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In an acid-base neutralization reaction, an acid reacts with a base to produce water and a salt.
Typology: Lab Reports
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GCC CHM152LL: SAMPLE LAB WRITE-UP 1 of 4
In an acid-base neutralization reaction, an acid reacts with a base to produce water and a salt:
acid base salt
The protons (H+^ ) from the acid react with the hydroxide ions (OH–^ ) from the base to form the water. The salt forms from the cation from the base and the anion from the acid. Because water is always formed, acids will always react with bases; whether the salt is soluble or insoluble does not determine whether the reaction occurs.
In this experiment, you will use a dilute solution of sodium hydroxide, NaOH (aq) to determine the molar concentration of acetic acid present in a sample of vinegar. You will measure out a small volume of vinegar and use a buret to determine the volume of sodium hydroxide required to completely neutralize the vinegar. The process of slowly adding one solution to another until the reaction between the two is complete is called a titration. The reaction between acetic acid, HC 2 H 3 O 2 (aq), and sodium hydroxide, NaOH (aq), is shown below:
When carrying out an acid-base neutralization reaction in the laboratory, you observe that most acid solutions and base solutions are colorless, and the resulting water and soluble salt solutions are also colorless. Thus, it is impossible to determine when a reaction has occurred, let alone when it is complete. To monitor the progress of a neutralization reaction, you use an acid-base indicator , a solution that changes color depending on the pH (or acid-content) of the solution. One commonly used indicator is phenolphthalein , which is colorless in acidic and neutral solutions and pink in basic (or alkaline) solution. During a titration, the indicator is added to the solution being titrated (the analyte ). The titrant (or standard ) is slowly added to the analyte until the endpoint , when the indicator changes color, signaling that the reaction between the two is complete. Note that phenolphthalein turns pink only when excess sodium hydroxide, NaOH (aq), has been added.
The equivalence point in a titration is when there are equal numbers of moles of acid and base present in the solution. Thus, for the reaction between KHP and sodium hydroxide, at the equivalence point
ANALYSIS OF VINEGAR VIA TITRATION PAGE 2
moles of HC 2 H 3 O 2 = moles of NaOH. (4)
You should recognize that the end point is determined by observation of a color change in the lab while the equivalence point is a theoretical point one can calculate or estimate given the amount of acid or base present to titrate.
Analysis of Vinegar
BE SURE TO WASH AND DRY YOUR LAB BENCH AFTER COMPLETING THE EXPERIMENT TO REMOVE ALL TRACES OF ANY SPILLED CHEMICALS.
ANALYSIS OF VINEGAR VIA TITRATION PAGE 4
Sample Calculations:
Molarity of HC 2 H 3 O 2 in vinegar:
Trial 1:
2 3 2
1 molNaOH
1 molHCHO L
0.216molNaOH
Trials 2 and 3: see results table
Average [HC 2 H 3 O 2 ] = 0. 839 M 3
Density of vinegar: density = 1. 005 g/mL 10.00mL
10.0503g volume
mass = =
Mass percent of HC 2 H 3 O 2 in vinegar:
1.005gsolution
mLsolution
3
molHC
0.839molHC × × × =
Vinegar Titration Results
Trial 1 Trial 2 Trial 3
Molarity of HC 2 H 3 O 2 in vinegar 0.842 M 0.838 M 0.838 M
Average Molarity of HC 2 H 3 O 2 0.839 M
Density of vinegar solution 1.005 g/mL
Mass percent of HC 2 H 3 O 2 in vinegar 5.01%
Questions:
#1. The mass percentage of the acetic acid in the vinegar solution was found to be
5.01%, which is in close agreement with the manufacturer’s reported acetic acid content
of 5.0%.
Conclusions:
In this experiment, a sample of Smiths brand vinegar was analyzed. Three trials were
carried out, and the average molarity of the vinegar was determined to be 0.839 M. The
mass percentage of the acetic acid, HC 2 H 3 O 2 (aq) , in the vinegar was found to be 5.01%,
which is only slightly higher than the manufacturer’s reported acetic acid content of
5.0%. One possible source of error was that NaOH was added past the endpoint for trial
#1, for which 0.20 mL more NaOH was used compared to trials #2 and #3 even though
the same volume of vinegar was titrated for all three trials.