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In this experiment, we monitored conductivity during the reaction that occurs between sulfuric acid – H2SO4, and barium hydroxide – Ba(OH)2, in order to figure the equivalence point, or stoichiometric point. The equivalence point is the point in a chemical reaction in which chemically equivalent quantities of acids and bases are mixed. In summation of this, the moles of the sulfuric acid are equivalent to the moles of barium hydroxide (base).

Gravimetric analysis, a method of quantitative chemical analysis in which the constituent sought is converted into a substance (of known composition) that can be separated from the sample and weighed. The steps commonly followed in gravimetric analysis are (1) preparation of a solution containing a known weight of the sample, (2) separation of the desired constituent, (3) weighing the isolated constituent, and (4) computation of the amount of the particular constituent in the sample from the observed weight of the isolated substance. There are five requirements to conduct gravimetric analysis on a sample.

• The compound formed must be pure and of known stoichiometry

• The precipitation reaction must be virtually complete, that is, the percent yield of the solid product should be 99.9% or better.

• The precipitation reagent should be specific for the sample being determined, interference by other types of compounds forming precipitates should be minimal.

• The solid that is precipitated should be in the form of reasonably large, well-formed crystals

• The molecular weight of the precipitated solid should be high enough that a reasonable weight of precipitate is generated even when the weight percent of substance being determined is low in the unknown sample.

Purpose –

The purpose of this lab is to analyze a solution of Ba(OH)

2

 of unknown concentration

so one can learn how to carefully handle, isolate, and analyze a chemical sample.

    The purpose of this lab is to analyze a solution of Ba(OH)2 with unknown concentration, so that one can learn how to carefully handle, isolate, and analyze a chemical sample. These skills are necessary and crucial for the chemistry lab setting, as chemical analysis Is the first step in numerous chemical, biochemical, and other applications. This lab will teach you how to quantitatively analyze Barium Hydroxide by using Conductimetric titrations and gravimetric determination.

Theory and Ionic Equations –

The reaction between barium hydroxide and sulfuric acid, yields water and an insoluble product, barium sulfate, as shown below in the following reaction:

2H+(aq) + SO42-(aq) + Ba2+(aq) + 2OH-(aq) ---> BaSO4(s) + 2H2O(l)  - Net ionic Equation

Ba(OH)2 (aq) + H2SO4 (aq)→ BaSO4 (s) + 2H2O(l) - Balanced Molecular Reaction

Materials

• LoggerPro

• Conductivity Probe

• Two 250 mL beakers

• 50 mL graduated cylinder

• 10 mL pipet and bulb

• 50 mL burette and clamp

• Ring stand

•  Funnel

• Balance

• Magnetic stirrer, stir bar

• Filter paper

• Distilled water

• 0.100 M Sulfuric Acid

• Barium hydroxide solution

2. Procedure

Conductimetric Titration

1. Once entering the laboratory, the titration apparatus should be set up beforehand.

2. Materials such as Barium Hydroxide and Hydrogen Sulfide, should already be set-up beforehand and are found underneath various hoods across the laboratory.

3. Obtain a 10 mL graduated cylinder in order to transfer 10 mL of the Ba(OH)2 solution into a 100 mL beaker. Next, add an additional 30 mL of distilled water into the graduated cylinder containing the 10 mL of barium hydroxide solution.

4. Connect the Drop Counter to DIG/Sonic 1 of the Vernier computer interface, and lower it onto the ring stand.

5. Connect the Conductivity Sensor to Channel 1 of the interface. Set the selector switch on the Conductivity Probe to the 0-20,000 range. Now connect the interface to the computer with the correct cable. Finally, plug the interface AC adapter in.

6. Start the Logger Pro program and open file “16b Conductivity Titration (drop count)” from the Advanced Chemistry with Vernier folder.

7. Measure of 60 mL of .100 M H2SO4 into a 250-mL beaker. Record the precise concentration of sulfuric acid into your data table.  

8. Obtain the plastic 60 mL reagent reservoir. Close both valves by turning the handles to a horizontal position. Set up the reagent reservoir for the titration by testing for leaks with water, by rinsing it with a few mL .100 M H2SO4, filling it with slightly more than 40 mL. 100 M Sulfuric Acid

9. Place the beaker in which the rinsed sulfuric acid was poured into below the tip of the reservoir.

• Drain a small amount of the sulfuric acid solution into the 250 cm3 beaker so that it fills the reservoir’s tip. To do this, turn both valves to the vertical position for a couple of seconds then turn them both back to the horizontal.

• Turn the top valve to the vertical and then slowly open the bottom valve until you have attained a drop rate of one drop per second (Achieving this rate is important so that the drop counter can accurately detect the number of drops). Once you have set the bottom valve to this drip rate, do not touch it again for the remainder of the experiment. Close the Top Valve (Horizontal). At this point you can use just the top valve to start and stop the flow of sulfuric acid in the beaker below.

