hydrogen ion on the outside so that a
small but measurable change to the voltage is observed. In this lab, the pH probe used is
calibrated by using two solution of known pH, one at pH = 3 and the other at pH = 11, and
measuring the small but different voltages coming from the probe when it is inserted into those
known solutions. Then any pH reading around or between those two known values will be a
pretty decent estimate of the solutions pH. Using equations from the pH Cycle, the
concentration of hydrogen ion associated with a measured pH can be determined. If an
equilibrium in water produces either H+ or OH?, the concentration can be measured and used in
the ICE scheme for a Situation 2 calculation. That is the crux of this lab.
In this Demo Lab we will pretend to use of lactic acid in water as the weak acid solution of
interest and pretend to use morpholine as the weak base of interest. Remember a weak acid
has its own hydrogen ion as part of its molecular structure that it donates to water molecules
thereby changing the acidity. A weak base generates hydroxide ions, like all bases do, but as a
result of a reaction with water itself. Weak bases rarely bring along their own hydroxide ions.
Weak base molecules usually possess nitrogen atoms in their structures that have lone electron
pairs to share, which they do with the small number of hydrogen ions formed in water. This
bonding action between hydrogen ion and nitrogen lone pair removes hydrogen ion from the
balanced water environment resulting in excess hydroxide ion building up and a pH value above
7. So the basic solutions will have pH values above 7 and the acidic solutions will have pH values
below 7; a value of 7 being neutral.
A weak acid has a reaction similar to this general layout: HA(aq) ? H+(aq) + A?(aq). An ICE
scheme can be set up using this general reaction scheme. Use the symbol LaH for lactic acid and
La? for the lactate ion in place of the general symbols. The general weak base reaction is: B(aq)
+ H2O ? BH+ + OH?. Use the symbol Mrp for the molecule morpholine and MrpH+ as the
morpholinate ion in place of the general symbols.
The measured pH values for both solutions are: pH of a 0.100 M LaH solution is 2.21 and the
pH for a 0.100 M Mrp solution is 10.35. Set up the scheme for both reactions and write down
the Law of Mass Action Kc expressions for each. Find lactic acid in Table 1 and fill in the line.
You will need to take the pH in Table 1 and convert it to hydrogen ion concentration. Find
morpholine in Table 2 and fill it in. You will need to convert pH to pOH and then convert pOH to
the hydroxide ion concentration. The balanced reactions for both solutions show why these
measurements and calculations were done. The weak acid establishes an equilibrium
concentration of hydrogen ion which we measure directly with the pH probe. The weak base
solution sets up an equilibrium concentration of hydroxide ion which we measure indirectly
with the pH probe.
Table 3 on the front page of the Report Page is an example ICE table to use. Because this is a
Situation 2 problem, our measured [H+]eq for the lactic acid or the measured [OH?]eq for the
morpholine are equal to the x values in each of those schemes. The bottom lines of the ICE
schemes give the Law of Mass Action quantities in terms of algebra and now, because we have
measured x, we can substitute and solve for an experimental value of Ka for lactic acid or Kb
For analysis, convert you measured, experimental Ka or Kb values to pKa or pKb, which means
to take the negative log of your Ka or Kb values. Get the literature K values for both molecules
from the front of the lab manual pages or elsewhere like from the textbook, and convert them
to literature pKa and pKb values. Perform a percent error analysis for each of your experiments.
%Error = (Literature pKa or pKb ? Experimental pKa or pKb) / Literature pKa or pKb
Answer Question 1 on the back of the Report Page.
Acid = Source of it in water
SA in H?O
Hiep HBr H2 Hmo, 4,504 1804
“WA in the
SB into naolt, 250H, KOH, CapH), Solt),
Base & Any source of Off in water /
(Krog) + Odlag
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