Experiment 4 Post-Lab Report PDF

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you say yee, I say haw. I repeat, you say yee, I say haw. Haw!...

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Experiment 4: The Analytical Preparation of a Buffer and A Primer of Essential Lab Skills Trevor Du 9/29/2021 Mikaylin Nogler, CHEM 241L, Section 417

"I pledge that I have not used someone else’s old or current lab report when writing this lab report. I pledge that I did not collaborate with any other students, except where allowed, and that the report I submitted here contains my own ideas, thoughts, observations, calculations, data, conclusions and answers. Lastly, I pledge that the data presented in this report was my own collected during lab or provided to me by my TA.”

Materials and Methods: Calibration of the 5 mL Micropipette: A large beaker was filled with water and the temperature was measured with an alcohol thermometer. The mass of an empty 100-mL beaker was measured, and the 5 mL micropipette was set to 0.500 mL. This amount of water was dispensed into the empty beaker. The beaker was weighed and this sequence was repeated with increasing amounts of water until 5.000 mL was reached. This entire process was completed twice. Preparation of the 0.05 M NaOH and HCl Solutions: Stock concentrations of 0.1 M HCl and NaOH were used. Small amounts of stock solution were put into 100 mL beakers while two 100 mL volumetric flasks were obtained. The stock solution was then transferred to the volumetric flasks and diluted with water. Theoretical Preparation of the 0.05 M Buffer Solutions: 0.243 g of sodium acetate and 6.43 mL of acetic acid were combined in a beaker. The beaker was placed on a stir plate, and after stirring, the stir bar was removed and the beaker contents were transferred to the 100-mL volumetric flask. The solution was then diluted to a 100 mL volume with DI water. Practical Preparation of the 0.05 M Buffer Solutions: 10 mL of 0.50 M acetic acid was added to a 100-mL beaker and diluted with DI water. 23.5 mL of 0.1 M NaOH was added to the acetic acid, and a pH of 2.40 was measured by the calibrated Vernier GoDirect pH Sensor inserted into the beaker. Determination of the Buffer Capacity: Two 100-mL beakers were filled with 20 mL of the practical buffer solution and stir bars were inserted into both beakers. Then, 0.1 M NaOH was added until a pH of 12 was reached in the solution. This process was repeated with the 0.1 M HCl instead of NaOH until a pH of 2 instead of 12 was reached.

Results and Calculations: Figure 1. Calibration curve for a 5 mL micropipette 0.150

Correction Factor (mL)

0.100 0.050 0.000 -0.050 -0.100 -0.150 -0.200 0.000

1.000

2.000

3.000

4.000

5.000

6.000

Micropipette Reading (mL)

Table 1.

Total Volume of 0.100 M HCl Added (mL) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8

Buffer Capacity Data for the Addition of 0.1 M HCl to 0.05 M NaOH Corrected Formal Volume of Amount of HCl Buffer Concentration of pH 0.100 M HCl Added (mol) Capacity (M) HCl (M) Added (mL) 0.000 0.000E+00 0.000 12.35 0.584 5.840E-05 0.003 12.39 0.07 1.168 1.168E-04 0.006 12.37 0.13 1.752 1.752E-04 0.008 12.35 0.13 2.336 2.336E-04 0.010 12.34 0.24 2.920 2.920E-04 0.013 12.32 0.11 3.504 3.504E-04 0.015 12.3 0.11 4.088 4.088E-04 0.017 12.28 0.10 4.672 4.672E-04 0.019 12.24 0.05 5.256 5.256E-04 0.021 12.2 0.05 5.840 5.840E-04 0.023 12.14 0.03 6.424 6.424E-04 0.024 12.11 0.06 7.008 7.008E-04 0.026 12.08 0.05 7.592 7.592E-04 0.028 12.04 0.04 8.176 8.176E-04 0.029 12 0.04 8.760 8.760E-04 0.030 11.94 0.02 9.344 9.344E-04 0.032 11.86 0.02

8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18

9.928 10.512 11.096 11.680 12.264 12.848 13.432 14.016 14.600 15.184 15.768 16.352 16.936 17.520 18.104 18.688 19.272 19.856 20.440 21.024