• Discard the sulfuric acid poured into the 250 cm3 beaker into the waste beaker kept in one of the fume hoods of the Laboratory.

• Before conducting the experiment, it is important to calibrate the volume of drops that wil be delivered so that the Vernier apparatus can obtain proper readings.

1. On the top row of the logger Pro tool bar open the EXPERIMENT menu and choose CALIBRATE.

2. Select the automatic button and continue as follows:

3. Place a clean and dry 10 cm3 graduated cylinder directly below the slot on the drop counter, lining it up with the tip of the reagent reservoir.

4. Click the start button and immediately open the top valve on the reagent reservoir (turn to the vertical) and do not touch the bottom valve as it’s set to the desired flow rate. You should see the drops being counted on the screen

5. When the Volume of the Sulfuric acid solution in the graduated cylinder is between 5 and 6 cm3 close the top valve of the reagent reservoir.

6. Enter the precise volume of Sulfuric Acid in the cylinder (nearest 0.1 cm3) in the edit box. Record the number of Drops/cm3 displayed on the screen for possible future use.

7. Click OK. Discard the sulfuric acid solution in the graduated cylinder into the waste beaker and set the cylinder aside.

• Assemble the Apparatus.

1. Insert the Conductivity sensor through the large hole in the drop counter.

2. Adjust the positions of the drop counter and reagent reservoir so they are both lined up with the center of the magnetic stirrer.

3. Place the magnetic stir bar in the beaker of the Barium hydroxide solution and lift the Conductivity sensor and slide the 100 cm3 beaker containing the barium hydroxide solution that was taken from the fume hood at the beginning of the experiment.

4. Plug the stirrer’s AC adapter and turn it on so the stir bar spins, but not too rapidly.

5. Adjust the reservoir so that its tip is just above the drop counter slot and drops will fall directly through the drop counter into the beaker. The drops MUST fall through the drop counter in the exact center (front to back AND left to right) of the opening in the drop counter.

• Conduct the titration

1. Click COLLECT to begin monitoring conductivity. No data will be collected until the first drop goes through the drop counter slot. Fully open the top valve (the bottom valve should still be adjusted so drops are released at a rate of one drop per second

2. Observe your graph; the conductivity will drop below 100 μS and then rise again. The titration curve will be V-shaped. After the conductivity reaches about 5000 μS, click STOP. Turn the top valve of the reagent reservoir to the Horizontal position.

• Examine the data on the graph to find the equivalence point, that is, the volume when the conductivity value reaches a minimum. To do this, highlight the linear portion of the data where the conductivity is decreasing only. Do the same for the linear portion of the graph that is increasing only. Highlight the area where the two linear fit lines intersect and zoom in for a better view. Go the ANALYZE menu and select “interpolate”. Move the mouse cursor to the volume reading where both linear fit display boxes show the same conductivity-the volume displayed is below the equivalence point volume for the titration.

• Place the beaker with the titration solution on a hot plate and begin heating to a near-to-low boil. While the solution is heating to flocculate the barium Sulfate, weigh the now cool filter crucible to the nearest cm3, and then place the vacuum filtration apparatus securely to form a good seal between the filter crucible and the rubber cap.

• Once the solution has been near boiling for a minute or two, remove it from the hotplate and allow it to cool close the room temperature

• Before filtering the solution, obtain a squirt bottle of methanol. Just before filtering your solution, spray about a cm3 of the methanol into the crucible to wet the filter disk. Turning on the water to create a vacuum will now seat the filter disk and prevent the loss of sodium Barium Sulfate.

• Quickly stir your solution to suspend the barium sulfate solid in the liquid, and immediately begin filtering your solution, being careful not to overfill the crucible or spill any of the solution Do not tilt the beaker steeply enough to allow the magnetic stir bar to enter the crucible-it should stay in the beaker.

• Once all the liquid is out of the beaker, you will need to rinse any barium sulfate remaining in the beaker into the crucible using the methanol squirt bottle.  Again, be careful not to rinse the stir bar into the crucible. After all the precipitate, has been transferred to the filter crucible, wash the precipitate with a small portion of methanol. Allow the crucible to sit on the vacuum apparatus for about a minute

• Get a beaker ready to put your crucible in the drying oven.

• Place the crucible in the beaker and place it in the drying over for 20 minutes.

• At the end of drying time, carefully remove the beaker with your crucible from the drying oven and place it in your desi cooler to cool. Once cool, record the weight of the crucible with the barium sulfate and then determine the mass of barium sulfate collected. With this information, one can calculate the original concentration of the barium hydroxide solution.

• At the end, discard your barium sulfate solid in the solid wastes container. Clean the filter crucible thoroughly to remove all traces of barium sulfate solid, place a new filter disk in the crucible, and return it to the oven to dry.

