PRECIPITATION TITRATION OF COBALT CHLORIDE Part 1: Perform a Coarse Titration Ta

Question

PRECIPITATION TITRATION OF COBALT CHLORIDE

Part 1: Perform a Coarse Titration Take a 150 mL Erlenmeyer flask from the Containers shelf and place it onto the workbench. Take the cobalt chloride solution from the Materials shelf and add 10.00 mL to the Erlenmeyer flask. Record this volume in your Lab Notes. Take water from the Materials shelf and add 10 mL to the flask to dilute the solution. This dilution makes it easier to visualize the end point. Take thymolphthalein from the Materials shelf and add 2 drops to the Erlenmeyer flask. Take a burette from the Containers shelf and place it on the workbench. Take 0.100 M sodium hydroxide from the Materials shelf and add 50.00 mL to the burette. Double-click on the burette and select show close up in the Item Properties window to get a closer view of the volume of titrant in the burette. Pass the mouse cursor over the burette and a gray tool tip will briefly display the total volume of NaOH in the burette as well as the volume dispensed. Record these volumes in your Lab Notes. Move the Erlenmeyer flask onto the base of the burette. Click and hold the stopcock for a 1 – 2 seconds. You should observe a rapid stream of drops dispensing from the burette. The volume dispensed should be about 2.00 mL. If the volume dispensed is above 2.50 mL, try holding the stopcock for a shorter period of time. Find the length of time you should click and hold the stopcock to deliver approximately 2.00 mL of titrant. Continue to add titrant in 2.00 mL increments. Pause after each dispensation. Record the dispensed in your Lab Notes. Check if the end point has passed after each dispensation. When the reaction reaches end point the solution changes from pink to blue. Stop adding NaOH once you reach the end point. Record the last burette volume at which the solution in the flask was still pink as well as the volume at which the solution turned blue in your Lab Notes. You now know between which two volumes the end point occurred. Record the volume dispensed before the end point as the final volume in your Lab Notes. Clear your station by dragging your containers to the recycling bin beneath the workbench.

Part 2: Perform a Fine Titration Prepare a burette and Erlenmeyer flask as described in steps 1 – 7 in part 1. Add the final volume (before the end point) determined in the coarse titration. To do this, click and hold the stopcock for longer periods of time. The solution in the Erlenmeyer flask should still appear pink. This is near, but not yet at, the titration's end point. Add NaOH one drop at a time. When the solution in the Erlenmeyer flask changes color from pink to blue, stop adding titrant. Record the final volume of titrant dispensed in your Lab Notes. Repeat the fine titration two more times for accuracy, and record the results in your Lab Notes. Clear your station by dragging your containers to the recycling bin beneath the workbench. Remember to press Save Notes.

NOTES: Part 1: 1. 10 mL of cobalt chloride added to erlenmeyer flask 2.10 mL of water added to the cobalt chloride 3.2 drops of the thymolphthalein added to the water and cobalt chloride 4. 50 mL of 0.100 M sodium hydroxide added to the burette 5. 50 mL in the burette. 0 dispensed so far. 6. 20.1 mL initially in the burette 7. 2.98 mL total dispensed first 8. 3.96 mL total dispensed second 9. 4.90 mL total dispensed third 10. 5.96 mL total dispensed fourth 11. 8.31 mL total dispensed fifth 12. 9.18 mL total dispensed sixth 13. 10.25 mL total dispensed seventh 14. 12.23 mL total dispensed eighth 15. 14.23 mL total dispensed ninth 16. 16.26 mL total dispensed tenth 17. 17.96 - 18.02 total dispensed when color changed 18. Was still pink at 17.95 and was blue at 18.05 19. At 18.00 mL dispensed color was sort of grey looking. 20. At 18.05 color was blue again.

QUESTIONS - Please show work, thank you!

11. In what order do the color change and the precipitation occur?
a. the percipitation occurs both before and after the color change
b. the color chagne occurs simultaneously with the percipitation
c. the percipitation occurs after the color change
d. the color change occurs after the percipitaton is complete

12. Suppose you were to use a 0.100 M solution of magnesium hydroxide (Mg(OH)2) instead of the 0.100 M solution of sodium hydroxide. How would this affect the experiment?
a. the stochiometry would be different and there would be 2 mol of the CoCl2 to every 1 mol of the Mg(OH)2
b. the stochiometry would be different and there woulbe be 1 mol of CoCl2 to every 2 mol of Mg(OH)2
c. the results of the experiment would be identical because of the stochimetry is the same
d. the stochiometry would be different and there would be 1 mol of CoCl2 to every 1 of Mg(OH)2

13. Suppose you were to use 0.100 M potassium hydroxide as the base instead of 0.100 M sodium hydroxide. How would this affect the results of the experiment?
a. potassium hydroxide would react with the cobalt chloride, casuing premature precipitation of the cobalt
b. it would not effect the results because the molarity of the base and the stochiometry of the reaction are unchanged
c. it would effect the results since the stochiometry of potassium hydroxide causes a different change in ph
d. it would effect the results because pf the stochiometry of potassium hydroxide reacting with the cobalt chloride is different

14. What kind of reaction occurs when sodium hydroxide is added to cobalt chloride?
a. double displacement
b. single displacement
c. hydrolosis
d. oxidation-reduction

15. Suppose a student misunderstood the procedures and added 10.00 mL of 0.100 M sodium hydroxide to the Erlenmeyer flask and 50.00 mL of cobalt (II) chloride to the burette as shown in the figure below. How will he be able to identify the end point of the titration?
a. he will not be able to identify it because you cannot reverse the titration
b. the solution in the erlenmyer flask will turn pink
c. the solution in the erlenmeyer flask will turn white
d. the solution will stay blue so he will not be able to identify it.

Explanation / Answer

ans)

from above data

11) option d...

The color change occurs after percipitation is complete

12) option d,,,,,

stoichiometrically the valency of magnesium is +2 .. and calcium is also +2 so for every one mole of cacl2 is react with one mole of Mg(OH)2

CaCl2+ Mg(OH)2 -----> Ca(OH)2+ MgCl2

13) option  b..,...

it would not effect the results beacuse the molarity of the base and the stochiometry of the reaction are unchanged

14) option a,,,

calcium and sodium are displaces each other

15)option b....

titration will take reverse..

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