9.928E-04 1.051E-03 1.110E-03 1.168E-03 1.226E-03 1.285E-03 1.343E-03 1.402E-03 1.460E-03 1.518E-03 1.577E-03 1.635E-03 1.694E-03 1.752E-03 1.810E-03 1.869E-03 1.927E-03 1.986E-03 2.044E-03 2.102E-03

0.033 0.034 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.048 0.049 0.050 0.051 0.051

11.78 11.69 11.59 11.38 11.06 10.58 9.74 7.33 2.91 2.53 2.41 2.29 2.22 2.18 2.13 2.11 2.09 2.04 2.02 2

Table 2a. Buffer Capacity Data for the Addition of 0.1 M NaOH to the Theoretical Buffer Total Corrected Amount of Formal Volume of Volume of Buffer Capacity pH NaOH Added Concentration 0.100 M 0.100 M (M) (mol) of NaOH (M) NaOH NaOH Added Added (mL) (mL) 0 0.000 0.000E+00 0.000E+00 4.39 0.5 0.584 5.840E-05 2.837E-03 4.48 0.031523945 1 1.168 1.168E-04 5.518E-03 4.59 0.02436916 1.5 1.752 1.752E-04 8.054E-03 4.69 0.025366691 2.5 2.920 2.920E-04 1.274E-02 4.85 0.029284583 3 3.504 3.504E-04 1.491E-02 4.99 0.015486683 3.5 4.088 4.088E-04 1.697E-02 5.17 0.01146114 4 4.672 4.672E-04 1.894E-02 5.42 0.007861361 4.5 5.256 5.256E-04 2.081E-02 5.85 0.004359187 5 5.840 5.840E-04 2.260E-02 10.64 0.000373637 5.5 6.424 6.424E-04 2.431E-02 11.41 0.002221575 6 7.008 7.008E-04 2.595E-02 11.66 0.00654654 6.5 7.592 7.592E-04 2.752E-02 11.81 0.01044903 7 8.176 8.176E-04 2.902E-02 11.91 0.015023818 7.5 8.760 8.760E-04 3.046E-02 11.96 0.028827343 8 9.344 9.344E-04 3.184E-02 12.02 0.02306659

0.02 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.02 0.02 0.04 0.04 0.01 0.04 0.04

Table 2b. Buffer Capacity Data for the Addition of 0.1 M HCl to the Theoretical Buffer Total Volume of 0.100 M HCl Added (mL)

Corrected Volume of 0.100 M HCl Added (mL)

Amount of HCl Added (mol)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

0.000 0.584 1.168 1.752 2.336 2.920 3.504 4.088 4.672 5.256 5.840 6.424 7.008

0.000E+00 5.840E-05 1.168E-04 1.752E-04 2.336E-04 2.920E-04 3.504E-04 4.088E-04 4.672E-04 5.256E-04 5.840E-04 6.424E-04 7.008E-04

Formal Concentration of HCl (M) 0.00E+00 2.84E-03 5.52E-03 8.05E-03 1.05E-02 1.27E-02 1.49E-02 1.70E-02 1.89E-02 2.08E-02 2.26E-02 2.43E-02 2.59E-02

Figure 2. Buffer capacities 0.30

Buffering Capacity (M)

0.25 0.20 0.15 0.10

0.05 0.00

0

5

10

15

20

25

30

Volume of 0.1 M HCl or NaOH added (mL)

35

40

pH

4.31 4.2 4.02 3.83 3.58 3.14 2.62 2.39 2.26 2.16 2.07 2.03 1.99

Buffer Capacity (M)

0.025792319 0.014892264 0.01335089 0.009616084 0.005185255 0.004169492 0.008969588 0.015118002 0.018744502 0.019885809 0.042765325 0.040915877

Example Calculations:

