LAB 4 - Lab Report 4: Chem 221 PDF

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Lab Report 4: Chem 221...

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Determination of Chloride by the Mohr Titration Department of Chemistry, Binghamton University, Binghamton, NY 13902

Introduction: This lab tested various basic skills and procedural operations in the analytical chemistry laboratory. In this lab. The major techniques tested were to ability perform a titration, perform a blank titration, and creating solutions of various concentrations. In addition, statistical analysis was necessary to effectively analyze the experimental data collected. One of the skills tested was the ability to analyze the data collected in each part of this experiment through statistical analysis. This is a fundamental skill required in the laboratory since it gives the data collected significance and allows for the experimenter to draw conclusions from the data they collected previously. In this lab specifically, statistical analysis was necessary in part A where the mean molarity, standard deviation, and relative standard deviation (in ppt) using the following formulas below: MEAN FORMULA: μ = ( Σ Xi ) / N

SAMPLE STANDARD DEVIAION FORMULA:

√

Σ (X i−μ)2 n RELATIVE STANDARD DEVIATION FORMULA: Sx=

RSD=

Sample Standard Deviation mean

×10

3

Another chemical technique tested in this lab was the use of a burette and performing accurate titrations. There are many preliminary steps one needs to take before beginning their titrations to ensure the accuracy of their collected data. Before starting the titration, it is necessary to rinse the inside of the burette with DI water. This helps to clean out any chemical substance that was in the burette previously that would affect the data collected. In addition, after rinsing with DI water, students must take some of their titrant and rise the inside of the burette again. This helps to ensure the titrant placed in the burette to perform the titration will be purely the chemical substance added with no residue of previous chemicals that might have might have been in the burette. After these steps one can perform a correct and accurate titration. In addition, a new laboratory technique introduced to students in this lab was the concept of a blank titration. A blank titration utilizes the titrant in the experiment along with distilled water and CaCO3. By performing this titration, it helps to stimulate the AgCl3 precipitate to form. The resulting volume of titrant till end point is relatively small, but is a key value. This value can help correct experimental titrant volume necessary to reach the end of the reaction by subtracting from the volume required to reach the end point when the solution of unknown chloride is titrated. Finally, this lab required students to prepare different solution with various concentrations. Although this is not a newly introduced laboratory technique, it is nevertheless fundamental to the accuracy of the experiment and collected data values. Some important aspects to consider when creating various solutions throughout the course of one lab is to make sure all solutions created and provided are properly labeled and that measurements of volume are accurate to ensure the molarity calculated is extremely close to the true value for the molarity of the created solution.

In this lab, the major objectives were to practice precipitation titrations, become familiar with the concept of a blank titration and understand its experimental importance in the laboratory, and finally calculate the percent of chloride in an unknown sample of water using the Mohr titration method.

Experimental Section: Part A: 1. In first part of this laboratory exercise, students had to first standardize a solution of AgNO3 with NaCl. To accomplish this, students first weighed out 1-1.5 grams of NaCl by difference and added the NaCl to a small beaker. After adding a small amount of DI water to dissolved the solid sodium chloride, this solution was transferred to a 250-mL volumetric flask and diluted to the calibration line with DI water. After doing this, three 25 mL aliquots of the NaCl solution were transferred into three separate Erlenmeyer flasks, labeled 1 2 and 3, and prepared for titration by adding exactly 8 drops of K2CrO4 solution which acts as an indicator for the titration. The burette for the titration is the prepared by rising out the burette first with DI water then a small amount of silver nitrate and then filled with silver nitrate. Students then, slowly, added AgNO 3 from the burette until the solution in the flask had a permanent orange tinge, ensuring to avoid over titrating the sample. This process is then repeated for the other two aliquots of the NaCl solution. After completing the three titrations, students must then perform a blank titration to correct the volume of titrant necessary to reach the end point for precipitating chloride ions.

2. To determine the percent of chloride ion in the unknown sample, students must prepare a solution of the unknown by weighing out 1-1.5 grams of unknown by difference. Then this sample should be transferred to a small beaker with a small amount of DI water to dissolve the substance. After doing so, students can then transfer the solution in the beaker to a 250-mL volumetric flask and diluted to the calibration line with DI water. Then, students must perform three titrations of 25 mL aliquots of the unknown solution gathering at least three reproducible data points. Unlike the first part of part A, it is not necessary for students to perform an additional blank titration and can use the same value for the blank titration gathered in the first part of part A. Finally, after collecting this data, the percent of chloride ion in the unknown can be calculated.

