Purpose:
The objective of this experiment were: a) to review the concept of simple acid-base
reactions; b) to review the stoichiometric calculations involved in chemical reactions;
c) to review the basic lab procedure of a titration and introduce the student to the
concept of a primary standard and the process of standardization; d) to review the
calculations involving chemical solutions; e) to help the student improve his/her lab
technique.
Theory:
Titration was used to study acid-base neutralization reaction quantitatively. In
acid-base titration experiment, a solution of accurately KHP concentration was added
gradually to another solution of NaOH concentration until the chemical reaction between
the two solutions were completed. The equivalence point was the point at which the acid
was completely reacted with or neutralized by the base. The point was signaled by a
changing of color of an indicator that had been added to the acid solution. Indicator was
substance that had distinctly different colors in acidic and basic media. Phenolphthalein
was a common indicator which was colorless in acidic and neutral solutions, but reddish
pink was result in basic solutions. Strong acid (contained H+ ion) and strong base (
contained OH ) were 100% ionized in water and they were all strong electrolytes.
Procedure:
Part A.
Investigating solid NaOH for use as a possible primary standard
First of all, The weight of a weighting paper was measured in analytical balance, then
added two pellets of NaOH and reweighed the total amount of those. At the end of the lab,
reweighed the combination and recorded all results in the lab manual.
Part B.
Preparation and standardization of a solution of sodium hydroxide
A clean beaker, burette, three 250ml Erlenmeyer flasks, and florence flask were rinsed by
soap and distilled water. Poured 1.40g of NaOH into florence flask and added 350ml
distilled water, then swirl it and inverted flask five times with parafilm on the top of
it. Next, obtained a vial of KHP from the instructor, and poured about 0.408g into three
different Erlenmeyer flasks by measuring with analytical balance. Then, filled up about
25ml of distilled water, added 3 drops of phenolphthalein into it and mixed them well by
a glass rod. Labeled all solutions to prevent mix them up. Before the titration began,
the buret should be rinsed with NaOH solution and recorded the initial buret reading.
Titrated the solutions until the reddish pink color appeared. Recorded the final reading,
and calculated the change of volume.
Part C.
Determination of the molar mass of unknown acid
Repeated the procedure above, but this time KHP was replaced with an unknown acidic
solution and concentration. Demanded the number of replaceable hydrogen from the
instructor.
Conclusion and Discussion:
From the titration results of three trial, the average of molarity of NaOH is 0.1021 .
The percentage deviation in molarity of NaOH had 0.20% error. The possible error in this
experiment were: the error in taking the buret readings, the error in measuring amount of
elements, and the NaOH was not stable under air.
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