• Rinse the conductivity probe tip with distilled water and blot dry. Also, thoroughly rinse the solvent reservoir with distilled water (including the tip). Return the equipment kit to the stockroom. Leave the drop counter, reservoir and stir station set up on the lab bench.

DIAGRAM OF SET-UP

Conductimetric Analysis

Moles of barium = Moles of H2SO4 used to reach equivalence point (Based on stoichiometric ratios of reactants and products)

Mol H2SO4 = [equivalence point Vol(L)x acid molarity (M)]

concentration of barium hydroxide = moles of barium Hydroxide / volume of Ba(OH)2 used

Gravimetric Determination

mass of BaSO4 precipitate = mass of ppt and crucible – mass of crucible

moles of Barium Sulfate = mass of BaSO4 precipitate / Molecular weight of BaSO4

concentration of barium hydroxide = moles of barium Hydroxide / volume of Ba(OH)2 used

OBSERVATIONS

Equivalence point from Titration (cm3) = 12.402 cm3

Molarity of the H2SO4 solution = 0.100 M

Volume of original Ba(OH)2 solution (cm3) = 10 cm3

 Mass of Crucible (g) = 38.839 g

Mass of Crucible and precipitate (g) = 29.180 g

CALCULATIONS

Moles of Barium from Titration (mole)= .100 M x 0.017926 L=0.00124 moles

Concentration of Ba(OH)2 from Titration (M) = 0.0017926 / .01 L = 0.179 M

 Mass of precipitate= 26.266g – 25.921g =0.345 g

Concentration of Ba(OH)2 from gravimetric (M) = 0.345 g / 233 g = 0.00148 moles Ba(OH)2

M = 0.00148069 / 0.010 L = 0.148 M 

Figure 1.2 – Data and Calculations.

Figure 2 – Data Collection from LoggerPro. Graph displaying Volume vs Conductivity.

ERROR ANALYSIS

DATA ANALYSIS AND ERROR

By comparing the results of my calculations of the Concentrations, it seems that the Equivalence point method is more accurate, as gravimetric method involves loss of precipitate during filtration, proper drying is required, proper temperature and experimental conditions are required. Errors tend to add up thus the method is not accurate when compared to Conductimetric titration.

SOURCES OF ERROR IN THE LAB

Sources of error could have stemmed from the scales that used to measure the mass of the crucible with, and without the precipitate. One could also read the values from the graduated cylinder wrong. This is known as the Parallax error, or not looking at the lower meniscus. Moreover, some of the precipitate could have been left in the beaker and not completely washed out by the methanol used, which in turn, could have resulted in the measurement having a lower mass. Thus, affecting the calculation of the concentration of the precipitate.

Another source of error, could be that the precipitate was not kept in the drying oven for long enough to rid of the moisture, which could lead to inaccurate concentration calculations. This could be rectified by keeping the crucible in the drying oven for a longer time to ensure all the moisture was removed.

A major source of error in this lab setting, could have been the failure to properly handle the solutions, thus introducing the likelihood of contamination, and altering the actual measurements of the data.

The second error is that it is possible for the reservoir tip to begin leaking – or the titrator still containing water -  as the titration due to movements that can offset it, and this could lead to a false equivalence point in that the conductivity graph would show a lower value as it is linked to the drop counter’s rate of drops. This would lead to a lower concentration than instead of a greater one. This could have been prevented, by thoroughly checking for leaks, or by using a surface for the bottom valve to remain secure at the ideal rate.

The two different values of the concentration of barium hydroxide from titration and gravimetric analysis, are valid proof that multiple random or systematic errors could have easily affected the results of this experiment.

Conclusion

The research question for this laboratory revolved around the determination of an unknown concentration. The goal of this experiment is to determine the concentration of a known solution, by titrating it with another strong electrolyte that will react with the first substance, and enter a non-ionic state. By doing this, we change the number of ions in the solution, and thus change its conductivity. This was done by determining the equivalence point with the Conductimetric method.  In the experiment, the Barium Hydroxide solution was titrated with a sulfuric acid solution to find the equivalence point.  The equivalence point was found at the minimum conductivity by analysis of a graph obtained from a logger pro.  Using the equivalence point, the concentration of the barium hydroxide was found.  The equivalence point from titration was 17.926 mL.  Using the equivalence points, the concentration of the Barium Hydroxide solution was 0.179 M. Gravitation analysis was used as well to determine the concentration of the precipitate. The mass of the precipitate was 0.345 g and the obtained concentration was 0.148 M. Sources of error which could explain the difference in concentration values of Barium Hydroxide from Conductimetric and gravitational Analysis have been considered as well. The lab served its purpose in teaching us how to isolate, handle and analyze chemicals through the techniques of Conductimetric titration and gravitational analysis.

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