October 20: Back-Titrations

October 20 : Back-Titrations

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Today in class, we conducted an experiment which dealt with new concepts of stoichiometry: titrations. We were able to choose between two alternatives (eggshells or aspirin analysis). The one that we decided to work on was the eggshell experiment. 

Brief Description:

The major component of eggshells is CaCO3, known as calcium carbonate. We had to react ≈ 0.40g of CaCo3 with excess 1M of HCl, and when the reaction was complete, we also had to titrate the unreacted HCl with 1M of NaOH (acid-base titration). With further calculations, we were able to figure out the amount of calcium carbonate present and the amount of HCl that actually reacted with it.

Objectives:

  1. Determine the amount of CaCo3 present in the eggshell used

  2. Relate experimental titration measurements to a balanced chemical equation

  3. Infer a conclusion from experimental data

  4. Apply reaction-stoichiometry concepts

Procedure:

  1. Prepare your data table.

  2. Wash the pipette with the 1.0M HCl supplied.

  3. Wash the burette with the 1.00M NaOH supplied and fill it up.

  4. Weigh accurately 0.40g (this means weigh around 0.4g, but record the actual reading to 2 decimal places) of the powder and put it into the conical flask. 

  5. Add a few drops of ethanol to help the acid react with the calcium carbonate.

  6. Pipette onto the powder, 10cm³ of HCl and add a few drops of phenolphthalein indicator. Swirl the flask to assist reaction.

  7. Now, titrate the excess acid in the flask against the NaOH from the burette until a faint pink colour appears.

  8. Repeat steps 5-7 as necessary.

Collected Data:

  * Accurate mass of shells 

  * Volume of NaOH before and after titrating

Calculations!:

  1. First, find the change of volume of NaOH. You can figure this out by subtracting the before-volume from the after-volume. This will give you the amount of NaOH used to neutralize the unreacted acid.

  2. Then, find the number of moles of the initial HCL used in this experiment. 

  3. Next, looking at your titration data, work out the number of moles for NaOH; this will also give you the # of moles of HCl (molar ratio used).

  4. Calculate how much of the acid has reacted!

  5. Convert out of moles :) Good job, you've solved the amount of the reacted HCl!

  6. Now, we have to solve how much of CaCO3 is present. To start off, look at the molar ratio of your balanced equation. 

                               2HCl(aq) + CaCO3(s) → CaCl2(aq) + CO2(g) + H2O(l) + HCl(aq)

We can see that the ratio is 2:1 (HCl : CaCO3). And since we already know the # moles of the reacted HCl, you can easily divide that value by 2 to get the # moles of calcium carbonate. Again, convert out of moles and answer the question correctly!

  

Conclusion:
  
See if you can adress the problem that the farmer was concerned about, possibly by calculating the percent of CaCO3 present in the actual eggshell. And also remember to state the limitations and certain errors (random / systematic)!


Assignments:

Finish this lab and STUDY FOR THE TEST! GOOD LUCK!