Angelica Rodriguez 05/14/13 Period 4 Acid-Base Crime Scene Titration Introduction:
Titration is a lab technique used to determine the exact concentration of an acid or base. In this laboratory experiment, the crime scene ana lyst will use their knowledge of acids acid s and bases to determine the concentration of each acid found as evidence in a murder. The titration technique will be used in the investigation to neutralize neutralize each HCl acid; that of the crime scene and of the samples samples belonging belonging to three suspects. suspects. In order to neutralize neutralize each each acid, it it must be known that when an acid and base react with one another, they neutralize and produce a solution that is not acidic or basic, but neutra l. Thus, the crime scene analyst can use u se a known concentration of a base, 1.0 M NaOH, to neutralize the unknown concentrations of each acid found. The reaction of an acid and base is known as a neutralization reaction, where where neutral water and salt are the products. Phenolphthalein indicator is a molecular substance that changes color when it comes into contact with acids acid s and bases and will be used used to show when the reaction has completely neutralized. Phenolphthalein is colorless when in an acidic solution and turns pink in a basic solution. Once enough base is added to neutralize the acid, the reaction reaches the equivalence point. The point at which the moles of base is equal to the moles of acid. The equivalence po int can be used to determine determine the initial concentration of an acid. acid. The goal of the titration is to get as close as possible to the equivalence point by careful addition of the the base. This will ensure that the calculated acid concentration is as close to the true value as possible. Having the calculated acid concentration for each of the d ifferent acids found in the investigation investigation will allow the crime scene analyst to identify who killed Mr. Meck by matching the molarity molarity of the crime scene HCl acid sample to the molarity of the suspect’s acid sample.
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Procedures:
1. Goggles Goggles were were put on. The following following material materialss were were obtained: obtained: 50 mL graduated cylinder. cy linder. 50 mL burette, stand, and c lamp. 125 mL Erlenmeyer flask. f lask. Phenolphthalein indicator. Glass funnel. · · · · ·
2. The stopcock stopcock at the bottom bottom of the burette burette was turned such such that it it was perpendicular perpendicular to the the tube of the burette in a c losed position. 3. The burette burette was carefully carefully filled filled near the top with with the 1.0 M NaOH base. base. It was was made sure sure to not go above the graduations. The initial volume of NaOH was recorded. 4. Exactly Exactly 20.0 mL mL from one of the the acid samples samples was measure measured d using a graduated graduated cylinder. cylinder. This was then poured into the Erlenmeyer flask. The flask was positioned on a white paper towel, directly beneath the burette so that when the color changed, it was easier to see. 5. 3 drops of of phenolphthal phenolphthalein ein indicator indicator was added to the acid acid (it stayed stayed clear). clear). 6. The unknown unknown acid was titrated titrated with with the NaOH by having opened opened the stopcock stopcock until there there was a slow stream of NaOH. 7. As the the drops came came out, the flask flask was gently gently swirled swirled to make make sure the coloring coloring disappeared. Observations were recorded in the observations/notes section in Table 1. 8. It was evident evident when when the solution solution was approaching approaching neutraliz neutralization ation because because the solution solution became pink faster and took longer to swirl before it became clear again. 9. When this happened, happened, the the stopcock was turned turned very gently gently to the “partly “partly open” open” position. position. Smaller, individual drops were coming from the burette one at a time instead of having a stream of NaOH. 10. As soon as the the coloring did not go away and the solution stayed stayed pink, the stopcock was immediately turned to the “closed” position. po sition. 11. The final volume volume of NaOH in the burette was was recorded. 12. The “ Total Total Volume (in mL) of of NaOH Used” Used” to neutralize neutralize the suspect’s acid sample was was calculated and recorded in the data table. 13. The burette was refilled with NaOH and the the procedure was repeated for the other 3 samples. Two trials were done for each sample. 2
Data:
Table 1: Data obtained through various va rious titration trials and the calculated molari mo larity ty of HCl. Exact Volume of HCl (mL)
Trial Suspect A 1 Kirkpatrick Trial HCl Acid 2 Suspect B Larkin HCl Acid Suspect C Nguyen HCl Acid Crime Scene HCl Acid
Trial 1 Trial 2 Trial 1 Trial 2 Trial 1 Trial 2
Initial Initial
Final
Volume of NaOH (mL)
Volume of NaOH (mL)
Total Volume of NaOH Used in titration (mL)
20
49. 55
40
9.55
