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The purpose of this lab is to teach us three new methods and give us
a better understanding of acid-base reactions. We will be learning how
to standardize a solution, determine % acid in a solution (Vinegar),
and how to determine neutralization capacity (antacids).
Discussion:
The first part of this lab was a required step for the next two
parts. We standardized NaOH so that we could determine its exact
Molarity. We used phenolphthalein as the indicator for neutralization.
We titrated KHP with the NaOH until we reached the equivalence point
which told us the moles of H+ ions equaled that of OH- ions. By knowing
moles of KHP, we knew moles of NaOH titrated. Dividing moles NaOH over
liters titrated we found the average Molarity for three trials to be
.290M. This is very close to the goal of .3M, and now we have a
standardized solution of NaOH for the next parts of the lab.
Part two was to determine the moles of acetic acid and its percent
in vinegar. Once again we used phenolphthalein as the indicator. We
titrated 20mL of Vinegar with 34.6mL and 34.3mL of NaOH for trials 1
and 2 respectively, until the equivalence point was reached.
Multiplying by the concentration found earlier of NaOH, we found .00986
and .00995 moles for each trial. Since only one H+ ion from acetic acid
(Only one is bonded to the electronegative oxygen, giving up its
electron) bonds with OH- we know that the moles of acid are the same as
NaOH. Dividing by .020L of Vinegar gives us the concentration of .493M
and .498M. We found an average of 2.85% acetic acid in Vinegar.
Part three was the most involved piece of the lab. The goal was to
find the moles, mass, and % composition of the active ingredient in the
antacids, Tums and Rolaids. We weighed each antacid to find total grams
so we could find % composition later. We dissolved the antacids in
excess HCl because they are insoluble in water. We added Thymol blue
indicator for this solution. We then titrated the solution with NaOH
until the equivalence point was reached. Rolaids required considerably
less NaOH to reach this point. Since NaOH only reacts with the excess
HCl not consumed by the base in the antacid, the number of moles of
NaOH equals the excess moles of HCl. Knowing that 70mL of .3M HCl is 21
millimoles, subtracting from this the excess millimoles HCl, we found 8
millimoles HCl reacted with CaCO3 in Tums, and 14 millimoles reacted
with CaCO3 in Rolaids. This means that the antacid in Tums weighs about
.801g while Rolaids contains 1.40g of antacid. Tums was 60.6% antacid,
while Rolaids was 95.2%. Theoretically, all of this means that Rolaids
is better since more acid will be consumed by it.
If the concentration of HCl was not correct, then the millimoles of
HCl would be off, which would adversely affect the amount of HCl
calculated to react with the antacid. However, both antacids would be
off by the same amount, so if the point was to determine which one
reacts with more acid, this would be mostly a non-issue. If the antacid
was not fully dissolved then not all of the HCl would react with it,
making it appear that it consumes less acid than it can.
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