For the addition of 4.0 mL of 0.1 M HCl to the theoretical buffer solution as in Table 2b, 4.0 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 0.100 𝑀 𝐻𝐶𝑙 𝑎𝑑𝑑𝑒𝑑 = 4.0 + 0.084 ( ) = 4.672 𝑚𝐿 0.50 1𝐿 0.1 𝑚𝑜𝑙 ) = 4.672 × 10−4 𝑚𝑜𝑙 𝑚𝑜𝑙𝑒𝑠 𝐻𝐶𝑙 𝑎𝑑𝑑𝑒𝑑 = 4.672 𝑚𝐿 ( )( 1𝐿 1000 𝑚𝐿 𝑓𝑜𝑟𝑚𝑎𝑙 𝑐𝑜𝑛𝑐. 𝑜𝑓 𝐻𝐶𝑙 =

4.672 × 10−4 = 1.89 × 10−2 𝐹 4.672 + 20 ( 1000 )

∆𝐶 0.0189 − 0.0170 | = 0.0151 𝑏𝑢𝑓𝑓𝑒𝑟 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = | |=| 2.26 − 2.39 ∆𝑝𝐻 End of Lab Report Questions: 1) Describe how you would use the calibration curve (Figure 1) you created for your micropipette to correct your measurements. Also consider how you would use your calibration curve to adjust a measurement that falls in between two points. What would this assume? The calibration curve is used to correct error the volume of strong acid or base added. For example, in our experiment the 5 mL micropipette was used. When the micropipette was set to 500 μL, the actual volume of water measured was 84 μL higher than the reading on the micropipette. The y-values on the calibration curve represent the correction factor, or the difference between the actual volume and what the micropipette says. This factor must be used to correct for the volume in calculations where the micropipette may be involved. If a measurement falls in between two points, one could still follow the graph up and find a correction factor, but

this assumes a linear relationship between all points on the graph, which we know isn’t true. It’s a “calibration curve”, not “calibration lines”. 2) In creating your calibration curve, you completed two trials. What would happen to the uncertainty if you increased the number of trials? There is a specific mathematical function that describes this. What is this function? As more trials are completed, the uncertainty decreases. The standard error function contains a variable for the total number of data values, and if this is increased, error goes down. 3) In one paragraph, describe how would you calibrate a 100-mL volumetric flask. First, the flask should be rinsed, dried, and weighed. The flask should then be filled to the line with DI water and weighed again. The temperature of the water should be measured with a thermometer. Volume can be calculated by taking the difference of the two weights and dividing by the density of water at the recorded temperature. This should be repeated a few times to ensure accuracy of volume. 4) In your own words, give an overview of how a pH meter operates. What does a pH probe directly measure? How? What must you do to convert your measured quantity to a pH? A pH meter is basically an electrode; thus, it operates by measuring a potential difference. These probes often have tips made of a special glass containing alkali metal ions. Hydrogen ions in aqueous solution participate in an ion exchange reaction with these metal ions, which creates the difference in electric potential that is measured by the pH meter. With our current technology, we don’t need to perform any extra conversions as modern pH probes interpret the voltage to give us a pH value automatically, as we saw in the Vernier Graphical Analysis software.

5) Compare and contrast the buffer capacity as a function of pH of the control experiments and the buffered solutions (Figure 2) over the pH range of 2-12. The buffering capacity of the theoretical buffer solutions (Tables 2a and 2b) is highest when the pH of the solution is closest to the pKa of acetic acid (4.76).2 Conversely, the buffering capacity at this pH for the control experiments (Table 1) is practically zero. At very low and very high pH (just above 2 and just below 12), the control experiments had a better “buffering capacity”, although this is not a true buffering capacity because there is no conjugate weak acid + weak base pair. 6) Excluding human error, consider one error that may have occurred during your experiments and describe how it would have affected your final result. Was this error random or systematic? One possible source of error that could have occurred is an inaccurate reading for the volume of the buffer solutions. For example, reading the meniscus from above eye level would produce a value less than if the meniscus were viewed from eye level. This systematic error would in turn affect the calculated values of moles HCl/NaOH added, formal concentration of NaOH, and therefore buffering capacity. 1. Experiment 4 Lab Manual 2. Quantitative Chemical Analysis- “Preparing a Buffer in Real Life”...