Murder by Molarity

BACKGROUND: A crime has been committed and you, as the forensic detective, as going to use your knowledge of solution chemistry to solve the crime. Miss Scarlet was found dead on the floor of the ballroom. Beside her body police found a clear liquid, undoubtedly the murder weapon. Detectives were quick to ascertain that the murder weapon was one of five chemicals. Police discovered the following evidence.

Professor Plum, an eccentric chemistry teacher, was working in the study with hydrochloric acid solutions of molarities 0.1M-0.5M. Miss Scarlet was Plum's worst student, arriving to class late, being responsible for lots of broken glassware, and rarely cleaning up after her experiments. The policeman in charge, never having had a chemistry class, did not seriously consider this to be a motive for murder.

Mrs. White, an asthmatic, has a prescription for hydrobromic acid in the bathroom. Her pharmacist assures us that her hydrobromic acid solution has a molarity in the range of 0.6-1.0M. That Mrs. White was presently rooming with Miss Scarlet's ex-boyfriend led the police to suspect her. Neither she nor her boyfriend could be found in the lounge for comment.

Mr. Green, a photographer, had solutions of hydroiodinic acid between 1.1M-1.5 M in the darkroom. Mr. Green apparently had been caught scarlet-handed at his own blackmail game. Pictures of him with Miss Scarlet were found hidden in Scarlet's bedroom. Mrs. Green should be spared seeing the pictures if at all possible.

Mrs. Peacock, wearing the same dress as Miss Scarlet, was found in the stable, administering hydroastatic acid solution to her horse. When questioned, she admitted that her horse had a severe case of bronchitis. Her veterinarian informed detectives that the strength of the solution was between 1.6 M-2.0 M. It is likely that she knew of Miss Scarlet's violent allergy to hydroastatic acid, since they had been lab partners in Professor Plum's chemistry class. Professor Plum recalled how angry Mrs. Peacock would get when Miss Scarlet came late to class, unprepared for the experiments, and how she invariably got sick a few days before the labs were due.

Colonel Mustard, had hydrofluoric acid solutions in excess of 2.1 M, in the conservatory. The Colonel, who despised the left-wing politics of Miss Scarlet, enjoyed his leisure hours making explosives. Colonel Mustard had been the interim assistant principal at the high school and was largely responsible for closing the campus when Miss Scarlet, then president of the student body, overthrew the administration and took control of the school.

YOUR TASK: You can obtain a sample of the murder weapon found at Miss Scarlet's side by asking your teacher for it, in the classroom. If you can determine its molarity, then you can narrow the list of suspects considerably and solve the case!

Who was the murderer? What was the murder weapon? What was its molarity? Provide a short, formal lab report giving evidence and analysis useful for the prosecution of the murderer. Your evidence will be Exhibit A at the trial, and will be subjected to the scrutiny of the defense attorney. On the basis of your evidence, a murderer might be set free or an innocent person executed.

PRELAB QUESTIONS:

1. What is molarity? 2. Draw your lab setup and label all pieces of lab equipment. 3. Define titrant and titrate. 4. Write and balance all 5 chemical equations. (Hint: they are all double displacement reactions)

PROCEDURE:

You will perform a titration in order to calculate the molarity. To perform a titration you will use the setup in Figure 1. In titration, one solution (solution 1) is added to another solution (solution 2) until a chemical reaction between the components in the solutions has run to completion. Solution 1 is called the titrant, and we say that it is used to titrate solution 2. The completion of the reaction is usually shown by a change of color caused by a substance called an indicator. Obtain 50mL of 1M NaOH, this is your titrant. Put an empty beaker under your buret and open the stopcock and run the NaOH through the buret to clean it. Dispose of the NaOH and get a new beaker. Obtain uknown solution from you teacher and record the number in your results section. In the new beaker add 25mL of your uknown solution AND 1 drop of indicator. If you don’t add the indicator stopcock your experiment will be ruined and you will not be able to run a second titration. You will receive a zero for the lab. Make sure the stopcock is closed and fill the buret back up with 50mL of the NaOH solution. Gently open the stopcock and allow the NaOH to drop one drop at a time into the beaker. Make sure to close the stopcock as soon as you see the color change. Record the amount of mL of NaOH that you added to the beaker. Subtract the new amount of NaOH in the buret from your intial 50mL NaOH and record this amount used. From this you will calculate the molarity of the solution.

***NOTE: Your calculations will be similar to #19 on your basic skills sheet.

After you have determined the identity of the murderous solution and its molarity you should be able to conclude which of the suspects is the murderer and where the murder was committed in order to write up your report for the court trial.

Post Lab Questions: 1. What is the molarity of the murderous solution?

2. Supply a fictional account (3 paragraphs) identifying the murderer and the murder weapon based directly upon the episode of you as chemists or detectives in pursuit of a murderer. It should use the characters from above, and be based on the correct solution to the problem. Humor is encouraged and extra credit points will be awarded based on the following scale:

Your science teacher: smiles (sm)……………..1 point chuckles (ch)……...…….2 points laughs out loud (lol).…....3 points

Admittedly, your science teacher is a poor judge of fiction, so extra credit points are based largely on luck.