Chemistry

posted by Jessica

I did a lab the other day where we separated a mixture of NaCl, SiO2, and CaCO3. The Mixture was made by my professor and we did not know the original amounts of each compound. After recovering all of the compounds we found that there was..
36.4% NaCl
36.4% SiO2
40.9% CaCO3
the total % recovered was 113.7% with a 13.7% error. I do not understand how we recovered more that the original sample. One of the post lab questions ask
1. List possible manipulation errors that might contribute to the following errors:
-obtaining a significant larger amount of NaCl than as present in the original sample;
-Obtaining a significant small amount SiO2 than was present in the original sample.

help me please! I thought maybe the physical form of the NaCl changed the mass after the boiling off process, I have also thought about the chemical bonding process, but it doesn't make an sense to me.

  1. DrBob222

    First, I don't know what process you used for separating the components. One common error is this type lab procedure is not drying each and that always contributes to greater than 100% recovery.
    If you care to type in the procedure you used perhaps I can help with the other questions.

  2. Jessica

    Beaker with the mixture, add distilled water, stir, pour through filered funnel= residue and filtrate

    filrate is boiled over a hot plate and the water boils off leaving just the NaCl

  3. Jessica

    I think I understand the SiO2 having a smaller amount present because the SiO2 most will absorb into the atmosphere due to the amounts of oxygen in the mixture to the oxygen level present in the atmosphere.

    I think the NaCl mass changes due to the heating process and alters the mass, not that theres actually more present.

    I really don't know! :(

  4. DrBob222

    I don't think your professor will buy either argument you gave above. The mass of NaCl will not change and SiO2 won't absorb into the atmosphere.

    Frankly, I don't understand how this separates the CaCO3. Did you omit a step? In the process you give the CaCO3 and SiO2 will end up together.
    A possible answer to obtaining a larger amount of NaCl could be two reasons:
    1. Not drying the NaCl completely.
    2. If some of the SiO2 escaped the filtration process (either by going through (or around) the filter paper that would account both for an increase in the NaCl and a decrease in SiO2.

  5. Jessica

    The CaCo3 and SiO2 are both solids so they remain in the filter. From there transfer it to a preweighed evaporation dish and add 3M HCl. From there we have a solid SiO2 and the supernatant liquid that we pour into a beaker and heat off to boiling and add 1M K2CO3. From there we filtrate and the residue is the CaCO3.

  6. Jessica

    Dr. Bob, I think youre right. I think I am over thinking this!

  7. DrBob222

    I'm glad to know you omitted those last steps.
    Do you know what the actual percentages are. If we knew that we could make some educated guesses about why may have gone wrong although that won't answer either of the two question posed.

  8. Jessica

    NaCl 36.4%
    SiO2 36.4%
    CaCO3 40.9%
    Percent recovery 113.7%
    Percent error 13.7%

    Original mass sample is 2.2g
    recovered masses
    NaCl 0.8g
    SiO2 0.8g
    CaCO3 0.9g
    total mass recovered 2.5g

  9. Jessica

    The question about the percent error and the reason for recovering more than the original mass is not asked in my lab questions. Being a math wiz, I know I did my calculations right and I even asked a fellow classmate and they too had a similar outcome. I am really just curious!

  10. DrBob222

    But this is what you started with, not what the prof gave you as an unknown. The most likely reason for >100% recovery, as I've already said, is not drying the samples sufficiently so you weigh all of the sample plus some extra water. And you had to dry three samples; i.e., the NaCl, the SiO2, and the re-precipitated CaCO3. Care is required in drying CaCO3; otherwise CO2 is lost and you weigh CaO and not CaCO3.

  11. Jessica

    That makes sense! Thank you so much for your help! You really have no idea how much I appreciate it!

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