Part B: For the second part of this lab, the objective was to determine the chloride content in surface water. To accomplish this, students first had to prepare a diluted solution of the titrant by adding 5-mL of AgNO3 to a 50-mL volumetric flask and diluting with DI water to the calibration line. Before performing the titration, it is again necessary to rinse the burette with DI water and the titrant before students fill the burette with the diluted solution of silver nitrate. Then, students must obtain a 100-mL sample of water and add 8 drops of K2CrO4 indicator and perform the titration. After completing this, students must perform a blank titration to correct the data value collected and calculate the chloride content in the water (in ppm).

Results/Discussion: Part 1:

Table 1: This table contains the values for the first three titrations carried out in Part A of this experiment. Trial Mass Molarity Volume Volume Correct # NaCl(s) of NaCl NaCL of ed (g) solution solution AgNO3 Volume (M) (mL) titrant of used to Titrant reach (mL) endpoin t (mL) 1 15.2 15.0 2 1.0916 .069 25 15.3 15.1 3 15.3 15.1

Table 2: This table contains the values for the three titrations carried out in the second Part A of this experiment where the objective was to find the percent of chloride ion in the unknown sample. Trial #

1 2 3

Mass Unknow n (g)

1.0408

Volume Unknow N Solution (mL)

25

Mean Volume Silver Nitrate:

Sample Standard Deviation: S=

Volume of AgNO3 titrant used to reach endpoin t (mL) 18.2 18.3 18.3

x=

√

Correct ed Volume of Titrant (mL)

18.0 18.1 18.1

18 + 18.1+ 18.1 3

(.0067) (3−1)

= .058

=

18.07 mL

The sample standard deviation for the volume of silver nitrate delivered for this set of data was calculated to be 0.058

Relative Standard Deviation: RSD=

0.058 18.07

3

×10

= 4.704

The relative standard deviation for this experimental data set was calculated to be 4.704 parts per thousand signifying a high level of experimental precision.

1 mol Ag+: 1 mol Clmol Ag+= (.01807 L)(.1 M)= 1.807e-3 mol Cl-= 1.807e-3

Based off the balanced equation, for every one mol of Ag+, one mol of Cl- reacts with it. So, to calculate mol of Cl- in the unknown sample, one can calculate the mol of Ag+ and use the relationship to find mol of Cl- which was calculated to be 1.807e-3.

Percent Cl- in Unknown: 1.807e-3 mol Cl- x (35.5 grams Cl-/1 mol Cl-)= 6.415e-2 grams Cl% Cl- in Unknown= (6.415e-2 grams Cl-/1.0408-gram sample Unknown) x 100= 6.16 % Cl-

To calculate the percent chloride in the sample, first the moles of chloride reacted must be converted to grams of chloride. Then, using the percent formula, the grams of chloride was divided by the entire mass of the sample and multiplied by 100 to get the percent chloride in the solution which came out to be 6.16% chloride.

Part B: Table3: This table contains the values for the titration performed in part B of this experiment and contains the data necessary to calculate the concentration (in ppm) of chloride ion in a water sample. Volume of water Sample (mL)

Volume Silver Nitrate to end point (mL)

Corrected Volume after Blank Titration

Concentration Cl- In Solution (Ppm)

100

30.3

29.3

104

Concentration Cl- (ppm): 1 mol Cl-: 1 mol Ag+ .000293 mol Cl- x (35.5 g Cl-/ 1 mol Cl-) = .0104015 g ppm= (mass solute/ mass solution) x 1,000,000= (.0104015g/ 100.0104015g Solution) x 1,000,000= 104 ppm

The experimental ppm of Cl- in this water sample is 104 ppm. This is an extremely high concentration leading me to believe there was some source of error for this experiment either in the procedure or the collection of data.

Conclusion: This lab was a very good introductory experience to the Mohr Method of titrations to find out the chloride ion concentration in various solutions. By using this method, it is possible to find the chloride ion in a variety solutions and has practicality beyond the laboratory and in the real world. This method is extremely useful in many aspects of chemistry and can help analytical chemistry accomplish a wide variety of tasks. However, there are some solutions in which the Mohr method does not work. One example of this are solutions that are highly acidic or basic or for solutions with iodine or thiocyanate ion. This is because these solutions cause the formation of colloid solutions with Ag+ and absorb CrO 4-. In addition, I believe that this lab effectively completed the objectives of introducing students to a new method of titration and the importance of utilizing blank titrations to correct titrant volume. Although this lab did meet its objectives, there absolutely was a source of error in my experiment which skewed my data and caused my value for the concentration of Cl- in a 100-mL water sample to be incredibly large at 104 ppm.

References (1) Instrumental Methods of Analysis Laboratory Manual, Department of Chemistry, Binghamton University, Binghamton, New York, 2017, pp. 57-62...