19.5
40
29. 85
10. 15
Molarity Observations/ of HCl Notes (M)
Pink faded 0.48 after a minute 0.52 Barely pink
20.5
35.1
31. 05
4.05
0.19
21
31
26. 55
4.45
0.20
20
45
29. 2
15. 8
0.79
20.5
29.2
12. 5
16. 7
0.81
19.5
30
26. 2
3.8
0.19 Little too pink
20
43.2
38. 7
4.5
0.22
Calculations:
“See attached” Conclusion:
It was concluded that the murderer of Mr. Meck was Suspect B, Mr. Larkin. Larkin. This conclusion was reached when the calculated molarities of the acid samples belonging to each suspect were were compared to the calculated molarity of HCl acid acid found in the crime scene. Based on on the comparison, it was seen that the molarity molarity of HCl acid used to kill Mr. Meck matched the th e molarity of HCl acid sample belonging be longing to Suspect B, Mr. Larkin. As seen in Table 1, Trial 1 of Suspect B reveals that their acid sample had a concentration of 0.19 M HCl. HCl. Likewis L ikewise, e, the acid 3
sample used to kill Mr. Meck also a lso had a concentration of 0.19 0. 19 M HCl. The molar ities of acid samples pertaining to Suspects A and C were much higher than the molarity of the HCl acid used to kill Mr. Mr. Meck. The only molarity of acid sample that matched the molarity of the crime scene acid was Mr. Larkin’s. The exact concentration of each acid was determined by using a lab technique called titration. In a titration, a measured volume of a strong base, NaOH, with a known concentration, 1.0 M NaOH, was delivered from a burette into a flask containing the unknown concentration of an HCl acid sample being analyzed. In this neutralization reaction, the base was added to the acid until they neutralized each other and produced neutral water water and salt. Phenolphthalein indicator was used to show when the reaction was fully fu lly neutralized. neutralized. In an ac idic solution, phenolphthalein was colorless and in a basic solution, pink. When the the acidic solution, HCl, was neutralized with the basic ba sic solution, NaOH, the reaction reached the e quivalence point. In the neutralization reaction of HCl and NaOH, the equivalence point occurred when one mole of HCl reacted with one mole mole of NaOH. NaOH. The equivalence equivalence point was then used to determine determine the concentra concentration tion of each acid found as evidence evidence in the the investig investigation ation.. Furthermore, the exact concentration of an HCl acid sample being analyzed was calculated by measuring the exact volume of HCl (mL) put in the flask. Then, the total volume of NaOH base used in titration to neutralize neutra lize the HCl acid was calculated. The total volume volume of NaOH base used in titration and the molarity of NaOH (1 M) were used to calculate the moles of NaOH used in the trial. After, After, using the mole ratio from the balanced che mical equation, the moles of HCl neutralized with NaOH were were calculat ca lculated. ed. Finally, F inally, the molarity of the HCl sample was calculated. The molarity of HCl was found by dividing the moles of HCl by the liters of HCl used.
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Moreover, a possible source of error in this laboratory experiment would be that when the NaOH base was being delivered from the burette to the flask containing an unknown concentration of HCl acid ac id,, not all a ll of the base released fell into the flask to neutralize the acid. ac id. Rather than going into the flask, drops of base remained around the sides of the burette. This could have altered the results and calculations obtained because the crime scene analyst assumed all of the base released fell into into the flask. f lask. However, because drops of base didn’t d idn’t go in the flask, f lask, the total volume of NaOH used u sed in titration measured by the crime scene scene analyst analy st was wrong. To have had an accurate measurement of the total volume of NaOH (mL) used in titration, the crime scene analyst would have had to subtract the volume of the NaOH drops that never went into the flask. Another possible source of error would be the possibility of carbon dioxide in the solution. When the base was added to neutralize the acid in the flask, this neutralization reaction produced water and salt. However, because the NaOH base can easily absorb carbon dioxide, carbon dioxide from the atmosphere diffused into into the t he basic solution. This potentially added more acid to the solution because when carbon dioxide dissolves in water, it creates carbonic acid. This affected affected the experiment because the presence of carbonic acid interfered with the endpoint detection. Because carbon dioxide combined with water added more acid to the solution, not only was it easily analyzed in titration, but more NaOH base was requ ired to neutralize the acid. The problem with this was that the unknown concentration of acid appeared to have a higher molarity than it actually was. If CO2 was removed from the solution, it would have a llowed the solution to be titrated to a more accurate endpoint.
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